Champagnon, J. 2011. Consequences of the introduction of

Transcription

UNIVERSITE MONTPELLIER II
SCIENCES ET TECHNIQUES DU LANGUEDOC
THESE
pour obtenir le grade de
DOCTEUR DE L'UNIVERSITE MONTPELLIER II
Discipline: Biologie de l’évolution et écologie
Ecole Doctorale: Systèmes Intégrés en Biologie, Agronomie, Géosciences, Hydrosciences, Environnement
CONSEQUENCES OF THE INTRODUCTION OF INDIVIDUALS WITHIN HARVESTED POPULATIONS: THE
CASE OF THE MALLARD ANAS PLATYRHYNCHOS
(Translation of the French original version)
Jocelyn CHAMPAGNON
Soutenue publiquement le 15 décembre 2011 devant le jury composé de:
William SUTHERLAND, Professeur, Université de Cambridge, Royaume-Uni
Jean-Michel GAILLARD, Directeur de recherche, CNRS, Lyon, France
Isabelle OLIVIERI, Professeur, Université Montpellier 2, France
Johan ELMBERG, Professeur, Université de Kristianstad, Suède
Matthieu GUILLEMAIN, Ingénieur, ONCFS, Arles, France
Michel GAUTHIER-CLERC, Directeur de recherche, Tour du Valat, Arles, France
Jean-Dominique LEBRETON, Directeur de recherche, CNRS, Montpellier, France
, Rapporteur
, Rapporteur
, Examinatrice
, Examinateur
, Directeur
, Co-directeur
, Invité
Note: This version is a quick translation of the original thesis wrote in French and it is
destined to non-speaking French reviewers. It has not been corrected by native English
speaking
Acknowledgements
First a warm thank to Matthieu, who initiated this thesis, ever available and supporting
director. With his sharp eye on the studied topics and his resources to answer questions, I
could not have expected a better supervisor.
A great thank to Michel who co-supervised me with competence and clever advice. Thanks
to him, the Tour du Valat hosted me generously.
I thank Jean-Dominique, co-supervisor who made himself available despite his numerous
solicitations. His experience and advice were of great value. His sympathy always made our
meeting very pleasing.
I wish to thank the jury members Bill Sutherland, Jean-Michel Gaillard, Isabelle Olivieri and
Johan Elmberg for having accepted this reading during a busy period, hoping to have raised
some interest for this work.
The whole of this thesis was followed from abroad by Johan Elmberg, member of the thesis
committee, through an efficient and friendly collaboration.
Thanks to the Office National de la Chasse et de la Faune Sauvage for funding and support to
this project. A great thank to Jean-Marie Boutin, Vincent Schricke, Valérie Guérineau and all
the CNERA Avifaune Migratrice staff.
The Tour du Valat is a foundation with a happy, dynamic and determined staff that I want to
thank. I greatly liked this sympathetic and stimulating atmosphere. I personally thank Jean
Jalbert, Patrick Grillas and Jean-Jacques Bravais for having largely complemented my PhD
grant and some of the research, as well as Marie who makes everything so simple.
Thanks to the SIBAGHE doctoral school for the financial support to my stay in the Czech
Republic, which made some of the work presented here possible.
Many people contributed to this thesis scientifically. In particular, the expertise of CEFE
researchers was of paramount importance. First, thanks to Pierre-André Crochet to train me
to population genetics analyses in a nice and efficient context. Thanks to Olivier Gimenez for
making himself available, his advice and sympathy. Lastly, I thank Roger Pradel and Rémi
Choquet, two super-competent scientists who always answered my multiple questions.
I thank Pierre Legagneux and Olivier Devineau for our collaboration and friendly exchanges. I
also appreciated working with Jakub Kreisinger and Dasa Čížková, in addition to being
overwhelmed with a memorable week with them.
A large part of the experiments took place at the Marais du Vigueirat thanks to Jean-Laurent
Lucchesi, as well as Grégoire Massez, Rémi Tiné, the managing wardens and all the staff
working in a friendly atmosphere on this nice marsh.
Another important part of the fieldwork was on twenty communal and private hunting
estates in Camargue. We wish to thank the landowners, managers and wardens of these
domains who helped us during our experiments, especially those who accepted to let us
sample their hunting bags. All hunters largely collaborated through Mallard ring reporting,
and we sincerely thank them for this.
We warmly thank the curators of museums and owners of private naturalised bird
collections for letting us sampling Mallard DNA. We also thank the owners of Mallard farms
for letting us sample their birds or directly sending DNA samples.
Jean-Baptiste Mouronval showed a great involvement and helped me all along this thesis. I
thank him for his points of view, vision, doubts and insight, in the field or regarding our
shared subjects of thought.
Thanks to Marion Vittecoq, for our arrangements on the hunting as well as scientific
grounds, and mostly for her energy and good mood.
Thanks to Franck Latraube and François Bourguemestre for having been involved in the Loire
Estuary and the Brenne, respectively.
A great thank to the “released Mallards” students: Perrine Lair, Quentin Charel, Aurélien
Villard and Mathieu Famette. I would also like to acknowledge Pär Söderquist, PhD student
on Mallard releases in Sweden who did a great job during Camargue fieldwork, sometimes
under harsh conditions.
A special final thank to those who shared great irrational moments in my office, just for fun:
François Cavallo, Guillaume Gayet, Anne-Laure Brochet, Jonathan Fuster. All were of great
scientific, proofreading, and administrative or fieldwork help.
For many a PhD thesis is a rite of passage where the candidate has to demonstrate his
abilities through day and night work during three years. For a few others, it is just short-term
contract. The work of the doctorate is certainly between these extremes, sometimes causing
deep depression or joy… I tried to find a stable and meaningful way, which was made
possible by the wonderful people I met over these three years. The closest ones first, then
the music companions, the happy daily workers, the Camargue naturalists as well as all those
who happened to visit Tour du Valat. Simply thanks to those who share some bits of life I
Arles, in l’Ecole, in the Rendez-vous are here forever, for a while or are already gone.
To the NACICCA spirit and all its actors,
To all my growing family,
To Magui and Nature
.
ABSTRACT
The consequences of releasing captive-bred game animals into the wild have received little
attention, despite their potential impact for target populations in terms of demography,
behaviour, morphometrics, genetics and pathogens. The present study considers Mallards
Anas platyrhynchos released for hunting purposes, an increasing practice in Europe over the
last 30 years. Because of domestication process in game farm facilities, our study shows high
natural mortality of these ducks once released compared to wild Mallards, in addition to
high vulnerability to hunting. A clear genetic differentiation allows discrimination of released
and wild Mallards. Hybridization with wild Mallards exists, but did not result into significant
introgression. Generally, genetic as well as demographic contributions of captive-bred birds
to the natural population were low, but a morphological modification associated with
releases was recorded over 30 years in natural population. Ecological consequences of the
releases for the wild population seem to be limited, but caution should be maintained on the
possible transmission of pathogens (occasionally high prevalence of avian Influenza A in
some breeding facilities) and the genetic risks associated with long-term releases.
Keywords: Population dynamics, Population genetics, Restocking, Hunting, Duck,
Domestication
CONSEQUENCES DES INTRODUCTIONS D’INDIVIDUS DANS LES POPULATIONS EXPLOITEES:
L’EXEMPLE DU CANARD COLVERT ANAS PLATYRHYNCHOS
RESUME
Le renforcement des populations naturelles exploitées par des individus captifs est rarement
évalué, bien qu’il puisse induire des modifications notables sur la population naturelle à de
nombreux niveaux: démographie, comportement, morphologie, génétique, pathogènes. Ce
travail de thèse concerne les introductions de canards colverts Anas platyrhynchos réalisées
à des fins cynégétiques. Cette pratique est très répandue en Europe, depuis plus de trente
ans. Du fait de leur domestication en élevage, les canards lâchés subissent une mortalité
naturelle très forte comparée aux oiseaux sauvages, à laquelle s’ajoute une plus grande
vulnérabilité à la chasse. Une différenciation génétique marquée permet de discriminer les
oiseaux lâchés de leurs congénères sauvages. Des croisements entre les deux groupes sont
détectés, mais l’introgression reste limitée. Globalement, la contribution démographique
and génétique des individus d’élevage à la population sauvage est faible, même si une
modification morphologique attribuable aux lâchers a été constatée dans la population
sauvage en trente ans. Les conséquences écologiques pour la population réceptrice
semblent donc limitées, mais une vigilance continue doit s’exercer concernant la diffusion de
pathogènes (forte prévalence occasionnelle de virus Influenza A dans les élevages) and les
risques génétiques associés au renforcement sur le long terme.
Mots clés: Dynamique des populations, Génétique des populations, Renforcement, Chasse,
Canard, Domestication
CONTENTS
1.
INTRODUCTION .............................................................................................................................. - 8 MAN IN ITS ENVIRONMENT ................................................................................................................................. - 9 Addition of individuals ............................................................................................................................... - 9 Harvest .................................................................................................................................................... - 10 Restocking for hunting............................................................................................................................. - 10 RESTOCKING OF EXPLOITED POPULATIONS: THE CASE OF THE MALLARD ..................................................................... - 11 History of Mallard releases...................................................................................................................... - 12 Survival and breeding abilities of released individuals ............................................................................ - 14 Potentially deleterious impact on the natural population....................................................................... - 16 AIMS OF THE THESIS ........................................................................................................................................ - 19 -
2.
EFFECTS OF CAPTIVITY.................................................................................................................. - 21 GENETIC DIFFERENTIATION................................................................................................................................ - 22 Neutral markers....................................................................................................................................... - 22 Markers under selection .......................................................................................................................... - 23 MORPHOLOGICAL DIFFERENTIATION ................................................................................................................... - 23 Digestive tract.......................................................................................................................................... - 23 Body condition ......................................................................................................................................... - 24 BEHAVIOURAL DIFFERENTIATION ........................................................................................................................ - 25 Time-budget ............................................................................................................................................ - 25 Diet .......................................................................................................................................................... - 25 Response to predators............................................................................................................................. - 25 CONSEQUENCES IN TERMS OF FITNESS ................................................................................................................. - 26 -
3.
SURVIVAL AND REPRODUCITON OF CAPTIVE-BRED MALLARDS.................................................... - 27 MARKING METHOD ......................................................................................................................................... - 28 RINGING - RECOVERY....................................................................................................................................... - 29 Ring return rate ....................................................................................................................................... - 29 Temporal distribution .............................................................................................................................. - 30 Dispersal .................................................................................................................................................. - 31 MARKING-RESIGHTING-RECOVERY ..................................................................................................................... - 31 Effect of hunting on survival .................................................................................................................... - 31 Effects of hunting on movements ............................................................................................................ - 33 FITNESS......................................................................................................................................................... - 35 Survival .................................................................................................................................................... - 35 Contribution to the breeding population................................................................................................. - 36 Evidences of reproduction ....................................................................................................................... - 36 -
4.
CONTRIBUTION OF RELEASED MALLARDS TO THE DYNAMICS OF THE NATURAL POPULATION .... - 38 MODEL AND PARAMETERS ................................................................................................................................ - 39 General model ......................................................................................................................................... - 39 Contribution of released ducks ................................................................................................................ - 41 Scenarios.................................................................................................................................................. - 41 ANALYSIS OF SCENARIOS ................................................................................................................................... - 42 Reproductive value .................................................................................................................................. - 42 Predicted population size......................................................................................................................... - 42 Discussion ................................................................................................................................................ - 44 PERSPECTIVES IN CAMARGUE ............................................................................................................................ - 45 Changes in harvest .................................................................................................................................. - 45 Are the releases benefitting the wild Mallard population? ..................................................................... - 45 -
5.
CONSEQUENCES OF RESTOCKING FOR THE NATURAL POPULATION ............................................. - 47 POTENTIAL SOURCES OF PATHOGENS .................................................................................................................. - 48 POTENTIAL SOURCES OF GENE FLOW ................................................................................................................... - 49 POTENTIAL SOURCES OF MORPHOLOGICAL CHANGE ............................................................................................... - 51 Changes in mean body weight................................................................................................................. - 51 Changes in bill morphology ..................................................................................................................... - 52 -
6.
DISCUSSION.................................................................................................................................. - 54 MODERATE EFFECTS OF THE RELEASES ................................................................................................................. - 55 Domestication of released Mallards........................................................................................................ - 55 Fitness of the released Mallards.............................................................................................................. - 55 Introgression............................................................................................................................................ - 55 Individual heterogeneity.......................................................................................................................... - 56 SOME POSSIBLE EFFECTS REMAIN ....................................................................................................................... - 57 Pathogen reservoirs................................................................................................................................. - 57 Genetic perspectives................................................................................................................................ - 57 Management perspectives ...................................................................................................................... - 58 RECOMMENDATIONS ....................................................................................................................................... - 58 Socio-economic context........................................................................................................................... - 58 Recommendations ................................................................................................................................... - 59 CONCLUSION ................................................................................................................................................. - 61 -
REFERENCES ............................................................................................................................................... - 62 PUBLICATIONS ........................................................................................................................................... - 82 APPENDICES ............................................................................................................................................... - 93 -
CHAPTER 1
1. INTRODUCTION
1. Introduction
Man in its environment
Like all species, humans interact with their environment. However, Man is different from
other species owing to the extent and the diversity of ways by which he affects his
environment (Vitousek et al. 1997). These changes occur at all scales, from ecosystems to
genes. At the worldwide scale, an estimated third of the earth area has to date been
modified by human activity (Vitousek et al. 1997). At the landscape scale, urbanisation and
other human activities are profound factors of change. Habitat loss has contributed to the
disappearance of some species. Lastly, the action of Man can affect the distribution of genes
within a population, thus affecting its natural evolution. For example, the human selection of
tamer individuals leads to domestication of animal species (Diamond 2002).
Numerous studies aim at understanding how the action of Man affects the
functioning and hence the dynamics of populations, i.e. by which mechanisms he can make
population size to vary over time (Williams et al. 2002). For example, the introduction of an
alien species will affect the dynamics of native species through changes in predation
pressure, competition, via hybridization, by introducing parasites or through changes of the
ecosystem (Hulme 2009; Spear and Chown 2009; Goodenough 2010). Globaly, broad
changes caused by human such as climate changes of changes in use influence the dynamics
of populations (Lebreton 2011). It is however through exploitation or through the
introduction of individuals that humans can modify the dynamics of a population the most
directly.
Addition of individuals
[Article 1]
According to World Conservation Union (IUCN 1987), restocking is the addition to an existing
population of individuals from the same species (Armstrong and Seddon 2008). Population
restocking can be intentional of accidental, from individuals bred in captivity or wild
individuals captured and moved. Restocking is practiced for a wide range of aims and in a
wide range of situations (Article 1).
