THE DUBLIN BAY PRAWN AND ITS FISHERY

THE DUBLIN BAY PRAWN AND ITS FISHERY
(Norway Lobster)
Dr Richard Briggs
(AFBI – Retired)
Overview
1. Biology
2. The Fishery
3. Stock Assessment and Management Advice
4. Technical Conservation
5. Conclusions
1. Biology
• Nephrops norvegicus
• Dublin Bay prawn
• Norway Lobster
• Scampi
• Langoustine
• Cigala
Probably not Nephrops
Nephrops is NOT a fish!!
Phylum: Arthropoda
Class: Crustacea
Order: Decapoda
Super Family: Nephropsidea
1.
A lobster with typical hard
exoskeleton which is
periodically shed (ecdysis).
There are no persistent hard
structures with growth rings
2.
Kidney shaped eye (from the
Greek nephros = kidney and
op = eye)
3.
Live within burrows in
seabed sediment .
4.
RangeFrom Iceland to
Mediterranean
Dublin Bay prawn
(Nephrops norvegicus)
Cod
(Gadus morhua)
Life Cycle
2. The Fishery
Captured by otter trawls (single or multiple) or by creels (pots)
About 60,000 tonnes caught of which 10,000 tonnes from Irish Sea
Irish Sea Nephrops grounds
55.0N
North
Channel
Cumbria
54.5N
35 2
30
54.0N
17
15
207
8
7
109
20
10
101
102
107
200
53.5N
106104
306
302 303
301 307 309
308 304
305
1
209 208
108
250
IRISH SEA
103
105
Nephrops Grounds
53.0N
6.5W
6.0W
5.5W
5.0W
4.5W
4.0W
3.5W
3.0W
Nephrops landings from the western Irish Sea
12,000
10,508
10,000
8,491
Tonnes Landed
8,370
8,000
7,441
6,793
10,162
9,198 8,963
7,561
7,065 7,270
6,554
6,000
4,000
2,000
0
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
Year
Value of Northern Ireland Marine Fisheries
Value of top 15 trawled species
14
Value of all trawled species = £21.5 millions
12
Nephrops = 53.5% of total 1st sale value
10
6
4
2
Turbot
Cockles
Velvet crab
Monks or Anglers
Horse Mackerel
Haddock
Queen Scallops
Hake
Lobsters
Brown Crab
Cod
Scallops
Herring
Mackerel
0
Nephrops
Value (£m)
8
3. Stock Assessment
Functional Units as defined by ICES
1
2
11
10
7
9
12
32
4
8
13
18
15 14
17
16
19 20-22
3
33
6
5
23
24
31
25
26
27
36
37
35
28
29 30
34
41
38
39
40
Irish Sea Nephrops TAC: Sub-Area VII
Western Irish Sea = FU15
Sub Area VII
MA
FU
J
14 15
L
16 17 18 19
M
20 21 22
55.00
18
54.00
53.00
16
15 14
17
52.00
51.00
19
20-22
50.00
49.00
48.00
-18.00 -16.00 -14.00 -12.00
-10.00
-8.00
-6.00
-4.00
-2.00
0.00
2.00
Assessment Methods
Catch (millions)
(1) Commercial Data
Nephrops Size Compostion
35
30
25
20
15
10
5
Weakness in that ages were unreliable.
Growth
Parameters
(K, Linf )
L2AGE
SLICING
MODEL
length/weight
relationship
Nominal Age composition
200
150
100
50
0
1
2
3
4
5
Age in years
6
7
7+
48
46
44
42
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
Carapace alength mm
Number
(millions)
Computer-based models using growth
parameters were used to slice size
frequency data into age classes. Which
were input to similar procedures to
those used to assess whitefish stocks.
Such as Virtual Population Analysis
(VPA).
10
0
(2) Larval Production
Using fecundity data to back calculate the number of females
required to produce the amount of larvae observed during surveys.
Labour intensive and costly. Completed one-off as an EU funded project.