The case concerning the most species is restocking aiming at conserving diversity
(Keller et al. 2000; Bishop et al. 2007; Pérez-Buitrago et al. 2008; Roche et al. 2008;
Venkatesh et al. 2008). The goal is then to support the demography of a threatened
population (Frankham 1995; Bowkett 2009). Another source of restocking, then
unintentional, is by escapees joining their wild conspecifics. This is particularly frequent in
open fish farms: for example, two million fish escape in the Northern Atlantic annually
(McGinnity et al. 2003). Such accidental restocking also occur in other taxa such as insects
escaped from greenhouses where they are introduced for plan pollination (Otterstatter and
Thomson, 2008), mammals escaped from fur farms (Norén et al. 2005; Kidd et al. 2009),
plants dispersing from botanical gardens (Hulme 2011) or from genetically modified
organism fields (Piñeyro Nelson et al. 2009). Furthermore, some feral populations (domestic
animals back in nature) such as cats Felis silvestris (Beaumont et al. 2001) or dogs Canis lupus
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1. Introduction
can get in contact with their wild counterparts (Elledge et al. 2008). Less common but
sometimes of local importance, restocking is also practiced for aesthetical (e.g. butterfly
release; Hodder and Bullock 1997), scientifically (e.g. study of passerine dispersal; Skjelseth
et al. 2007) or religious reasons (e.g. Buddhist ceremonies; Kwok 2007). Concerning a smaller
number of species but a larger number of individuals, restocking is also regularly practiced
for the management of hunted and fished populations.
Harvest
Hunting and fishing are ancestral activities implying the harvest of individuals living in the
wild. The exploitation of individuals by hunting is legally ruled. The aim of such legislation is
to maximize harvest without irremediably affecting species (Nichols et al. 1995; Commission
Européenne 2008). In this context, understanding the consequences of harvest for the
dynamics of exploited species is of crucial importance (Nichols 1991; Elmberg et al. 2006).
Without legislation or such understanding, hunting leads in some cases to drastic decreases
of populations, or even drives some species to extinction. For example, the passenger Pigeon
Ectopistes migratorius, which once probably was the most abundant species Man had known
on earth, got extinct within a century of unregulated hunting (Halliday 1980). Such a rapid
extinction was probably due to the high hunting mortality rate compared to natural
mortality (Sandercock et al. 2011), combined with the limited growth rate of this species,
where individuals produced one single egg per year (Lebreton 2011).
In addition to bag or period regulation (e.g. Duriez et al. 2005), hunting management
practices are sometimes specifically set up so that the population can sustain a maximum
harvest rate (Mathevet and Mesléard 2002): supplementary feeding (Draycott et al. 2005;
Knox 2011), habitat management (Tamisier and Grillas 1994), predator control (Watson et al.
2007; Smith et al. 2010) or restocking (Arroya and Beja 2002). The latter however only
concern a limited number of species, for which captive-bred individuals are released within
the wild populations (Figure 1.1).
Restocking for hunting
When exploited populations are restocked, a large number of individuals are released to
either i) increase exploited populations temporarily or over the longer term, or ii) increase
harvest opportunities within viable populations (Sokos et al. 2008). In the former case, the
aim is close to that of restocking carried out in a conservation perspective, restocking is
practiced to small populations and the term reinforcement is frequently used (Bro et al.
2006). In both cases, Man modifies the dynamics of the population through both the
introduction of individuals, often of captive origin, and the harvest of these individuals as
well as that of the individuals from the natural population (cf. Watson et al. 2007). In the
context of conservation programmes, the factors of success for restocking schemes (i.e.
those leading to a viable and sustainable population) are relatively well studied (Article 1). In
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1. Introduction
particular, in this context, the mechanisms by which restocking positively or negatively
affects the whole population have been documented (Article 1). Conversely, in the context
of restocking for hunting, these factors and mechanisms have been little studied, probably
because the potential consequences are less crucial, since the natural population is generally
not threatened (Article 1 but see Matson et al. 2004). Furthermore, the relative harvest of
released as opposed to wild individuals is generally unknown.
Species
Latin name
Area
References
Mallard
Anas platyrhynchos
North America, Europe
This study
Pheasant
Phasianus colchicus
North America, Europe
Musil and Connelly 2009
Houbara Bustard
Chlamydotis undulata
North Africa, Middle East
Lesobre et al. 2010; Pitra et
al. 2004
Willow Grouse
Lagopus lagopus
Great Britain
Storch 2007
Wild Turkey
Red-legged
Partridge
Grey Partridge
Meleagris gallopavo
North America
Latch et al. 2006
Alectoris rufa
Europe
Blanco-Aguiar et al. 2008
Perdix perdix
Europe
Putaala and Hissa 1998
Common Quail
Northern
Bobwhite
Brown Hare
Coturnix coturnix
Europe
Derégnaucourt et al. 2005
Colinus virginianus
United States
Buechner 1950
Lepus europaeus
Europe
Angelici et al. 2000
European Rabbit
Oryctolagus cuniculus
Spain, France
Delibes Mateos et al. 2008
Roe Deer
Capreolus capreolus
Europe
Randi 2005
Red Deer
Cervus elaphus
Europe
Randi 2005
Alpine Ibex
Capra ibex
Europe
Biebach and Keller 2009
Chamois
Rupicapra spp.
Europe
Crestanello et al. 2009
Bighorn Sheep
Ovis canadensis
North America
Buechner 1960
Reindeer
Black-faced
Impala
Wild Boar
Rangifer tarandus
Amérique du Nord, Eurasie
Baskin 2000
Aepyceros melampus petersi
Namibia
Matson et al. 2004
Sus scrofa
Europe
Randi 2005
Figure 1.1. Examples of hunted species for which restocking programmes have been carried out (from
Article 1).
Restocking of exploited populations: the case of the Mallard
The Mallard Anas platyrhynchos is certainly the most common duck worldwide (Anatidae;
del Hoyo et al. 1992). This bird naturally has a Holarctic distribution, with a total number of
individuals estimated to a minimum of 32 million wild birds (sum of breeding or wintering
population monitoring schemes depending on flyways; del Hoyo et al. 1992). Mallard was
domesticated several times since over 3000 years, initially in Asia (Price 2002; Li and al
2010). The current captive population is estimated at 681 million individuals (Tanabe 1995),
mostly for human food. Furthermore, the Mallard is a synanthropic species common in
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1. Introduction
gardens, parks and urban environments (Baratti et al. 2009). Owing to its excellent ability to
adapt to new environments (Bellrose 1985), cities and breeding facilities, the Mallard has
colonised the world following Europeans settlers (Fox 2009). As early as in the XIXth century,
it was introduced outside in natural range throughout the world (Callaghan and Kirby 1996),
leading to hybridization of Mallard with many other native duck species (Mallet 2005).
Within its natural range, Mallard releases are carried out for hunting purposes. Such
hunting releases are common, especially in Europe and North America. Farm ducks are then
used to produce individuals later released in the wild populations (see Cheng et al. 1978).
Such Mallards are very different from those bred for their meat. The terms “captive-bred” or
“released ducks” will hence be used throughout this document for « bred in captivity to
restocking exploited populations ».
Despite this practice having been used for close to a century, few studies to date
have tried to evaluate the ability of these released individuals to survive in the wild. Fewer
still are the studies which aimed at documenting the (potentially negative) effects onto the
natural populations, as well as documenting the contribution of these individuals to the
dynamics of such populations.
History of Mallard releases
Restocking in North America
Mallard releases within their historical habitat were initiated during the first half of the XXth
century, probably in the United States, to increase production rate of hunted populations
(Brakhage 1953). Such practice was then used to counterbalance the decrease of
populations due to overharvesting or particularly cold winters (Leopold 1933). Releases were
however kept at a modest scale since « The high cost, as well as aesthetic limitations of
artificially reared birds, makes it doubtful whether such stock will ever become important for
release as shootable game» (Leopold 1933; p. 356).
Traditionally in North America, shooting preserves release birds from towers.
Approximately 70% of these birds are immediately killed on site (US Fish and Wildlife Service
[USFWS] 2003) and a particular attention is given to the recapture of birds not killed, so as to
avoid their dispersal. Since 1985, a change in law in the United States allows private hunting
estates to free-flighted captive-bred Mallards to compensate the observed decline in wild
Mallards (Department of Game and Inland Fisheries [DGIF] 2007). The number of released
Mallards has increased (Smith and Rohwer 1997). In 2001, 314 private hunting estates
annually released, for the whole of the United States, ca. 270 000 Mallards. Mallard release
is far more limited in Canada, where it is virtually not practiced any more (USFWS 2003).
Restocking in Europe
In Europe, restocking of Mallard population through the release of captive-bred individuals
started in the 1950s, mostly in Denmark (Fog 1964), Czechoslovakia (Havlin 1991) and United
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1. Introduction
Kingdom (Wardell and Harrisson 1964). The number of released birds was initially very
limited: 5 236 between 1950 and 1960 then 1 539 between 1963 and 1968 in Denmark (Fog
1964; Fog 1971). In Great Britain, the release organised by the Wildfowlers’ Association of
Great Britain and Ireland started with 110 Mallards in 1954, before reaching 8 949 in 1962.
The goal was to get these ducks to join the wild populations. The birds were hence released
in non-hunted areas (Wardell and Harrison 1964).
From the 1970s, the number of released birds started to increase dramatically in
Europe. In Southern France, the development of commercial hunting estates (also called
“daily hunts” or “shooting hunts”; close to “put and take” practices sensu Sokos et al. 2008)
lead to the development of this practice, which eventually reached the other private hunting
estates where hunters pay for an annual hunting lease (Tamisier and Dehorter 1999: p.43;
Mathevet 2000: p.258). In 1989, 752 000 Mallards were produced in French game bird
facilities, two-thirds of which being so only in Sologne, central France (Fournand 1992).
North America
Minimal
estimation of
number of
Mallards
released
270 000
France of which
1 400 000
311 000
100 000
Camargue
> 50 000
40 000
> 5 000
30 000
12 000
8 000*
Sweden
200 000
150 000
200 000
Denmark
500 000
50 000
35 000
Czech Republic
250 000
160 000
35 000
Germany
100 000
250 000
400 000
United Kingdom
500 000
500 000
125 000
> 2 850 000
7 500 000
4 500 000
Country or Area
Brenne
Total Europe
Winter
Estimated
population size number of
References
estimate
breeding
(January)
pairs
-
9 200 000* USFWS 2003; USFWS 2011
Deceuninck and Maillet 2011; FNC-ONCFS
2008
M. Gauthier-Clerc, pers. com.; Kayser et al.
2008
F. Bourguemestre, pers. com.; C. Fouque,
pers. com.; Chapter 3
P. Söderquist, pers. com.; Nilsson 2008;
Birdlife International 2004
Noer et al. 2008; Gilissen et al. 2002;
Article 4; Musilová 2009; Birdlife
International 2004
J. Mooij, com. pers. ; Gilissen et al. 2002 ;
Harradine 1985; Kirby 1995; Birdlife
International 2004
Delany
and
Scott
International 2004
2006;
Birdlife
Figure 1.2. Estimated number of released Mallards and population size in winter (January counts) or number
of breeding pairs (except * estimation of total population size).
Current estimates (since 1996) are of ca. 1.4 million Mallard being produced and
released for hunting purposes in France (Mondain-Monval and Girard 2000; for a description
of the release technique see Appendix 1). In Denmark, 350 000 to 400 000 presumed wild
Mallards were harvested in the early 1970s. Ten years later, the national hunting bag for this
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1. Introduction
species was of 700 000 ducks, the increase likely being due to the rapid increase in the
number of released Mallards (Søndergaard et al. 2006). Since the end of the 1990s, ca.
500 000 Mallards are released annually in this country (Noer et al. 2008). In the other
European countries the practice has also become more common: an estimated 200 000 to
300 000 Mallards are released annually in the Czech Republic (Hůda et al. 2001), and over
200 000 in Sweden (P. Söderquist, pers. com.). Releases also occur in Great Britain (500 000
in the 1980s; Harradine 1985), as well as in unknown numbers in Italy (Baratti et al. 2009),
Latvia, Spain, Belgium, Germany and most likely in other European countries. These numbers
show the importance of duck releases for the wild populations that receive them, especially
when these figures are compared to the breeding and wintering populations (Figure 1.2).
Survival and breeding abilities of released individuals
It is crucial to assess the fate of released individuals once in the wild, so as to estimate their
demographic contribution to the natural populations. This allows, on the other hand,
estimating the potential of these birds to disseminate pathogens and to cross with their wild
conspecifics. When one wants to increase a population through a significant input of
individuals, it is necessary to rely on captive populations (Bowkett 2009). The control of
productivity, phenotype, and trade are indeed easier in captivity. Captivity hence potentially
affects the ability of released birds to survive and breeding in nature.
Effect of captivity on survival
In the long-term, keeping populations in captivity lead to genetic changes. Owing to small
funding populations (Robert 2009), genetic drift is intense and captive populations are
subject of inbreeding depression (Kraaijeveld-Smit et al. 2006) and loss of genetic diversity
(Lande and Shanonn 1996). In addition, some alleles towards which selection pressure is
released in captivity can get promoted (Tufto 2001; Frankham et al. 2004). This can lead to
profound behavioural and morphological changes (Robert 2009), up to for example a
reduction of brain volume in captive birds (Guay and Iwaniuk 2008).
Behavioural deficiencies are common for released captive-bred individuals in
reintroduction programmes (Rantanen et al. 2010a). These can result from genetic changes
or from the captive environment encountered during growth. Released individuals are for
instance particularly naïve towards predators (Kraaijeveld-Smit et al. 2006; Houde et al.
2010; van Heezik et al. 1999). They are also likely to rely on poorer habitats than wild
individuals (Rantanen et al. 2010b). Behavioural changes have been observed in released
Mallards, in particular loss of migratory propensity (Lincoln 1934; Errington and Albert 1936
in Brakhage 1953). Osborne et al. (2010) observed that released ducks were hardly afraid by
Man and gathered in urban parks where they are fed, which increases feral wildfowl
populations (Osborne et al. 2010).
- 14 -
1. Introduction
Diet being made of artificial food in captivity, the digestive tract can become
maladapted to the digestion of natural foods (Liukkonen-Anttila et al. 2000). A lower body
mass resulting from an inability to forage would be detrimental to released ducks. For
example, when they get food-stressed owing the freeze of ponds, making food temporarily
inaccessible, survival of wild Mallards was estimated to be seven days for males and five
days for females (Whyte and Bolen 1984). If released Mallards have a poorer body condition,
they could show a greater sensitivity to temporary food deprivation, which may increase
their mortality in case of extended cold.
To evaluate the survival of released Mallards, it is necessary to estimate the share of
mortality due to hunting. The close proximity of Mallards and humans in game bird facilities
may lead, through domestication, to a particularly high hunting vulnerability. Two studies
have compared the survival rates of released and wild Mallards where hunting was
practiced, and showed lower survival for released ducks, including in those which already
survived a previous hunting season (Soutiere 1989; Dunn and al 1995). Soutiere (1989) also
suggetss that natural mortality is higher in released than in wild Mallards. Despite the crucial
need to understand the vulnerability of released Mallards to hunting if one wants to
estimate their demographic contribution to the population, no study to date has compared
the survival of released Mallards between hunted and non hunted sites.
Effect of captivity on reproduction
Captivity could decrease the reproductive potential of released Mallards. For example, a
study has showed that the breeding capacity of salmonids Oncorhynchus spp. introduced in
the wild was decreased by 40% for each generation in captivity (Araki et al. 2007). In captivebred pheasants, the low hatching success (22%) compared to that of pheasants born in the
wild (49%) is likely due to a greater nest abandonment rate by released females (Sage et al.
2003). This could result from selection in captivity of genetic lineages producing more eggs
but with lower brooding instinct (Guéméné et al. 2001).
Captive-bred birds may also be poorer breeders for physiological reasons, such as a
poorer sanitary status or an inability to build-up reserves (Mayot 2006). In Mallards, an
earlier study has shown that testis size of captive males was lower than in wild males
(Stunden et al. 1999). Devries et al. (2008) also demonstrated that a lower body condition of
the (wild) Mallard female translates into a lower investment and breeding success. Because
of captivity, released Mallards could show a poorer body condition than wild ducks, which
may negatively affect their ability to transmit their genes.
Studies on released Mallards generally suggest a lower survival and a low
contribution to reproduction, even when releases are in non-hunted areas (Sellers 1973; Batt
and Nelson 1990; Stanton et al. 1992; Osborne et al. 2010). Only the study by Lee and Kruse
(1973) shows a significant contribution of released Mallards to the wild population. However
all these studies were done in the North American release context, where the relative
number of released birds compared to the whole population is small compared to the
European situation (Figure 1.2). The previous studies carried out so far in Europe were so
- 15 -
1. Introduction
before the 1970s, and concerned a limited number of birds (Boyd 1957), without the use of
powerful statistical tools (Fog 1964; Wardell and Harrison 1964; Havlin 1991).
Contribution of released individuals to the natural population
Restocking generally aims at improving the demographic trend of target populations.
Restocking indeed contributed to halt the extinction of the White Stork Ciconia ciconia in
France (Massemin-Challet et al. 2006). However, the contribution of released individuals to
the wild population depends on their individual ability to breed, which may be deficient. The
expected increase in population following releases can then be limited, as shown in Crested
Coot Fulica cristata released for conservation purposes (Martínez-Abraín et al. 2011). In the
same context, it was suggested that captive Mallards had a low potential to increase wild
populations over the long term (Bailey 1979; Bednarik and Hansen 1965 in Batt and Nelson
1990).
The aim of Mallard releases is to increase shooting and hence harvest opportunities
more than increase wild population size. Depending on the reproductive value of released
birds, the wild populations could either benefit from a demographic support, or at minima
not experience reduction in numbers despite increasing hunting bag. Modelling is an
appropriate tool to estimate the effect of a change in harvest on the demography of a wild
population. No attempt had however so far been made to set up such a model and evaluate
the contribution of released individuals to the dynamics of an exploited natural Mallard
population.
Potentially deleterious impact on the natural population
Some factors associated with restocking can have negative indirect demographic
consequence for the receiving population. Because restocking implies the use of foreign
individuals it can i) increase competition with resident individuals, ii) introduce pathogens, or
iii) lead to genetic changes with behavioural or morphological consequences for the
population. These effects can alter evolutive processes. The different possible consequences
for the receiving species, especially when introduced individuals were captive-bred, are
listed in turn below (see Article 1 for further details).
Competition
Captive breeding always occurs at higher individual density than density in the wild, so that
agonistic behaviours can get promoted. Once released into the wild, these individuals can
therefore directly compete with the resident individuals for access to food or breeding sites.
For example, bumblebees escaped from greenhouses compete for food with native
conspecifics (Ings et al. 2006). A longer breeding period has also been demonstrated in
captivity for salmons. Once their reach the breeding grounds, escaped fish can therefore
- 16 -
1. Introduction
destroy the nests of wild females and replace the former eggs by their own (Webb et al.
1991).
Pathogen transmission
Captivity conditions are favourable to pathogens like viruses and bacteria, sometimes
leading to the apparition of particularly virulent strains (Lebarbenchon et al. 2008; Mennerat
et al. 2010). The massive releases of captive-bred ducks can therefore lead to the
introduction of new pathogens in the natural habitat (Cunningham 1996). Transmission
probability between released and wild birds in then increased by their shared use of the
same resources or habitats (Millán 2009). The vectors of some diseases being absent from
the site before releases, wild populations can lack adequate immunity (IUCN 1998). For
example, the salmon parasite Lepeophtheirus salmonis infects wild fishes in the surrounding
of fish farms (Morton et al. 2004). Similarly, the intestinal protozoa Crithidia bombi would be
transmitted by escaped bumblebees to their wild counterparts (Colla et al. 2006). Lastly,
restocking of red-legged partridges and European rabbits would be responsible for the
transmission of parasitic worms and sarcoptic mange to the target population, respectively
(Villanúa et al. 2008; Navarro-Gonzalez 2010).
In one of their reports (USFWS 2003), the US Fish and Wildlife Service documented
the risk caused by released Mallards for transmission of duck virus enteritis to wild
counterparts (Converse and Kidd 2001). Some correlations indeed exist between the
presence of this pathogen and the areas where high densities of released Mallards interact
with wild ones. Further, the presence of vaccine against this virus was detected in wild ducks
living close to estates releasing those Mallards (USFWS 2003).
The Mallard is one of the main natural hosts for low pathogenic influenza A viruses,
which cause moderated symptoms on this duck species (Latorre-Margalef et al. 2009;
Jourdain et al. 2010). However, this virus can evolve towards highly pathogenic forms,
causing high mortality rates in poultry farms and being potentially transmissible to humans
(Keawcharoen et al. 2008). In Europe, a former study detected influenza A virus in a farm
producing Mallards for releases (Bragstad et al. 2005). This virus potentially dispersing from
human infrastructures to wildlife, hunting Mallard farms could potentially be good
transmission vectors (Gauthier-Clerc et al. 2007).
Genetics
Hybridization, considered as the interbreeding of individuals from genetically different
populations (Rhymer and Simberloff 1996), can occur when wild individuals come in contact
with: escaped individuals (Norén et al. 2005; Kidd et al. 2009), individuals deliberately
released for conservation purposes (Ciborowski et al. 2007) or individuals released to
increase harvest (Blanco-Aguiar et al. 2008; Barbanera et al. 2010). If hybrids are fertile and
back-cross with one of the parental populations, introgression can occur (Rhymer and
Simberloff 1996). Introgression is then defined as gene flow between populations of
- 17 -
1. Introduction
hybridizing individuals. Introgression has been demonstrated between domestic and wilds
strains of many taxa: plants (Keller et al. 2000), fish (Winkler et al. 2011), birds (Barbanera et
al. 2010) and mammals (Kidd et al. 2009). The negative consequences of hybridization
depend on the degree of introgression (Allendorf et al. 2001; Fitzpatrick et al. 2010), from
minor or restricted effects (Scandura et al. 2011; Winkler et al. 2011) to worrying fitness
declines (McGinnity et al. 2003). Two studies even suggested that introgression can lead to
extinction of native populations (Saltonstall 2002; Hegde et al. 2006).
Box 1.1: Genetic consequences of massive restocking
Broadly speaking, the negative consequences of intraspecific hybridization are less acute
than those of interspecific hybridization (Allendorf et al. 2001). It is even conversely
considered that intraspecific hybridization could improve genetic diversity (Allendorf et al.
2001). For example, restocking of Atlantic salmon in Spain indeed brought genetic diversity
(Ciborowski et al. 2007). However, it has been demonstrated that the negative genetic
consequences of massive restocking could come from gene flow potentially leading to loss of
adaptation, change in population composition or decreased genetic variability (Figure 1.1;
Laikre et al. 2010a).
Furthermore, even when gene flow is limited, in small wild populations a loss of
genetic diversity can be observed through a demographic reduction of the population
leading to decreased effective population size.
Massive Release
No gene flow
Gene flow
Decreased number of breeders owing to:
Disease transmission
Wasted reproduction
Competition
Change in genetic
composition
Disruption of genetic
adaptation
Loss of genetic
diversity
Figure 1.3. Pathways by which massive releases can affect the genetics of the target wild population.
Modified according to Laikre et al. (2010a)
Furthermore, gene introgression potentially leads to behavioural changes in the
population target. This could be the case in Common Quail Coturnix coturnix and Little
Bustard Tetrax tetrax where migratory behaviour is threatened by massive releases of
captive-bred individuals (Derégnaucourt et al. 2005; Villers et al. 2010). Finally, genetic
introgression by captive-bred individuals can lead to morphological changes detrimental for
- 18 -
1. Introduction
the wild population (Tufto 2001). This has for example been demonstrated for the eggs of
chinook salmon Oncorhynchus tshawytscha in rivers close to fish farms (Heath et al. 2003).
An effect of Mallard releases has been demonstrated on the genoptype of the Black
duck Anas rubripes with which Mallard hybridize. Mallards have been released massively in
traditional Black duck breeding areas, leading to hybridization and decrease of the genetic
differentiation between these two species in less than a century (Mank et al. 2004).
Similarly, several duck species are threatened by hybridization with Mallards released or
introduced for hunting (Fowler et al. 2009; Fox 2009; Kulikova et al. 2004; Pérez-Arteaga et
al. 2002; Rhymer et al. 1994; Williams et al. 2005). Introduction of maladapted genes, which
developed in released Mallards and were transmitted to wild ducks, has also been suspected
although could not be demonstrated (USFWS 2003). In their review of the genetic
consequences of massive releases of plants and animals within exploited populations, Laikre
et al. (2010a) highlighted the lack of knowledge for Mallard. Deleterious genetic effects were
conversely largely demonstrated in wild fish populations submitted to hybridization: loss of
genetic diversity, loss of adaptation, changes in population composition and/or genetic
structure of the population (Box 1.1).
Aims of the thesis
[Article 2]
Mallard releases have not so far received adequate attention. Despite the commonness of
this practice in Europe, no in-depth study has been carried out to precisely evaluate the
ability of released Mallards to survive, the factors affecting such survival, their contribution
to the target population or the consequences for the health, genetics and morphology of
wild ducks. Considering such gaps in knowledge, this thesis aims at providing new insight on
the consequences of this massive – in terms of number of individuals released compared to
the size of the target population – practice that has occurred continuously over the last 30
years (Article 2). This work relied on modern statistical, genetically and modelling tools
recently developed in these various disciplines. This thesis has four parts.
In the first part (Chapter 2), we aimed at assessing the consequences of captive
breeding conditions for the Mallards to be released. Since released birds cannot be
distinguished from wild ones according to their phenotype only (Byers and Cary 1991; Manin
2005), we assessed the genetic characteristics of the captive-bred Mallards (Article 3). In
order to detect differences that may later affect survival of these birds in the wild, we
focused on markers related with individual immune response (Article 4). Considering the
ability of the birds to forage in the wild, we compared some behavioural and morphological
traits linked to food acquisition between wild Mallards and ducks just released. We also
evaluate the ability of captive-bred individuals to adapt to natural foods, several months
after release, through the study of their digestive tract, diet and body condition (Article 5).
The second part (Chapter 3) considers movements, survival and reproduction of the
Mallards once released, using capture-mark-recapture data. These data first inform about
the general hunting harvest patterns of these birds. Dispersal and mobility of released
- 19 -
1. Introduction
Mallards before and after the onset of the hunting season were then analysed (Article 6).
After that, the relative importance of hunting and other mortality causes on survival rate of
the Mallards until their first breeding season was examined. Finally, we described the
evidences of released Mallards reproduction in the wild.
The third part (Chapter 4) uses the previous data to model the contribution of
released individuals to the demography of the exploited natural Mallard population. This
calculates population trends depending on the hunting pressure over released and wild
ducks. From these results, and also taking into account the increasing hunting bag in
Camargue over the last two decades (Mondain-Monval et al. 2009), we assessed and
compared the survival rates of wild Mallards before massive releases started and today,
after 30 years of massive releases (Article 7).
Lastly, the fourth part (Chapter 5) aims at evaluating the sanitary, genetically and
morphological consequences of Mallard releases for the target population. We studied the
potential of released Mallards to:
• Be vectors of Influenza A viruses in the wild (Article 8)
• Alter the genetic integrity of Mallard populations (Article 3 and 4)
• Cause morphological change in the wild population over the medium term
(Articles 9 and 10).
The results of this thesis are then discussed (Chapter 6) within a broader context, also
bearing in mind socio-economical constraints, so as to provide new options properly
considering the releases of birds within the management of the species.
- 20 -
CHAPTER 2
2.
EFFECTS OF CAPTIVITY
2. Effects of Captivity
[Article 3]
[Article 4]
Genetic differentiation
No attempt to discriminate released and wild Mallards morphologically has so far been
successful (Byers and Cary 1991; Fournand 1992; Manin 2005). The use of several molecular
markers recently developed for Mallard (Sorenson and Fleischer 1996; Sorenson et al. 1999;
Maak et al. 2003; Denk et al. 2004; Huang et al. 2005; Kraus et al. 2011) allowed us to look
for potential genetic differences between the two bird categories with greater power.
Captive Mallards generally come from strains developed over more than 30 years through
the interbreeding of wild strains and strains developed for duck meat production (Cheng et
al. 1978). Owing to the rare introduction of wild Mallards in game bird facilities, genetic drift
should lead to a specific signature of captive birds on a wide range of neutral markers.
However, if the released birds survive and are able to breed and hybridize with their wild
conspecifics, the continuous input of individuals in the wild population over 30 years could
lead to a lack of genetic differences between modern wild and released Mallards.
We therefore sampled (stuffed) wild Mallards harvested before 1975, before massive
releases started, so as to establish a control wild genotype before potential gene flow. The
results below come for one part from a study in France based on 4 types of samples (i) from
museums and private collections of birds collected before 1975, (ii) from five modern game
bird facilities, (iii) from a protected (non-hunted) site of major importance for wintering
ducks in Camargue, and (iv) from the hunting bag of two Camargue estates (Article 3). Some
other results come from another study carried out in the Czech Republic where six game bird
facilities and seven Mallard natural breeding sites were sampled (Article 4). DNA extraction
and sequencing for the two analyses were performed in the same Czech laboratory as part of
collaboration between Office National de la Chasse et de la Faune Sauvage (ONCFS) and the
Institute of Vertebrate Biology of the Czech Republic Academia of Sciences.
Neutral markers
The study in France allowed us detecting a clear genetic differentiation between released
and control wild Mallards (FST = 0.052), even if the two groups showed slight overlap (Figure
2.1). The exchanges practiced by breeders between different captive Mallard strains (Duck
breeders, pers. com.) lead to genetic homogenisation of these trains, resulting in a common
genetic signature for all released Mallards, wherever they come from. The same pattern is
detected in the other study based on only modern data: in the Czech Republic, released
Mallards can be genetically differentiated from local wild breeders (FST = 0.055; Article 4).