Number of Larvae in plankton
= Spawning Biomass (SSB)
Fecundity (number of eggs per gram of female)
Female Nephrops Spawning Stock Biomass
estimates from different methods
(vertical bar = 95% confidence range)
10000
9000
SSB (tonnes)
8000
7000
6000
5000
4000
3000
2000
1000
0
ALP
LCA
VPA
Assessment Method
Portaferry work
Briggs, R. P., Armstrong, M.J., Dickey-Collas, M., Allen, McQuaid, N. and Whitmore, J. (2002). Estimation of
Nephrops Biomass in the Western Irish Sea from Annual Larval Production. ICES Journal of Marine
Research, 59: 109-119.
Maturity and Fecundity (Nuala McQuaid, PhD)
Relationship between realised fecundity and carapace
length for eastern and western Irish Sea stocks..
Maturity ogive of females from western Irish Sea The dotted
curve represents maturity estimated from presence of
spermatophores and the solid line represents females with
ovaries of stage 3 or above
McQuaid, N., Briggs, R.P., Roberts, D., (2009) Estimation of fecundity in Nephrops norvegicus (L.) from the Irish Sea.
Journal of the Marine Biological Association of the UK 89(6), 1181-1188.
McQuaid, N. and Briggs, R.P. and Roberts, D., (2006). The Size at Onset of Maturity in Nephrops norvegicus from
the Irish Sea. Fisheries Research 81: 26-36
(3) Camera Surveys
Adopted by ICES since 2005
Underwater video used to count Nephrops burrow clusters

Adults inhabit burrows in
offshore sediment from
which they only emerge to
feed and to mate.

Research in Scotland
suggests that one adult
Nephrops occupies a
single burrow cluster and
unoccupied burrows are
quickly filled by sediment.

By counting burrow
clusters over a known area
it is possible to provide a
fishery independent index
of Nephrops abundance
and to map the Nephrops
grounds.
The Ships
AFBI & Marine Institute
Now 10 years data
RV Corystes
RV Celtic Voyager
Camera sledge deployment
The AFBI camera sledge being deployed at sea
Stations
• 3.5km apart
• Camera towed for 10
mins at each station
Results
Western Irish Sea (FU15) burrow abundance estimate
(Error bars = cv)
8
7
6
Billions
5
4
3
6.3
6.3
6.5
6.2
5.9
4.9
5.3
5.7
5.6
2010
2011
2
1
0
2003
2004
2005
2006
2007
Survey
2008
2009
Contour plots of burrow densities using geostatistics (2009 – 2011)
(krigged density plots)
REF: Annika Clements (2009) Broad-scale ecological investigation of Nephrops norvegicus (L.) burrow distribution in the
western Irish Sea. PhD Thesis Queens University of Belfast
Burrow densities from different areas
Moray Firth
Firth of Forth
Fladen Ground
Farn Deeps
Celtic Sea
Aran Grounds
Irish Sea West
Clyde
South Minch
North Minch
0
0.2
0.4
0.6
0.8
Burrow Density (number m-2)
1
1.2
Summary of data analysis
Landings
Size distribution
Bias Adjusted
Burrow Count
Catch Option
in numbers
Mean
Nephrops
weight in
catch
Harvest Rates
from
Model
Catch Data
Growth and
Maturity Parameters
Predicted
Landings = TAC
Justification for choice of Harvest Rate
ie proportion of stock safe to catch
Fmax
Frecent
4
F0.1
yield
3
2
1
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Fmult
Maximum Sustainable Yield (MSY): now forming ICES advice basis.
Not possible to estimate directly for Nephrops as no age or SSB trend data.