The genetic differentiation between the two groups was likely made possible by a founder
effect when the captive strain was created and the following genetic drift, which reduced the
frequency of rare alleles (Frankham et al. 2004). We also observed a reduced genetic
diversity in game bird facilities for neutral markers: allelic richness was 25% lower in France
and 39% lower in the Czech Republic compared to the historical and modern wild
populations, respectively.
- 22 -
0.8
0.6
Axis 2 (1.95%)
0.4
0.2
0
-1
-0.5
0
0.5
1
1.5
2
-0.2
-0.4
-0.6
-0.8
-1
Axis 1 (2.37%)
Figure 2.1. First plan of the Factorial Correspondence Analysis showing the relationships between the
genotype of Mallards collected in France before the development of massive releases (N = 15, black squares)
and five French captive Mallard strains (N = 98, grey squares). Done from the alleles obtained on 15 loci
(from Article 3).
Markers under selection
A loss of genetic diversity in game bird facilities compared to the natural population was also
observed on a non-neutral marker: a gene belonging to the Major Histocompatibility
Complex implied in the immune system (Article 4). This suggests a lower ability of the
captive-bred ducks to face the many pathogens they encounter once released (Jourdain et
al. 2007). The genetic modification observed in released Mallards could globally translate
into particular behavioural or morphological traits, potentially affecting their survival once in
the wild.
Morphological differentiation
[Article 5]
Digestive tract
Food being artificial in captivity, the digestive tract of captive birds may have somewhat lost
its ability to digest natural items (Liukkonen-Anttila et al. 2000). The oesophagus, intestines
and cæca of captive Mallards are actually similar to those of wild birds (Article 5).
Conversely, the gizzard is smaller in birds kept in captivity, probably because food in captivity
is already crushed. Furthermore, body condition (mass/size ratio) is poorer in captive birds,
despite ad libitum access to food.
The digestive tract is made of plastic organs, and the observed difference could have
an environmental as well as genetic origin. A few months after release, we indeed observed
that the gizzard of captive-bred Mallards developed and became similar to that of wild birds
(Figure 2.2). This is consistent with the known ability of the bird digestive system to change
in response to changes in diet (Miller 1975; Heitmeyer 1988; Kehoe et al. 1988).
- 23 -
Indice de poids du gésier sec corrigé
par la taille de l'individu
4
2
0
-2
-4
Contrôle
Lâché
Sauvage
Figure 2.2. Mean gizzard weight (corrected for individual size) of three groups of Camargue Mallards: captivebred kept in captivity (Contrôle, N = 20), captive-bred released as young in the wild, 3 or 5 months before
analysis (Lâcher, N = 25) and of presumed wild origin (N = 17). Groups with different colours are statistically
different (Group effect: F2,55 = 12,1; P < 0.0001; Bonferroni test P < 0.0004 between contrôle and the two
other groups). 95% CI is presented. Half of the sampled individuals in each group were sampled in September
2009, half in November 2009 (from Article 5).
Body condition
The body condition of captive-bred individuals improved after release but did not reach that
of wild birds (Figure 2.3). This suggests long-lasting constraints of captivity, probably of
genetic origin, onto released Mallards. It is also possible that domestication altered foraging
behaviour of these birds.
200
Condition corporelle
160
120
80
40
0
Contrôle
Lâché
Sauvage
Figure 2.3. Body condition of three groups of Camargue Mallards: captive-bred kept in captivity (Contrôle,
N = 20), captive-bred released as young 3 or 5 months before analysis (Lâcher, N = 25) and of presumed wild
origin (N = 17). Groups with different colours are statistically different [Group effect: F2,55 = 18,2; P < 0.0001;
Bonferroni tests show a significant difference of the Sauvage group and the Contrôle and Lâcher groups (P <
0.0001), as well as a marginal difference (P = 0.07) between Contrôle and Lâcher]. 95% CI is presented. Half of
the sampled individuals in each group were sampled in September 2009, half in November 2009 (from Article
5).
- 24 -
Behavioural differentiation
Time-budget
The young released Mallards may have difficulties in foraging owing to their inexperience of
natural conditions (Mathews et al. 2005). The analysis comparing the behaviour of released
and wild Mallards in the field however did not demonstrate any difference in the proportion
of time spent foraging, whatever the way this was considered (Figure 2.4). However, it
should be kept in mind that ducks mostly forage at night (Tamisier and Dehorter 1999) while
this study was conducted during daylight hours to try to demonstrate maladapted behaviour
of the released Mallards.
Behaviour
Released
Wild
Proportion ± SE
Paired Student t-test
t
dl
P
Active (feeding, swimming, flying,
social interactions…)
0.47 ± 0.06
0.48 ± 0.05
0.06
14
0.95
Feeding, swimming
0.46 ± 0.06
0.44 ± 0.06
-0.52
14
0.61
Feeding
0.23 ± 0.05
0.27 ± 0.06
0.98
14
0.34
Figure 2.4. Proportion of released and wild Mallards being active, foraging (feeding + swimming) of feeding
proper during daylight hours (from Article 5). Scan samples were performed in a protected are of Camargue
over 15 study days, covering 3 months after the release of 300 Mallards (17th June – 10th September 2010).
Diet
Released birds may be unable to use the same food resources as the wild individuals, or be
able to exploit only a part of these. The gut contents from hunted birds identified as released
or wild (using earlier marking, see Chapter 3) were compared for two Camargue hunting
estates, without any difference in diet being recorded (Article 5). Released birds however
contained more wheat (33% vs. 20% of wild Mallards), likely coming from hunting bait.
Response to predators
Behaviour towards predators can also be affected by captivity, which favours a lower
emotional reactivity and hence a lower ability to perceive danger (Desforges and Wood-Gush
1975; Rantanen et al. 2010a). Some elements suggest a high natural predation onto released
individuals during their first weeks in the wild: during two successive years, 4% and 8% of the
nasal saddles (used to identify ducks) of released ducks in a protected area in Camargue
were recovered close to a fox den. Most of these saddled were still closed, indicating they
had not simply been lost by the individuals (Guillemain et al. 2008a). Unfortunately, the
impact of natural predation could not be estimated more precisely because some saddles
- 25 -
were recovered long after the last visual observation of the marked individual (up to 18
months). The date of the predation event could hence not be precisely determined.
Consequences in terms of fitness
The results from this chapter suggest captivity has deleterious effects on the ability of
Mallards to show efficient immune response, to acquire enough reserves to breed efficiently
(owing to a poor body condition) as well as to show adapted anti-predator behaviour. The
next chapter considers the survival and breeding aspects from a quantitative point of view.
In particular, the deficient anti-predator behaviour may translate into a high vulnerability of
released Mallards at the onset of the hunting season.
- 26 -
CHAPTER 3
3. SURVIVAL AND REPRODUCITON OF
CAPTIVE-BRED MALLARDS
3. Survival and Reproduction
Marking method
To compare survival rates between released and wild Mallards, it is necessary to rely on the
same marking method and data analysis techniques. This was done for the first time in
France by Legagneux (2007) in Brenne, in the département of Indre, central France. We
followed on this method by also ringing released Mallards in the Camargue (Figure 3.1).
CAMARGUE
free-flighted release
BRENNE
free-flighted release
Year of ringing
(i)
CAMARGUE
release for shooting
ringed
saddled
Ring
return (%)
i
i+1
2002
1638
-
30.6 1.9
-
-
-
-
-
-
-
2003
1671
-
36.7 2.4
-
-
-
-
-
-
-
2004
362
-
30.7 1.9
-
-
-
-
-
-
-
2008
1848
942
30.1 1.4
1018
-
23.0 1.5
-
-
-
2009
838
414
35.8 0.2
1938
228
21.2 1.3
5428
38
0.5
2010
-
-
2129
202
20.7
100
13
?
TOTAL
6357
1356
5085
430
21.6 1.4
No individuals
-
-
32.8 1.6
No individuals
ringed
saddled
Ring return
(%)
i
i+1
?
No
individuals
5428
Ring
return (%)
i
i+1
25.7 0.5
Figure 3.1. Number of Mallards ringed and saddled before release in hunting estates of two French areas
following two modalities. The percentage of rings returned during the next hunting season (i) and the
following one (i+1) is given. This table does not include the 600 saddled Mallards released in a non-hunted
Camargue protected area in 2009 and 2010.
Birds ringed as part of restocking schemes in Camargue were so in communal or
private hunting estates, as well as experimentally in a non-hunted protected Camargue area
so as to evaluate the relative role of hunting as a mortality cause for these birds.
Furthermore, birds bred for a daily hunt commercial estate (“releases for shooting”
equivalent to “tower release” in USA, released on the same site they were bred, see
appendix 1) were also ringed in Camargue (N = 5428 in 2009). Fitting a nasal saddle in
addition to a ring allows identification of birds from a distance through the reading of the
saddle code, without the need to physically recapture those (Guillemain et al. 2008a). Such
saddles were fitted to 2386 birds to increase the quality of the information related to their
survival and movements after release without the need to recapture or recover them (Box
3.1), and to assess the potential reproduction of released females.
Box 3.1: Capture-mark-recapture methods used
Capture-mark-recapture (CMR) methods are based on the identification of individuals with
marks fitted at first capture (rings and nasal saddles in ducks, Guillemain et al. 2008a).
Regular sampling [monthly (Articles 5 and 6) or annual (Article 7)] allows recapture
(necessary for rings, given the small size of signs) or resighting (for nasal saddles) of the
- 28 -
individuals, later compiled as individual histories showing if a bird was contacted or not
during a sampling session. The model taking such information into account is known as the
Cormack-Jolly-Seber « CJS » model (Lebreton et al. 1992) and has been used in article 5 and
chapter 3. In the case of exploited populations, hunted individuals provide some information
and constitute the basis of the Brownie model (Brownie et al. 1985) used in article 7.
Burnham (1993) improved this latter model by combining the Brownie approach and that
based on recaptures (CJS) when both information sources are available. The Burnham
method was used in the poster presented in Appendix 2.
From the capture-recapture (and potentially recovery) histories for many individuals,
it is possible to model the population to get crucial parameters such as survival, hunting
recovery probability or live recapture probability. These parameters can then be wompared
between sex and age classes, periods or sites (Lindberg 2010).
In paper 6, the model from Lindberg et al. (2001) was used. This relies on a « robust
design » approach (Pollock 1982) which partitions the capture occasion into several
secondary occasions between which the population is not submitted to mortality,
immigration or emigration, and is therefore considered as being « closed ». Capture
probability is evaluated separately for primary and secondary sessions, then allowing
calculation of the probability for an individual to get detected, knowing it is present. This
model is useful to develop new approached related with movements or life history traits
(Kendall and Bjorkland 2001). The Lindberg model adds the information from ring recoveries
of hunted individuals. We used it to model survival and movements of released Mallards
during hunted and non-hunted months (Article 6).
Goodness of fit tests similar to χ² tests (Lebreton et al. 1992) are used to assess the
extent to which the model fits the data. The next step is to test a number of models
corresponding to different predictions, which differ from each other by the variables taken
into account (for example difference in survival rates between sexes during spring and
summer, when incubating females are more likely to get predated). Model selection is based
on the degree of appropriateness to the data (Quasi Akaike Information Criterion, QAIC).
When only young birds are ringed, as is the case for released Mallards (Figure 3.1),
juvenile survival cannot be estimated with a ringing-recovery model (Brownie et al. 1985).
Conversely, the use of nasal saddles increase the number of possible recaptures through
distance resightings, which allowed us using CJS, Burnham or Lindberg models.
Ringing - Recovery
Ring return rate
When a ringed bird is hunted, transmission of the ring details is a necessary requirement to
allow modelling of the population, but such transmission does not always occur if the bird is
not recovered (« crippling loss »), or if the ring is not sent back (Lebreton 2005). Figure 3.1
shows that ring return rate of released Mallards differs markedly between regions. A priori,
- 29 -
this could be due to differential survival between regions, for example due to differential
hunting pressures. However, survival modelling with the nasal saddles allows controlling for
this, and suggests similar survival rates in Camargue and Brenne (see below). Still, the
proportion of hunted bird rings returned is higher in Brenne. Variability in this proportion
therefore seems to mostly be linked to ring return rate once the bird has been killed, which
likely depends on the level of information received by the hunters, as well as their local
interest for local scientific programmes (Guillemain 2010).
Temporal distribution
Ring recoveries begin at the opening of the hunting season, hence two months after release.
In Camargue, most recoveries were on the release site itself (92%) and as early as in August
(56%) (Figure 3.2). An ANOVA (Complete Model: F15,116 = 58.4; R² = 0.15; P = 0.003) confirms
that on-site recoveries are more frequent (F1,116 = 9.1; P = 0.003). This analysis also shows
that the number of recoveries at the release site is significantly higher in August than from
October to January (F5,116 = 3.7; P = 0.004; Bonferroni tests; P < 0.05 for each month), while
the number in September is intermediate (P = 0.41 for the comparison between August and
September, P > 0.50 for the comparison between September and the other months). From
September onwards the recoveries occur with no significant difference in number between
months (Bonferroni test; P >0.05). Finally, the distribution of recoveries over time does not
depend on the release site in Camargue (F9,116 = 1.2; P = 0.31).
After the first hunting season, recoveries are rare, not exceeding 2% of the total
number of ringed birds except for year 2003 in Brenne (Figure 3.1). This could be due to a
lower ring return rate when birds are recovered further away from their release site.
However, the recovery distance for these birds is similar to that of the birds recovered
during the first hunting season (N = 26; 88% recovered on site; mean distance = 1.0 km, see
next paragraph).
Nombre d'individus repris
250
200
Ailleurs
Sur place
150
100
50
0
Août
Septembre
Octobre
Novembre
Décembre
Janvier
Figure 3.2. Temporal distribution of recoveries during the next hunting season (21 August – 31 January) of
1938 Mallards released at 11 Camargue sites in June – July 2010. Birds recovered where they were released
(“sur place”) are distinguished from birds released further away (« ailleurs »).
- 30 -
Dispersal
Our data confirm the pattern obtained in Brenne between 2002 and 2004, which showed
that 76% of recoveries were on the very lake where birds had been released, while only 0.2%
were outside the Brenne region (Legagneux et al. 2009). Mean Mallard dispersal in
Camargue was 610 ± 66 m (N = 861; releases in 2009 and 2010). Even when only the birds
recovered outside the estate they were released are considered (i.e. which did disperse to
some extent), this distance is only 6.0 ± 0.5 km (N = 87; dmax = 49.9 km).
Releases for shooting purpose (i.e. released the day of the hunt) are associated to
even shorter distances of 272 m on average (± 42 m; N = 2090; F1,2949 = 18.7; P < 0.0001), and
only 2.6 km if the analysis is restricted to the birds recovered outside the estate they were
released (± 0.3 km; N = 222; F1,307 = 33.7; P < 0.0001). Such a shorter distance is likely due to
the captivity of these birds at the same place they are released, since the age of one day,
while in free-flighted restocking, the birds are generally released in a different estate than
where they were bred. The same is observed for Fox Vulpes vulpes that systematically return
to the place they were kept in captivity if they somehow managed to escape (Trut 1999).
Marking-Resighting-Recovery
Ringing only juvenile birds before release does not allow estimating their survival
independently from that of adults (Box 3.1; Brownie et al. 1985). Consequently, despite the
ringing schemes in Camargue and in Brenne, data obtained from the wild Mallards cannot be
compared to those of released individuals. The following analyses are hence all based on
modelling after nasal saddle resightings.
Effect of hunting on survival
During two years, three groups of captive-bred females were released onto three Camargue
sites with contrasted hunting pressures. Such pressure was estimated as the mean monthly
density of hunters per 100 ha, respectively 0; 3.1 and 21.1 at the three sites. Goodness of fit
tests in a CJS parameterization suggested a model with a year effect (release in 2009 or
2010), a site effect and an age effect on survival should be considered (Figure 3.2).
Female Mallards released in a hunting-free area did not survive better during their
first year than those released in a hunting estate (annual survival rate 0.8 – 4.1% as opposed
to 2.6 – 15.9%). The estimates from a month-dependent model with a site and an age effects
provide some insight about likely mortality causes (Figure 3.4).
First, mortality is high right after release at the 3 sites. A cost of release (Tavecchia et
al. 2009) is frequently considered in restocking programmes for conservation, which
translates into a high mortality during the few days after release. Because of maladapted
behaviour (Chapitre 2) and high density of potential prey from the release (Gortázar et al.
2000; Kostow 2009), predation likely contributed to the mortality recorded during the first
- 31 -
months after release. In particular, fox control being a common practice in Camargue,
especially in hunting estates (Massez 2010), the impact of predator was probably higher in
the non-hunted area. This could explain why cumulative survival became stable as early as in
August in the hunting estates while it still decreased during this month at the protected site
(Figure 3.4).
Age
st
nd
1 year
Site
Protected area
Hunting
pressure
0
Release year
N
Survival
SE
Survival
SE
2009
0.767
0.018
0.880
0.038
2010
138
154
0.669
0.024
3.1
2009
79
0.858
0.016
0.931
0.023
82
21.1
2010
2009
2010
0.750
0.737
0.856
0.025
0.026
0.019
0.862
0.045
(Réserve)
Hunt 1
(Chasse 1)
Hunt 2
2 year
(Chasse 2)
77
54
Figure 3.3. Estimated monthly survival rates from the best model considering site, age and release year
effects on survival, and age and site effects on resighting probability [Ф(Age+Site)×Year PAge+Site; ĉ = 2.68; np = 11;
QAICC = 1595.6]. From the poster in Appendix 2 completed with data from year 2010.
After this high mortality during the month following release, a high natural survival
rate was expected (Sarrazin et al. 1994), potentially with a negative effect of hunting.
Survival conversely continues to decrease at all sites, including the non-hunted one. As
suggested in the previous chapter, it is likely the intrinsic constraints such as poor body
condition or depressed immune system which could explain such low annual survival during
the first year. Individuals which survived this first year then had a higher monthly survival
(16.4 to 41.9%; Figure 3.3), similar to that of wild ducks (Figure 3.7).
1
Survie cumulée
0.8
Réserve
Chasse 1
0.6
Chasse 2
0.4
0.2
in
Ju
ai
M
Av
ril
b
N
ov r e
em
Dé bre
ce
m
br
e
Ja
nv
ie
Fé r
vr
ie
r
M
ar
s
ct
o
O
m
br
e
t
Se
p
te
Ao
û
ille
t
Ju
Ju
in
0
Figure 3.4. Cumulative survival of female Mallards released at the age of 7 weeks over two successive years
at three Camargue sites with no hunting pressure (Réserve), moderate hunting pressure (Chasse 1) and high
hunting pressure (Chasse 2). These estimates are from an age- and month-dependent model for survival and
age-dependent model for resighting probability [ФSite×Age(Month) PAge+Site; ĉ = 2,68; np=55; QAICC = 1641,6].
- 32 -
As opposed to the general model, after which hunting mortality tends to be
compensatory (Box 3.2) due to increased density-dependence without harvest or to
selective harvest, the present system is singular because it is likely the inexperience of the
birds once in the natural habitat that makes hunting mortality compensatory. The higher
survival rates recorded in the hunting estates are linked to management practices that tend
to limit the effects of such inexperience, through predator control, supplementary feeding of
the ducks or the creation of a non-hunting area within the estate.
Box 3.2: Compensatory and additive mortality
The ways by which harvest and population dynamics interact are a major theme of research
for the study of exploited populations. A key question, from both a theoretical and a more
applied perspective, is to determine if hunting mortality is additional to natural mortality (S),
or if the effect of harvest on population dynamics is compensated.
This figure from Sandercock et al. (2011) shows different hypotheses if a) hunting mortality is additive, i.e.
annual survival decrease with increasing harvest mortality (b = 1), or is even superadditive (b > 1) when for
example social relationships get broken; b) hunting mortality is partially compensated (0 < b < 1) until a
threshold c; c) harvest mortality has no effect on survival until a threshold c, or even has a positive effect on
survival (b < 0).
Two theoretical compensation mechanisms exist. Compensation by density dependence
predicts that individual reproduction and survival rate of surviving individuals are improved
when the population is harvested. In this case, in natural conditions without harvest densitydependent mechanisms could limit survival and reproduction, so that harvest removes these
constraints and improves survival and reproduction rates of surviving individuals (Lebreton
2005). However, evidences of such mechanisms are difficult to test so they are difficult to
demonstrate (Lindberg et al. in revision). Compensation by heterogeneity, can occur when
harvest mostly affect the individuals with a poor contribution to the demography of the
population.
[Article 6]
Effects of hunting on movements
The analysis of bird movements in Brenne (article 6) confirms the high fidelity of the ducks to
their release site (monthly probability that a surviving bird would still be present the
- 33 -
Probabilité de lecture (p) et de
relecture (c)
following month: F = 0.96 ± 0.05), as already suggested by the high recovery rates on the
release sites themselves (Figure 3.2). However, the Mallards do not become completely
motionless, and we especially recorded some movements during the hunted months. While
the probability to see the marked ducks at least once a month is generally higher during the
hunting period, the probability to see them more than once during the three successive
observations days of each monthly session is lower (Figure 3.5). Ducks therefore
preferentially use their release site (over three days, the probability to see the mat least
once is high, especially during the hunting season), but they nonetheless move between or
within sites, so that they are not resighted every day.
0.8
*
*
1
*
p
c
*
0.6
0.4
0.2
0
Août
Sept
Oct
Nov
Dec
Jan
Fév
Mars
Figure 3.5. Sighting (p) or resighting (c) probability of a Mallard released in Brenne during a monthly session
made of three successive days, over their first hunting season (shown in grey, from Article 6).
The movements observed in response to predation risk vary with duck body
condition. Legagneux et al. (2009) hence demonstrated that the probability to emigrate from
the release site before the hunting season was greater in smaller birds, probably owing to
intraspecific competition. In the article 6, we confirmed this pattern and went further with
our robust design protocol with nasal saddled ducks: our results provide more information
on the fate of birds temporarily not observed, and show that on the most heavily hunted
lakes, the birds in better condition has a lower survival than those with a poorer condition
(Figure 3.6).
- 34 -
1
50
Probabiblité de survie
0.8
40
0.6
30
0.4
N
20
0.2
10
0
0
-4
-3
-2
-1
0
1
2
Indice de condition corporelle
3
4
Figure 3.6. Survival probability in September (plain line) and its confidence interval (dashed lines) for female
Mallards released in Brenne in July 2009 depending on their body condition index. The histogram shows the
number of individuals in each class of body condition. These results are for the lakes where hunting pressure
is greater (from Article 6).
Previous studies have generally showed that Mallards with a poorer body condition
had a higher hunting mortality (Hepp et al. 1986; Dufour et al. 1993; Heitmeyer et al. 1993).
The present results show the opposite, i.e. among released Mallards the “best” ones are
more likely to be harvested. These birds with a better condition may monopolize the release
lakes before the hunting season starts, forcing poorer birds to surrounding lakes. These birds
in better condition are then subjected to a high hunting pressure from September onwards.
In this context, hunting causes a quicker decline of the released Mallard population, by
specifically affecting survival and breeding success of these birds.
Fitness
Survival
Annual survival of released Mallards is generally low during their first year, as shown by our
Camargue results (annual survival ranging from 1 to 16%). These rates are far lower than
those obtained for juvenile wild ducks by other authors (Figure 3.7), and are among the
lowest of those obtained for released birds in North America (9 to 33%; Dunn et al. 1995;
Soutiere 1989).
- 35 -
Sauvages Lit.
Adulte
Lâchés Lit.
Cette étude
Sauvages Lit.
Juvénile
Lâchés Lit.
Cette étude
0.0
0.2
0.4
0.6
0.8
1.0
Survie annuelle
Figure 3.7. Annual survival rates of released Mallards during their first year (Juvenile; grey) and following
years (Adult; white) from this study (Article 6 and results above) compared to data from the literature based
on three estimates for each age class (Soutiere 1989; Dunn et al. 1995). As a comparison, estimates for wild
Mallards have been compiled (black) from 16 studies for juveniles (Gunnarsson et al. 2008; Giudice 2003) and
111 for adults (O. Devineau, unpublished data). Boxes show lower and upper quartiles.
Contribution to the breeding population
Taking into account our Brenne survival rate (Article 6), 18% of released Mallards survives
until the next breeding season (March). Considering also that 30 000 to 50 000 Mallards are
released annually in this area, and that emigration outside the Brenne is of 0.2% only
(Legagneux et al. 2009), 5400 to 9000 released birds should theoretically survive their first
hunting season. Ducks counts in mid-February by the "Oiseaux d'eau and Zones humides"
network of ONCFS/FNC/FDC record ca. 8000 birds (7480 ± 1310 between 2001 and 2010. C.
Fouque, pers. com.) over the 300 most densely used Brenne lakes, most likely contributing
to the Brenne breeding population (Fouque et al. 2004). Considering that up to 50% of the
birds are not detected during such counts (Kirby 1995), our survival rates suggest that 33 to
56% of the Brenne breeders are likely to be Mallards released the previous year.
Evidences of reproduction
Evidences of reproduction by released females are scarce: only 8% of the 722 saddled
females were resighted after the end of their first hunting season in Camargue. In total, 66
different females were hence observed between 1st March and 1st September. Among these,
39 were observed while being paired at least once (59%) and 4 (0.6% of the total number of
- 36 -
saddled released females) were observed with 5±3 ducklings aged 1 or 2 weeks. Breeding by
released ducks has therefore indeed been observed, but is likely infrequent. Furthermore,
none of these families was observed more than once except for one female, observed alone,
suggesting a low survival of both broods and females with ducklings.
The contribution of released Mallards to the wild population therefore seems to be
limited. We do not have enough breeding attempts documented in the wild to assess the
survival of ducklings whose mother was released or their ability to later breed themselves,
which would have been necessary to properly assess the fitness of released Mallards.
However, the genetic data presented in chapter 5 allow estimating gene flow from the
released to the wild population. Before this, the reproductive value is considered in the next
chapter to model the contribution of released Mallards to the increase of the wild
population under various harvest scenarios.
Figure 3.8. Female Mallard code FNR with fifteen one day old ducklings. This female was released in Brenne
in August 2009. Photograph on 20 April 2010 by L. van Ingen.
- 37 -
CHAPTER 4
4. CONTRIBUTION OF RELEASED MALLARDS TO THE
DYNAMICS OF THE NATURAL POPULATION
4. Contribution to Population Dynamics
The previous chapter suggests that Mallard released for hunting purposes have a lower
fitness than wild ones. In order to better understand how the Mallard population responds
to harvest, it makes sense to model it as the mixing of two duck groups with different
qualities: released ducks with high mortality during their first year and low breeding success,
and wild Mallards with the opposite properties. In this chapter a population showing such
individual heterogeneity was modelled and its trends in numbers computed under different
scenarios. These scenarios represented different hunting harvest rates over the two groups
of birds. The hypothesis being tested is that a high harvest rate could not be sustained by the
wild population alone, while selective harvest of released individuals, with a lower
reproductive performance, could prevent population decline. In a second step, having
recorded an increase in harvest in the Camargue, real ringing data were used to test the
realism of the modelling exercise through the comparison of annual survival rate between
modern wild birds and wild ducks ringed before the development of release practice (i.e.
before the 1970s).
Model and parameters
General model
A simplified life cycle was build for Mallard from a « birth-pulse » model (Caswell 2001;
Figure 4.1). It considers two age classes: up to one year (‘‘1A’’) and more than one year
(‘‘+1A’’). Only females are considered, since they only condition the dynamics of the
population (Caswell 2001). The model considers that annual survival of juveniles until the
next breeding season (S1A) is lower than that of adults (S+1A) (Reynolds et al. 1995;
Gunnarsson et al. 2008). All juveniles surviving until their first breeding season do not
attempt to breed (a1A < 1), as opposed to older individuals (a+1A = 1; Devries et al. 2008).
Fecondity (F) was estimated based on an even sex-ratio at birth (Bellrose et al. 1961; Blums
and Mednis 1996; Gunnarsson et al. 2008; but see Denk 2005 for the first study suggesting a
male biased ratio right from hatching) and was not considered to differ between age classes.
The parameters used to run the model were of European origin whenever available.
- 39 -
Released
Annual release of
N0 individuals
S+1A
S1A(L).F(L).a1A(L).(1-α)
S1A(L)
1A
+1A
S+1A.F(L).(1-α)
S1A(L). F(L). a1A(L).α
S+1A.F(L).α
Wild
S1A
+1A
1A
S+1A.F
S+1A
S1A.a1A.F
Figure 4.1. Simplified life cycle for Mallards considering releases of captive-bred birds. The description and
value used for each symbol is given in Figure 4.2.
Group
Parameter
Wild
1st year survival without hunting
Wild
/
Released
Proportion of breeders among first-year
females
Number of young fledged females
produced per pair
After 1st year annual survival without
hunting
Released
1st year survival without hunting
Wild
Wild
Released
Released
Released
Proportion of breeders among females of
one year
Number of young fledged females
produced per pair
Proportion of wild individuals produced
by released females
Symbol
Value
S1A
0.51
Corrected from
Gunnarsson et al. (2008)
a1A
0.81
Devries et al. (2008)
F
1
S+1A
0.66
Article 7
S1A(L)
0.10
Chapter 3
a1A(L)
0.59
Chapter 3
F(L)
0.5
Yerkes and Bluhm (1998)
α
{0.8; 1}
Wild
Hunting harvest rate
H
0.13 ± 10%
Released
Hunting harvest rate
H(L)
{0.13 ± 10%; 0.5}
Source
Gunnarsson et al. (2008)
Chapter 4
Chapters 3 and 4
Figure 4.2. Description and value of the parameters used to model the Mallard population.