MSY proxy therefore established
Evidence for Harvest Rate at Fmax as Proxy for MSY
Commercial Fishery Data indicates that the fishery has been sustained at
about Fmax for many years
Landings - International
Effort - Different fleets
UK Northern Ireland Nephrops trawlers
250
15000
International Landings
Rep. of Ireland
Rep. of Ireland - All gears
9000
6000
3000
0
1950
1955
1960
1965
80
1970
1975
1980
1985
1990
1995
2000
2005
200
150
100
50
0
1980
2010
1985
LPUE - Different fleets
Mean size (mm carapace length)
50
40
30
Rep. of Ireland - LPUE
UK Northern Ireland - LPUE
10
0
1960
1995
2000
2005
2010
ROI - Catch - Males
ROI - Catch - Females
UK NI - Catch - Mal
UK NI - Catch - Fem
34
60
20
1990
Mean sizes - Different fleets
70
CPUE or LPUE (kg/hour trawling)
Landings (tonnes)
12000
Effort ('000 hours trawling)
UK - All gears
32
30
28
26
24
22
20
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
2010
Trawl Survey Data
E5
E4
E3
54.8N
38
54.6N
54.4N
1
37
35 2
54.2N
30
54.0N
207
8
53.8N
107
53.6N
106104
200
17
15
209 208
109
20
10
101 210
7
108
102
36
103
105
35
53.4N
6.4W
6.2W
6.0W
5.8W
5.6W
5.4W
Figure 1: Western Irish Sea Nephrops stations
5.2W
5.0W
4.8W
4.6W
Trawl Surveys: catch rates
Average catch (kg) per nm
140
(Vertical bars = SE)
120
80
60
40
20
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
0
1997
Kg per nm
100
Mean carapace length from August surveys
35
30
25
20
15
2007
2008
2009
2010
2011
2007
2008
2009
2010
2011
2006
2004
2003
2002
2001
2000
1999
1998
100
90
80
70
60
50
40
30
20
10
0
Mean percentage females in August
2006
2004
2003
2002
2001
2000
1999
1998
(error bars = SD)
1997
Percentage females (%)
1997
10
2005
August males
August females
2005
carapace length (mm)
(error bars = SD)
4. Technical Conservation
Mixed fishery
 The western Irish Sea is a whitefish spawning and nursery area enhanced by a gyre.
 Catches include cod, haddock, whiting and small non-commercial species
 Discards of undersized and non-commercial species is therefore a problem.
Michelle Allen (2009). An Investigation of sampling techniques within marine fisheries discards. PhD Thesis
Queens University of Belfast
Allen, M., Kilpatrick, D., Armstrong, M., Briggs, R., Course, G. and Pérez, N. (2002). Multistage cluster sampling
design and optimal sample sizes for estimation of fish discards from commercial trawlers. Fisheries
Research 55, 11-24
Square mesh escape panels
To develop more selective fishing
gears. Many of the novel net
designs include the incorporation
of square shaped meshes in trawl
nets which do not distort during
towing
Strip of diamond mesh in
centre (12 meshes wide)
8.85m
82mm
1.5m
4mm P/E
4.5m
84x5.6mm
compact
netting
120 round
Briggs, R.P. (2010) A novel escape panel for trawl nets used
in the Irish Sea Nephrops Fishery. Fisheries Research. 105: 118-124
Recent SEAFISH study– Spring 2012
Box Extension – 300mm SMP
Large square mesh top
panel
Diamond mesh side
and bottom panels
Standard two panel
extension and codend
4 panel (boxed) extension
section
Main body of the
trawl
Bulk (kg)
250
Boxed extension section
150
100
50
0
1
2
3
4
5
6
Tow
7
8
9
10 11 12
EXPERIMENTAL
CONTROL
14
Number of cod
Bulk - Kg
200
: Cod catch
16
EXPERIMENTAL
CONTROL
12
10
8
6
4
2
0
1
2
3
4
5
6
7
Tow
8
9 10 11 12
5. Conclusions
Irish Sea Nephrops are withstanding exploitation levels
• Historical data from 1960s when the fishery was in its infancy
shows little change in size composition of catches
• Larval production studies in 1995 showed that 440 billion larvae
were produced suggesting high recruitment potential
• The Irish Sea Gyre (whirlpool effect) retains both larvae and
sediment enhancing recruitment success
Why is this?
1. Management legislation - MLS, TAC, Minimum mesh size
2. Live within burrows in seabed sediment .
3. Crepuscular ie active at dawn and dusk and seldom emerge during
strong tides
4. Females remain in burrows whilst incubating eggs (9 months)
5. Juveniles (0-group) remaining burrowed for at their 1st year
6. When cod stocks were high (1980s) they could have consumed as
much Nephrops as the fishery (Armstrong, 1990). Cod are now at a
low level, though there are recent signs of a recovery!
QUESTIONS?