- 40 -
Contribution of released ducks
The model considers annual and constant releases arbitrarily representing one-fifth of the
initial population size. This is close to reality if we consider that the European population
comprises 2.8 million Mallards released annually (Figure 1.2) and 14 million flying birds at
the end of the breeding season (which is consistent with the recording of 7.5 million
individuals in January, a low mortality until January (1.5 million birds) and the harvest of 5
million ducks between the end of the breeding season and January; Delany and Scott 2005;
Mooij 2005).
The annual survival rate for released juveniles comes from our results (Chapter 3).
Released individuals, once adults (i.e. having survived one full hunting season), were
considered as having the same survival rate than adult wild birds (S+1A(L) = S+1A). This
assumption is optimistic considering that adult released birds may actually have a slightly
lower annual survival than proper wild ducks (Figure 3.7).
Estimates are lacking regarding the reproduction of released Mallards. We hence
used the proportion of females observed with a mate between 1st March and 1st September
(Chapitre 3) as the proportion of breeding released females. Similarly, productivity of
released females was assumed to be half that of wild females (thus 0.5 females produced
per released female), following a study comparing breeding success of released and wild
pheasants (Brittas et al. 1992).
0.0295 ⋅ (1 - H ( L ) ) ⋅ α
0.329 ⋅ (1 - H ) ⋅ α   N1 A (t ) 
 N1 A (t + 1)   0   0.4284 ⋅ (1 - H ) 0.658 ⋅ (1 - H )


   
 
N
(
t
1
)
0.51
(1
H
)
0.658
(1
H
)
0
0
+
⋅
⋅
 +1 A
  0  
  N +1 A (t ) 
=
+
⋅
N
(t + 1)   N 0  
0
0
0.0295 ⋅ (1 - H ( L ) ) ⋅ (1 − α ) 0.329 ⋅ (1 − H ) ⋅ (1 − α )   N1 A ( L ) (t ) 
 

 1 A( L )
   



 N

N

0
0
0.1 ⋅ (1 - H ( L ) )
0.658 ⋅ (1 - H )
  +1 A( L ) (t ) 
 +1 A ( L ) (t + 1)   0  
Figure 4.3. Matrix of the demographic model for Mallard including two age classes (1A and +1A) and two
groups of ducks with contrasted breeding performances.
Scenarios
The performance of young Mallards born in nature from released mothers is probably lower
than that of bird with wild parents, for example because of poorer maternal care in the
former case. Factor α, which is the share of “wild” individuals produced by released
Mallards, takes this into consideration. This factor varies between 0.8 and 1 so as to evaluate
its influence on the relative reproductive value of the two age classes in each of the two
groups.
Since the aim is to evaluate the consequences of changing harvest on the
demography of the population, we considered natural survival (SNat) values of wild Mallards
and applied an effect of harvest (H) so that survival S = SNat × (1 - H). This equation is an
approximation assuming that hunting mortality in concentrated in time (Lebreton 2005). For
the initial value of H, we used ring recovery probability f linked to H by equation H = f / δ
where δ is ring return rate. Ring return rate has never been evaluated for European hunters
- 41 -
of any waterbird species. We hence used the North American value δ = 0.32 provided by
Nichols et al. (1991) for Mallard. The initial harvest rate considered was therefore H = 0.13
from the value of f in article 7 for modern Mallards. We first applied this harvest rate to both
groups, then tested a 10% decrease of it separately for each group. We then applied an
extreme 0.5 harvest rate of juvenile released birds together with a 10% decrease of harvest
of wild Mallards.
Analysis of scenarios
Reproductive value
An increase of α from 0.8 to 1 increases the share of the reproductive value of the released
birds from 21% to 25% of the reproductive value both group (Figure 4.4). The reproductive
value of the juveniles is however very low (3%), while that of adult released ducks increases
to a level somewhat similar to juvenile wild Mallards (22%; Figure 4.4). For the rest of the
results, we set α to 1, which is more conservative since the reproductive value of released
Mallards is then considered at its highest, which is not necessarily the case in reality.
Group
Age class
Wild
Wild
Released
Released
1A
+1A
1A
+1A
Relative reproductive value
α = 0.8
α=1
0.32
0.33
0.44
0.46
0.03
0.02
0.22
0.19
Figure 4.4. Relative productive values for each group and age class.
Predicted population size
Based on the above parameters, the model suggests the population should decline when it
does not receive released individuals (λ = 0.986). This decline could be considered of limited
magnitude, considering the approximation of some parameters. Figure 4.5 shows that
released birds can contribute to population stability. Indeed, even if the released birds have
a low survival rate, a small share of these breed (particularly those birds initially released
which survived more than one year) and released are continuously carried out. In the case
when 200 birds are initially released in a population of 1000 wild ducks, continuous releases
prevents population extinction over 400 years, so that this population remains over 1200
individuals if harvest rate is similar over released and wild birds (Figure 4.5). In the more
likely scenario where harvest is more intense over juvenile released ducks, the same pattern
is observed but population size at equilibrium is lower.
- 42 -
HL = H = 0.13
HL = 0.5; H = 0.13
H = 0.13; No releases
Figure 4.5. Projection of a 1000 individual Mallard population following three scenarios: without releases
(plain line), with an annual release of 200 birds submitted to similar harvest than the wild population (HL = H
= 0.13) and with an annual release of 200 birds submitted to a higher harvest than wild ducks (HL = 0.5; H =
0.13)
Given the low reproductive value of the released birds, the population is hardly
affected by changes in their survival rate. This can be seen in the predicted population size
for a 10% decrease of harvest of the released juveniles, from 0.13 to 0.117 (case b in Figure
4.6). Conversely, the wild population is far more sensitive to changes in harvest. If its harvest
is reduced by 10% then the population increases by 0.1% each year, even without releases
(λ = 1.001). This general increasing pattern holds for a harvest of released birds increased to
H = 0.5 (which is a 387% increase, case d in Figure 4.6).
- 43 -
c) HL = 0.13; H = 0.117
++++++++++ d) HL = 0.5; H = 0.117
.. ..
b) HL = 0.117; H = 0.13
a) HL = H = 0.13
Figure 4.6. Predicted size change of a 1000 individual Mallard population receiving 200 birds annually (a)
under different scenarios: b) a 10% decrease in harvest of released birds; c) a 10% decrease in harvest of wild
ducks d) with an increase in harvest rate of released individuals from 0.13 to 0.5.
Discussion
This modelling exercise is limited by the availability of precise parameter estimates for wild
birds, and even more so for released Mallards. Furthermore, it considers a closed population
without density-dependent processes. Finally, it probably overestimates the ability of
released Mallards to breed. However, it allows simulating the general response of the
population to changes in some parameters. For instance, it shows that despite their low
reproductive value the released birds can maintain a minimum population size. This is
because some few juveniles are produced by the released Mallards, especially those that
survived more than one year, which after the model parameterization then survive like the
wild adult Mallards and are exposed to similar harvest, even if they are twice less likely to
breed.
Our model mostly shows that even a short decrease of harvest rate of the wild
Mallards quickly leads to a population increase, both with or without released birds.
Conversely, a harvest rate similar to that recorded on our results leads to a population
decline, whose sustainability is only possible by the input of released birds.
- 44 -
Perspectives in Camargue
Changes in harvest
Since 1992, Mallard hunting bag in Camargue has doubled (Figure 4.7) and harvest
opportunities (hunting bag corrected for hunting pressure) have increased further still (in red
in Figure 4.7).
Figure 4.7. Changes in hunting bag (blue line) and hunting opportunities (hunting bag corrected for hunting
pressure, red line) for Mallard in 35 Camargue estates from 1992 to 2010 (from Mondain-Monval et al. 2009)
Are the releases benefitting the wild Mallard population?
The above modelling exercise suggests that the wild Mallard population could likely not have
sustained such a high harvest rate without the releases. Furthermore, the population did not
decrease but actually increased, as suggested by the positive trend of the population to
which Western Mediterranean Mallards belong (AEWA 2008). It is therefore likely that the
survival of wild Mallards did not decrease despite increased hunting bag, because harvest is
preferentially onto released birds.
To test this hypothesis, annual survival rate of historical (before the massive
development of releases) and modern wild Mallards were compared. Similar data for Teal
Anas crecca were used as a control: no Teal releases are carried out, and Teal hunting bag
did not increase over the last decades (Mondain-Monval et al. 2009). All dataset were from
the same sites and periods for the two species. The simultaneous ringing of juvenile and
adult birds allowed a Brownie parameterisation of models with the hunting recoveries (Box
3.1).
The best model did not include a “period” effect for any of the two species, indicating
that current survival of Mallard and Teal are not different than that of ducks ringed before
the 1970s (Article 7). Current annual survival rate estimate for Mallard is 59%, which is
similar to the results of previous studies of adult wild Mallard (57% to 70%; Giudice 2003;
Figure 3.7).
- 45 -
[Article 7]
These results suggest that the increase in annual Mallard hunting bag mostly results
from the harvest of released birds, since no such increase in hunting bag is observed for Teal.
The genetic analyses in the next chapter (Figure 5.3) suggest the wild Mallards preferentially
use protected areas without hunting. The effect of harvest on the survival rate of wild
Mallard wintering in Camargue therefore seems to be similar to that in wintering Teal.
Indeed, wild Mallards and Teal seem to be preferentially shot in the close surroundings of
day roosts located in protected areas (Figure 5.3; Guillemain et al. 2008b), did not
experience a decrease in their survival rate over the last 30 years, and their Western
Mediterranean populations have increased over the last years (AEWA 2008).
- 46 -
CHAPTER 5
5.
CONSEQUENCES OF RESTOCKING
FOR THE NATURAL POPULATION
5. Consequences for the Natural Population
Potential sources of pathogens
[Article 8]
Because many birds stay or halt in the Camargue (Berthold 2001), this place is the focus for
the spread of many pathogens including influenza A viruses (Jourdain et al. 2007). A
monitoring of viral excretion frequency in 2006-2007 demonstrated a mean prevalence
(share of all individuals showing viral excretion) of 5% in Mallards in Camargue during winter
(Lebarbenchon et al. 2010).
The monitoring of Mallards in game bird facilities was set up to test for the presence
of either low or high pathogenic viruses. Further, a search in wild birds for the strains of
Influenza A viruses detected in the game bird facilities was set up to test the hypothesis of a
disease transmission from these farms to the natural habitat.
Low pathogenic influenza viruses were indeed found in the four Camargue game bird
facilities we studied, at prevalence ranging from 0 to 99% depending on sampling sessions
(Figure 5.1). The high prevalence (99%) in 2009 in one of the farms (E1) shows that the risk
caused by the captivity and release of Mallards in the wild is high. All the Mallards of this
farm had in 2009 the same new low pathogenic H10N7 strain.
222
99
Mallard age
(weeks)
6
6
No of days
before release
21
2
E2
100
7
0
0
E3
100
12
0
24
E4
100
8
0
0
Year
Farm
2009
E1
E1
2010
No samples
Prevalence (%)
99
8
Figure 5.1. Infection rates of captive-reared Mallards by avian influenza viruses in three game farm facilities
(from Article 8).
The sampling of 851 Mallards from Camargue hunting bags was carried out during
the previous (2008/2009) and following (2009/2010) winters to this contamination episode
of farm E1. No Mallard then sampled from the wild carried an H10N7 virus. These results do
not allow concluding regarding the existence of potential virus exchanges between game
bird facilities and the natural habitat. However, this result does not mean that such
exchanges do not occur: they may indeed have remained undetected due to sometimes very
short virus excretion periods in ducks (Jourdain et al. 2010).
- 48 -
[Article 3]
[Article 4]
Potential sources of gene flow
The studies presented in chapter 2 have shown that historical and modern wild Mallards
were genetically different from farm Mallards. This provides a good opportunity to test one
of the major concerns of hunters and land managers regarding the potential loss of the
« wild Mallard type » in France.
The samples collected on Mallards caught in a protected Camargue area (“Aire
Protégée”) show a close genetic proximity to the old museum specimens « Musées » (Figure
5.2). This suggests that despite massive releases of captive-bred Mallards over 30 years, the
genetic integrity of the wild population has not been deeply affected. This result allows
rejecting the hypothesis of a massive gene flow from the released to the wild population. It
is probably the consequence of a low fitness of the released Mallards (Chapters 2 and 3).
Elevage_2
Elevage_3
Elevage_4
Elevage_5
Elevage_1
Figure 5.2. Genetic distance
between the Mallard populations
studied (« Chasses » = hunting
estates, « Elevage » are farms
(from Article 3).
Chasses
Aire protégée
Musées
0.005
A more precise analysis within the modern populations shows that despite low
introgression, some released birds do breed in the wild (Figure 5.3C). Similarly, assignation
software STRUCTURE suggests there are some hybrids in all samples (Figure 5.2), even if they
represent less than 10% of the individuals. The presence of such hybrids is confirmed in
Camargue by more specific software NEWHYBRIDS (cf. Article 3).
- 49 -
A
Chasse Camargue
B
Réserve Camargue
Rep. Tchèque
C
5%
10%
6%
13%
Elevage
Sauvage
Hybrides
3%
19%
75%
87%
82%
Figure 5.3. Proportion of Mallards assigned to the Wild (« Sauvage »), Released (« Elevage ») or Hybrid
1
(« Hybride ») categories after the sampling of two Camargue hunting estates (A, N = 41), a protected area in
Camargue (B, N = 39) in winter and breeding birds in the Czech Republic (C, N = 139). Results were obtained
from the Bayesian assignation method of software Structure (Pritchard et al. 2000).
Our results therefore suggest real though limited risks of intraspecific hybridization.
This is consistent with previous studies. In Red Fox, for example, introgression of domestic
genes into a North American natural population was limited both geographically and
demographically (Sacks et al. 2011). In Brown Trout Salmo trutta the individuals born in
nature but of captive origin have a low breeding success, which limits introgression over
several generations (Wollebæk et al. 2010). The case of the releases Mallards seems to
belong to this category of limited introgression.
It is possible that our results only document a minimal rate of introgression of the
wild Mallard population by the released birds, while this rate is actually higher. First, the
study in Camargue is based on wintering ducks, hence a mixing of birds with different
geographic origins, some of which potentially be areas without Mallard releases. Hybrids
between released and wild birds would thus be diluted by the arrival of purely wild Mallards
in winter. Our sample and the recorded introgression rate are therefore only representative
of the situation for wintering ducks (including sedentary ones) in the South of France, and
should be considered as such.
Secondly, it is possible that the apparition of new selection pressures and the release
of other selection pressures in game bird facilities increase the difference in life history
traits, behaviour or morphometrics between wild and released populations more than they
do on neutral markers such as those used here. Indeed, selection in captivity can promote
the fixation of alleles that are deleterious in natural habitats (Lynch and O’Hely 2001), even if
counter selection generally prevents their spread (see Tufto 2001). Conversely, if these
alleles are promoted by natural selection in the wild, the introgression rate could be far
1
The percentage comprises the share of individuals whose captive origin could be told by the presence of a ring
and which were hence not considered in the genetic analyses. This explains the differences between the values
in this figure and the values in Figure 5 of Article 3.
- 50 -
higher than measured here on neutral markers (Fitzpatrick et al. 2010). Teeter et al. (2008)
hence demonstrated that measured introgression rate can differ by a magnitude of 50
depending on the markers used. The intense and continuous immigration of individuals with
captivity-derived traits could eventually affect significantly the mean morphometrics of the
individuals of the wild population, which may affect their ability to respond to global change
(Tufto 2001; McGinnity et al. 2009). The next part aims at assessing the existence of such
potential changes in morphometrics since the development of massive releases in France.
Potential sources of morphological change
Released Mallards cannot be discriminated from wild ones by visual inspection (Byers and
Cary 1991). Breeders indeed select a phenotype as close as possible to the wild one for their
hunting Mallards. For example, one of the game bird facilities we worked with has been
selected ducks considering bill colour, body weight and general appearance of birds of both
sexes, as well as the breast pattern of males (Duck breeder, pers. com.). A recurrent artificial
selection by the breeders could eventually lead to a general morphological change of the
released Mallards, eventually leading to changes of the wild population over the long term.
In addition to such deliberate selection by the breeders, the release in captive-bred ducks of
some selection pressures that occur in nature can also lead to morphological changes of
phenotypic traits not detected by the breeders (Lynch and O’Hely 2001). Such changes can
affect the population living in the wild, which is made of released individuals, wild individuals
and hybrids. We therefore tested the consequences of massive Mallard releases for the
morphology of the wild population by comparing the situation before and after 30 years of
massive releases (« Before After Control Impact »). Because there may be some confounding
effects such as habitat changes over the same period, global warming or other global
changes potentially affecting species morphology, we used Teal as a control. Teal indeed
have a niche globally similar to that of Mallard (Tamisier and Dehorter 1999), but no Teal
releases are carried out.
[Article 9]
Changes in mean body weight
Body condition (weight corrected for size of the individual) is a crucial trait affecting Mallard
survival and reproduction (Whyte and Bolen 1984; Devries et al. 2008). Duck body mass also
shows strong variations throughout the winter following changes in the birds’ activity
rhythms (Tamisier and Dehorter 1999; Guillemain et al. 2005).
Released Mallards have often been considered in the past as being “too fat”. The
body mass of birds of wild origin, even if kept in captivity, is indeed sometimes lower to that
of individuals from long release strains (Dubovsky and Kaminski 1994). Furthermore, a study
trying to find plumage anomalies in Mallard in Dombes over the 1976-1981 and 1993-1994
periods demonstrated heavier mass than reference values (Géroudet 1999) for close to 45%
- 51 -
of the Mallards examined, considering this was a consequence of nearby Mallard releases
(Manin 2005).
Over the last 30 years, the mean body mass and body condition of wild Mallards
considerably increased (Figure 5.4). A similar increase is observed in Teal, for which no
releases occur. It is therefore likely that this increase is not due to the release of heavier
Mallards, but to local habitat improvement for ducks, through the increase in areas
protected or a greater food availability (due to longer flooding of wetlands and/or spreading
of hunting bait) (Tamisier and Grillas 1994). It is worth noting, however, that in response to
this study Gunnarsson et al. (2011) performed the same analysis in Sweden an showed a
similar increase in Mallard body mass, but not in Teal. Their study supports the hypothesis
that Mallard releases, also frequent in this country, could be responsible for the observed
increase in mass.
5
Poids / Longueur Aile
Historique
Moderne
4.5
4
3.5
Mâles Juv.
Mâles Ad.
Femelles Juv.
Femelles Ad.
Figure 5.4. Comparison of modern (N = 1371) and historical (N = 6156) Mallard body condition when
captured in Camargue during winter.
Changes in bill morphology
[Article 10]
Subtle differences in bill morphology exist between dabbling duck species. These lie in bill
lamellar density, which are used to filter water and retain prey items (Pöysä 1983; Nudds et
al. 1994; Guillemain et al. 2002; Gurd 2007). In Mallards as in other ducks, lamellar density is
the evolutive product of two forces: i) one favours high lamellar density to retain a wider
spectrum of prey sizes, while ii) the other favours a lower density to avoid detritus from
clogging inter-lamellar spaces (Guillemain et al. 1999). In game bird facilities the former
selection pressure could be released since Mallards are fed with large pellets they do not
have to filter.
We indeed recorded a decrease in the number of lamellae at the proximal end of
Mallard bill, which was not observed in Teal (Figure 5.5). The captive-bred Mallards could
not be included in the analysis because they were all from the same strain and were hence
not independent from each other. They are only shown for information in figure 5.4. The
modern sample comes from protected areas and hunting estates in Camargue, and is
- 52 -
therefore a representative sample of hybrids, released and wild ducks that form the
population living in the wild. From the previous paragraph on genetics, we estimate the
probability of birds examined for their bill of being of wild origin to 69%. We hence cannot
conclude on the origin (genetic or due to a mixing of individuals) of the observed difference
between the historical and the modern samples, although we could demonstrate such a
difference.
11
12
11
10
Lamelles / cm
10
9
Femelle colvert
9
8
Mâle colvert
8
7
7
6
6
5
4
3
*
(12/41)
ns
(20/48)
ns
(20/48)
ns
(19/48)
ns
(15/46)
1
2
3
4
5
ns
(10/27)
2
5
*
(21/65)
ns
(28/73)
ns
(29/75)
ns
(28/75)
1
2
3
4
*
ns
(25/71) (24/61)
4
6
5
6
Parties du bec en cm
Parties du bec en cm
(de la partie pro ximale à la plus distale)
(de la partie pro ximale à la plus distale)
Figure 5.5. Number of lamellae per cm of bill (1 = proximal end, 6 = bill tip; upper mandible) in Mallards.
Museum specimens collected before massive releases started are in plain line, modern Mallards in dashed
line and the mean value from a game bird facility is represented as a grey triangle for information. Sample
sizes in parentheses, historical birds first, then modern birds.
A lower lamellar density may affect foraging ability on small seeds such as those
generally encountered by Mallards (which is consistent with the low survival of released
Mallards when they have to switch to natural food, Chapters 2 and 3). The observed change
in bill morphology could also partly explain why Mallards increasingly tend to rely on hunting
bait or cultivated fields (Osborne et al. 2010). The decrease in lamellar density could then
provide an advantage since the risk of lamellar clogging would be reduced while foraging
efficiency (on such artificial foods) would be maintained. A positive selection should then
occur, which could explain why this character changed so rapidly, i.e. over ca. 30 generations
(Fitzpatrick et al. 2010). This suggests that gene introgression from captivity to the wild
population could be greater than previously determined from the neutral markers.
- 53 -
CHAPTER 6
6. DISCUSSION
6. Discussion
Moderate effects of the releases
Domestication of released Mallards
Domestication of animal species implies the control by Man of their reproduction, habitat
use and foraging (Mysterud 2010). Domestication implies changes in morphology, behaviour,
physiology and life history traits (Mysterud 2010). Captive-bred Mallards derive from many
generations of captive birds. The present results demonstrate the genetic difference
between these and wild Mallards. Furthermore, the difference in bill lamellar density
between released and wild birds likely results from the release of some natural selection
pressures and the adaptation to a captive environment. Even if no difference in foraging
behaviour in natura could be demonstrated between wild and released birds, our results
suggest a preferential use of hunting bait as well as a lower body condition and a smaller
gizzard in released birds. Further, captivity seems to result in naïve behaviour towards
predators. Finally, our study demonstrated a lower MHC genetic variability, suggesting
released birds are more sensitive to natural pathogens, which generations of captive ducks
probably never encountered. All of the above show the high level of domestication of the
released Mallards.
Fitness of the released Mallards
We propose that domestication probably caused cascade effects leading to poor survival of
the released birds in the wild. Indeed, the morphological changes in captivity hamper
foraging efficiency in natural habitats, as demonstrated by the poorer body condition of
these birds. Energy-stressed individuals then have to expose themselves to hunting while
looking for baited sites, or to predators by diminishing their vigilance while foraging
(Tamisier and Dehorter 1999). Domestication therefore is a burden for released Mallards.
Further studies could be carried out to determine if the lower survival results from genetic
changes or to the environment in which these Mallards developed. For this purpose, game
bird facility eggs could be introduced into wild hen nests. Survival and behaviour of the
duckling could then be compared to that of wild ducklings raised in similar conditions.
In addition to a low survival, released Mallards seem to be poorly breed (only 0.6% of
the 722 saddled released females were observed with young over two years in Camargue).
Even if small number of these birds are however able to breed in the wild (Figure 5.3C),
released Mallards generally have a low fitness since they show poor ability to transmit their
genes to the next generations.
Introgression
Our results show a significant, although limited, rate of hybridization. The existence of a
strong selection pressure after release, even increased by the strong vulnerability of these
- 55 -
6. Discussion
birds to hunting and predation probably disadvantages the alleles of released birds strongly,
which can explain the low genetic introgression we recorded. It is also possible that released
birds preferentially breed with each other, as shown by Cheng et al. (1979). In their
experiments, Cheng and co-authors demonstrated that male Mallards preferentially mate
with females from the same strain, suggesting a barrier to panmixia. Some hybridization is
nonetheless possible because this barrier is not absolute (Cheng et al. 1980), maybe because
of the male-biased sex-ratio in duck populations (Blums and Mednis 1996): if the first males
to mate select females from their own strain, some additional males may hybridize with
females from a different origin. Our results demonstrate some interbreeding between wild
and released Mallards, but a limited introgression, which is consistent with the results from
Cheng et al. (1979). Gene flow from the released to the wild population is therefore limited
by strong counter selection in the wild. This limits the risk of breakdown to the genetic
structure of the wild population through genetic homogenization, for example (Olden et al.
2004).
Individual heterogeneity
Because of the low survival rate and breeding success of the released Mallards, Mallard
restocking introduces some individual heterogeneity in the global population, made of two
components: individuals born in the wild, from parents already living in the population
(either wild birds or some few released individuals) with "standard" demographic
performances, and individuals recently released. We did not demonstrate any change in wild
Mallard survival over the last 30 years despite the increase in Camargue hunting bag over
the last decades. In addition, released individuals seem to be harvested more by hunting
than wild Mallards (Figure 5.3). Restocking may hence have contributed to the increase in
global Mallard harvest without affecting the annual survival rate of the wild individuals.
Mallard restocking therefore differs from the red-legged partridge case, where
releases without hunting ban do not mitigate the existing threats over wild birds (Bro et al.
2006). This may be due to the higher vulnerability of wild partridges to hunting, while wild
Mallards would be less exposed because of their preferential use of hunting-free areas. It is
also likely that the size of the wild Mallard population (compared to that of wild partridges)
may have caused such differences of impact of the releases between the two species.
Our results therefore suggest there are two distinct categories, with wild Mallards
harvested in constant numbers, leading to a relatively stable survival rate over 30 years,
mostly breeding among themselves on one side, and on the other side released Mallards
quickly hunted or dying before the first breeding season, and hence hardly hybridizing with
their wild conspecifics.
- 56 -
6. Discussion
Some possible effects remain
The results of this thesis allow considering the consequences of Mallard releases over the
longer term, and evaluating the possible threats if restocking schemes continue at the same
scale.
Pathogen reservoirs
We confirmed that game bird facilities are a potential source of pathogens, which they will
continue to be since they are a favourable habitat for parasite development (Mennerat et al.
2011). Game restocking is a potential source for pathogen dispersal in general (Gortázar et
al. 2007; Slota et al. 2011). In the case of the Mallard, Influenza A viruses are among the
most closely monitored pathogens owing to their potential impact for economy and health
(Munster et al. 2007; Brochet et al. 2009). This study confirms that game bird facilities are a
potentially major source for virus dispersal (Gauthier-Clerc et al. 2007). In the case of farms
producing game for restocking such dispersal is promoted by i) duckling trade throughout
France and abroad, ii) exchanges of genitors between farms and, of course, iii) releases in
the wild. Other pathogens can spread through game farms (Millán 2009). In particular, the
spread of bacteria such as pathogenic Escherichia coli strains is a potential threat not only to
the wild Mallard population, but also to other duck species or humans (Ewers et al. 2009;
Díaz-Sánchez, in press).
Genetic perspectives
Even if introgression rate is so far low, the divergence between wild and released birds may
not have reached equilibrium, or could be modified by future demographic change in one or
the other population. It is also possible that our results only provided a minimum
introgression rate of the wild Mallard population by the released birds. Lastly, if the hybrids
have a fitness just slightly higher than the released Mallards (through a heterosis effect),
they may increase gene flow over time, which would increase the risk that the wild
population would loose its local adaptations. In particular the decrease in bill lamellar
density coupled with habitat change may benefit the released Mallards or their offspring:
the increasing use of hunting bait may therefore benefit the released birds by increasing
their survival during cold winters, compared to wild birds, so the former could eventually
participate to a greater extent to reproduction within the natural population.
The lack of major effects of the releases so far may not hold forever. Our results have
their geographical and methodological limitations (number and type of genetic markers
used). Future analyses by a Swedish team (J. Elmberg and P. Söderquist) combining the
present data (from France and the Czech Republic) with new data from Sweden and
Denmark are on their way, and will rely on a large number of SNP markers recently
- 57 -
6. Discussion
developed for Mallard (Kraus et al. 2011). The increase in the number of markers used and
study sites will likely provide finer estimates of introgression rate.
Management perspectives
This work focused on the consequences of Mallard releases for the natural duck population.
However, the impact of these practices for the rest of the ecosystem remain to be assessed
(see e.g., Draycott et al. 2011). It is likely that when hunting is practiced on « artificial » game
less attention is given to wetland management, since the habitat is then only the physical
ground onto which hunting occurs.
Recommendations
Socio-economic context
We mostly considered the threats to the natural duck population. However, in an applied
ecology context the ethical and economical questions also have to be considered to provide
adequate management recommendations for this species.
The Mallard has been domesticated for several thousand years. Its aesthetical
characteristics are greatly appreciated, especially for males, which likely contributed to its
worldwide spread (Fox 2009). Even if captive-bred Mallards do not differ enough from wild
Mallards to establish specific phenotypic criteria, some individuals released in the wild
sometimes show aberrations. Some authors therefore consider that the releases have
“mongrelised” the species (Manin 2005; Prompt and Guillerme 2011). The terminology
employed demonstrates the importance of such problems in the way we consider the
species and its aesthetical value. This somewhat reflects how much humans want to
preserve the genetic integrity and the phenotype of the Mallard (De Vries 2006). It may also
reflect how poorly considered is the practice of the releases itself. To better understand
these processes it is necessary to explain the type of hunting supported by Mallard releases.
Releases are currently carried out for economic reasons, but are contrary to the notion of
"fair chase" (Zink 2011).
The expectations in terms of hunting bag correspond, for the breeder and the
hunting estate manager, to economic requirements. Hunters paying a lease to hunt in a
private estate expect hunting bags matching their investment. Releases can therefore be
practiced by land managers without informing their hunters (customers paying for a hunting
lease onto a private estate; Mathevet 2000), so has to increase the estate reputation and
maintain the economic benefit. This may partly explain the extent to which releases are
practiced while most hunters themselves dislike it. Hunters sometimes also consider they
“have to” release Mallards to have decent bags, especially at the onset of the season.
Many hunters, especially the younger ones, are against game releases (Havet et al.
2007) and look for a more natural and “fair chase”. This notion of fairness comprises the
- 58 -
6. Discussion
ability for the game to escape. A “fair chase” also puts more value in wild than in released
game (Knox 2011). Nobody will claim that a bird hatched from an artificially incubated egg,
moved to a game bird facility at one day of age, released at seven weeks, supplementary fed
with hunting bait and shot the day of the opening of the hunting season on the very lake
where it was released has the same value than a bird born at high latitudes and shot during
its autumn migration. Aldo Leopold said (1933: p.394): « The recreational value of a head of
game is inverse to the artificiality of its origin, and hence in a broad way to the intensiveness
of the system of game management which produced it.». Releases are criticized by some
hunters because it makes hunting less noble and is not in harmony with the roots of the
hunting tradition (Sokos et al. 2008). Shooting estates, where game is released just prior to
the hunt, and where the number of pieces of game harvested is the main objective, are the
extreme situation (Sokos et al. 2008).
Recommendations
It is probably impossible to stop the releases without profound changes in the hunting
practices and an ethical consideration of the question. Population restocking by released
birds increases hunting bags, mostly on estates where releases are practiced given the quasi
inexistent dispersal of these Mallards. Such a low dispersal contributes to mitigating the risks
associated with restocking. Without a precise monitoring of the releases, the apparent
stability of demographic trends may hide changes in the genetic structure of the population.
From the results of this study we therefore make some suggestions to be able to
continuously monitor the consequences of this practice in the future. Broadly speaking, we
recommend maintaining a clear difference between the released and the wild populations.
Should the genetic strain be improved?
In a reinforcement perspective, i.e. to sustainably support the wild population, hunters may
want to develop new captive strains, from only wild birds, as was done for pheasant (Thémé
et al. 2006). This would allow improving survival and reproduction of the released Mallards
once in the wild, if harvest was stopped or temporary interrupted (Bro et al. 2006). The
current demographic trends in Western Europe however do not justify restocking practices,
since the wild populations can sustain a reasonable harvest (AEWA 2008). Furthermore, the
production of Mallards could not allow satisfying a large demand, given the low breeding
success of wild Mallards when kept in captivity (Stunden et al. 1999).
The introduction of wild Mallards in game bird facilities could also promote some
pathogens to which captive-bred ducks are not so far confronted. Furthermore, some few
generations in captivity would be enough to alter the genotype of wild duck strains (Bryant
and Reed 1999; Williams and Hoffman 2009). If survival and reproduction of the released
Mallards was increased, while these are not truly wild genetically, the risk of introgression of
non-native genes into the wild population would be increased.
- 59 -
6. Discussion
Considering that a large majority of the individuals are currently released to be
quickly harvested, and not to support the wild population, we recommend not improving the
fitness of these birds in the wild so as to limit the risk of exchanges with the wild population
(pathogens, gene flow). The sterilisation of captive-bred individuals as practiced for salmon
(Cotter et al. 2000), could also be considered.
Ringing
In France, the law called « Arrêté du 12 mai 2006 » (NOR: AGRG0600922A, Appendix 3)
states that Mallards over 20 days should be individually identified before being given, sold or
released. An instruction from the Ministry of Agriculture explains that the rings fitted to the
birds should in no case be taken off before release in the wild. This rule is however not
respected in practice, where released birds are virtually never marked. From our experience,
we propose to explanations to this situation. First, systematic ringing is a major constraint
for land managers practicing releases without informing their hunters. Ringed game could no
longer be considered as of wild origin, which would decrease its hunting value and hence the
value of the estate. The second reason lies in the costs this would be for the breeders to ring
all their birds, both financially and logistically, plus the constraint to keep record of the rings
so as to allow determining the origin of each bird (which is the aim of this law).
Strict law enforcement would allow evaluating the share of released bird in the
hunting bag. Hunting bag collection and compilation should also be generalised in France as
it is in several European countries (Mooij 2005), so as to better understand the
consequences of harvest for population dynamics (Elmberg et al. 2006; Mondain-Monval et
al. 2009). Systematic ringing of released birds could also allow assessing how many are
caught as part of ringing programmes in the wild. So far these are considered as wild birds,
which may bias the scientific study of the species. Finally, systematic ringing of the released
birds would allow estimating the demographic parameters of captive-bred and wild birds
separately.
Sanitary monitoring of captive-bred ducks
Sanitary monitoring has to be continued following the conditions defined in article 3 of the
Arrêté du 12 mai 2006 cited above, through regular samplings in game bird facilities
(Appendix 3). Pathogen spread is one of the main threats of released birds for the wild
population, which can be limited by appropriate veterinarian measures (Cunningham 1996;
IUCN 1998).
Genetic monitoring
We suggest setting up a regular monitoring of introgression rate of the wild population by
released ducks, through DNA collection in a representative sample of the wintering
population every fifth to tenth year. This could easily be done during Mallard ringing
operations in the wild. Some 30 birds in 10 protected areas without Mallard releases in
- 60 -
6. Discussion
France should be enough to detect any potential introgression increase and potentially limit
duck releases if judged necessary. This should be done in parallel to the ringing of released
ducks since this would allow estimating the proportion of hybrid birds.
Conclusion
Wildfowlers often consider that the wild Mallard has simply disappeared since the
development of massive duck releases. On the contrary, our results generally support the
hypothesis that massive Mallard releases in France and Europe have so far had true but
limited effects on the wild population. However, our results can also be taken as a warning
of the potential consequences if such practices were to continue at the same scale in the
future. In order to anticipate the possible threats identified, we suggest some measures to
help differentiation of released and wild Mallard.
Considering the limited impact of Mallard releases so far and the economic
importance of this practice on the one hand, and the associated ethical concern they cause
on the other hand, a national debate would be welcome to better determine the legislative
framework within which such releases should be carried out. In the context of such a debate
the present thesis provides some insight onto the potential ecological consequences of
Mallard releases.
It is very surprising that such a practice is not submitted to administrative recording.
Massive wildlife introductions in France are indeed rarely monitored. For instance,
international Mallard egg trade for releases in the wild, like many fish releases to restock
rivers, or pheasant or partridge releases for hunting are not recorded anywhere. Such
practices are also criticised in Sweden by Laikre et al. (2006), and are similar in all European
countries. National and European databases should therefore be set up and their access
made easy. They would provide a firm basis to evaluate the consequences of releasing
captive-bred animals in natural habitats. This would help preserving the genetic diversity,
which is an important part of global biodiversity (Hughes et al. 1997; Laikre et al. 2010b).
- 61 -
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PUBLICATIONS
1. Conspecifics can be aliens too: a review of effects of restocking
practices in vertebrates
Champagnon J., Elmberg J., Gauthier-Clerc M., Lebreton J.-D. and Guillemain M.
In revision for Journal for Nature Conservation
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2. Consequences of massive bird releases for hunting purposes: Mallard
Anas platyrhynchos in the Camargue, southern France
Champagnon J., Guillemain M., Gauthier-Clerc M., Lebreton J.-D. and Elmberg J.
Published in Wildfowl (2009) Special Issue 2: 184–191
- 84 -
3. Restricted genetic impact of massive restocking on wild mallard
Champagnon J., Crochet P.-A., Kreisinger J., Čížková D., Gauthier-Clerc M., Massez G.,
Söderquist P., Albrecht T. and Guillemain M.
In revision for Animal Conservation
- 85 -
4. Duck’s not dead: Evaluating the effect of stocking with captive reared
individuals on genetic integrity of wild mallard population
Čížková D., Javůrková V., Champagnon J. and Kreisinger J.
Submitted
- 86 -
5. Is it Worth Being a Fat Duck? Teal and Mallard Annual Survival Rates at
a Thirty Years Interval
Devineau O., Champagnon J., Guillemain M., Lair P., Lebreton J.-D., Massez G. and GauthierClerc M.
Submitted
- 87 -
6. Low survival after release into the wild: assessing “the burden of
captivity” on Mallard physiology and behaviour
Champagnon J., Guillemain M., Elmberg J., Massez G., Cavallo F. and Gauthier-Clerc M.
Article in press (published online) in European Journal of Wildlife Research
- 88 -
7. Combining robust estimates of survival and emigration reveals hunters’
unintentional directional selection in mallards
Legagneux P., Champagnon J., Souchay G, Inchausti P., Bretagnolle V., Bourguemestre F.,
Van Ingen L. and Guillemain M.
Submitted
- 89 -
8. High Influenza A virus infection rates in Mallards reared for hunting in
the Camargue (South of France)
Vittecoq M., Grandhomme V., Champagnon J., Guillemain M., Renaud F., Thomas F.,
Gauthier-Clerc M. and van der Werf S.
In preparation
- 90 -
9. Wintering French Mallard and Teal are heavier and in better body
condition than 30 years ago: effects of a changing environment?
Guillemain M., Elmberg J., Gauthier-Clerc M., Massez G., Hearn R., Champagnon J. and
Simon G.
Published in AMBIO (2010) 39: 170-180
- 91 -
10.Changes in Mallard Anas platyrhynchos bill morphology after thirty
years of supplemental stocking
Champagnon J., Guillemain M., Elmberg J., Folkesson K. and Gauthier-Clerc M.
Published in Bird Study (2010) 57: 3, 344-351
- 92 -
APPENDICES
Appendix 1: Release method
Release method
In France, the breeding method consists in producing wild-like ducks through strong
selection of the genitors (bill colour, body mass, general appearance + breast of the males).
Eggs hatch in an incubator, each hen producing 20 to 60 eggs. After hatching a large
proportion of the ducklings are sold at the age of one day to other breeders or directly to
the managers of hunting estates. Two main release methods are used depending on the
hunt practiced:
- Daily hunt, also called « shooting hunts » or « commercial hunts ». Ducks are bred
on site after reception at the age of 1 day if not produced on site. Birds are fed throughout
the winter. This type of hunts is infrequent and has apparently declined in the 1980s in
Camargue (Mathevet 2000. p.258). Conversely, they release more birds (several thousands
of individuals per estate annually).
- Communal or private hunting estates where birds are bought at 5 to 12 weeks of
age between mid-June and early August when unfledged. These are generally fed until the
opening of the hunting season only. The main aim is to have birds on the estate upon
opening of the hunting season (third Sunday of August).
There are five main duck producers in the country, producing each more than 80 000
Mallards annually. Exchanges between producers are frequent and up to 25% of the
production is exported (to Belgium, Spain, Germany, Denmark, The Netherlands or even
Marrocco or Qatar exceptionally).
- 94 -
Appendix 2: Poster on the effect of hunting on survival
Poster presented during two congresses in Nebraska, USA, in March 2011; meetings of the
Association of Field Ornithologists - Cooper Ornithological Society - Wilson Ornithological
Society (AFO/COS/WOS) and of the Waterbird society and North American Crane Working
Group.
- 95 -
Appendix 3: Arrêté du 12 mai 2006
- 96 -
Appendix 4: List of Communications
Communications in congresses
Champagnon, J., Guillemain, M., Gauthier-Clerc, M., Lebreton, J.-D. 2011. Ecological consequences
of massive duck releases for hunting purposes. Conservation Sciences in the Mediterranean Region.
Oral communication, Arles, France (Dec. 2011).
Champagnon, J., Guillemain, M., Gauthier-Clerc, M., Elmberg, J., Cavallo, F. and Massez., G. 2011.
Survival probability and morphological adaptation of captive-bred Mallard Anas platyrhynchos after
release into the wild. Association of Field Ornithologists - Cooper Ornithological Society - Wilson
Ornithological Society (AFO/COS/WOS). Oral presentation, Kearney, USA (March 2011)
Champagnon, J., Guillemain, M., Massez., G., Cavallo, F., Gauthier-Clerc, M. and Lebreton, J.-D. 2011.
Impact of harvest on survival of captive-bred Mallard released for hunting purposes. AFO/COS/WOS.
Poster, Kearney, USA (March 2011).
Champagnon, J., Guillemain, M., Elmberg, J., Folkesson, K. and Gauthier-Clerc, M. 2011. Do
restocking programs result in maladapted populations? Mallard bill morphology after 30 years of
massive releases. Waterbird society and North American Crane Working Group. Oral presentation,
Grand Island, USA (March 2011).
Champagnon, J., Guillemain, M., Gauthier-Clerc, M., Lebreton, J.-D. and Massez.,G. Le renforcement
massif d’une espèce d’oiseau d’eau pour la chasse dans les zones humides françaises: le canard
colvert. Société Française d’Ecologie. Oral presentation, Paris, France (Nov. 2010).
Champagnon, J., Guillemain, M., Elmberg, J., Folkesson, K. and Gauthier-Clerc, M. 2009. Do stocking
programs result in maladapted populations? Mallard bill morphology after 30 years of massive
releases. North American Duck Symposium. Poster, Toronto, Canada (August 2009; poster presented
by Matthieu Guillemain).
Champagnon, J., Guillemain, M., Elmberg, J., Folkesson, K. and Gauthier-Clerc, M. 2009. Large-scale
introductions alter average bill morphology in Mallard (Anas platyrhynchos). Ecology and Behaviour
Meeting. Oral presentation, Lyon, France (April 2009).
Champagnon, J., Guillemain, M., Gauthier-Clerc, M., Lebreton, J.-D. and Elmberg, J. 2009.
Consequences of massive bird releases for hunting purposes, the Mallard Anas platyrhynchos in
Camargue, southern France. Pan-European Duck symposium (PEDS). Poster, Arles, France (March
2009).
Champagnon, J., Guillemain, M., Gauthier-Clerc, M. and Lebreton, J.-D. 2009. Conséquences des
introductions d’individus dans les populations d’oiseaux d’eau exploitées: l’exemple du Canard
colvert, Anas platyrhynchos. Réveil du Dodo. Oral presentation, Montpellier, France (March 2009).
- 97 -

Champagnon, J. 2011. Consequences of the introduction of

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