Movement and population size of spiny lobsters in San Diego Bay

Movement and population size of spiny lobsters in San Diego Bay
Final Report
Assembled for the San Diego Unified Port District
January 10, 2011
Kevin A. Hovel1 and Douglas Neilson2
1
Department of Biology
San Diego State University
5500 Campanile Drive
San Diego, CA 92182
hovel@sciences.sdsu.edu
2
California Department of Fish and Game
4949 Viewridge Avenue
San Diego, CA 92123
Table of Contents
1.
List of tables ................................................................................................................ iii
2.
List of figures .............................................................................................................. iii
3.
List of appendices…………………………………………………………………….iv
4.
Executive summary ......................................................................................................1
5. Introduction ..................................................................................................................2
6.
Objectives .....................................................................................................................5
7.
Methods
Spiny lobster population size ....................................................................................5
Spiny lobster movement ............................................................................................9
8. Results
Spiny lobster population size ..................................................................................10
Spiny lobster movement ..........................................................................................13
8. Discussion ...................................................................................................................16
9. Literature cited ............................................................................................................20
10. Appendix 1. .................................................................................................................22
11. Appendix 2………………………………………………………………………….. 24
ii
1. List of tables
Table 1. Summary of captures and recaptures of spiny lobsters in lobster traps
11
Table 2. Summary of lobster captures and reproductive condition by month
11
Table 3. Summary of lobster population estimates for each zone in San Diego Bay
13
Table 4. Number and proportion of acoustic detections for each station
16
2. List of figures
Figure 1. Lobster fishery landings over the last 100 years.
3
Figure 2. Photo of a custom built lobster trap used in the study.
5
Figure 3. Sites for the study in San Diego Bay.
6
Figure 4. Photograph of a lobster with t-bar tag inserted into dorsal musculature.
8
Figure 5. Photograph of lobsters with acoustic transmitters attached to the carapace.
8
Figure 6. Spiny lobster size distribution in San Diego Bay.
12
Figure 7. Spiny lobster density at selected seagrass sites in San Diego Bay
14
Figure 8. Histograms for the number of stations visited by spiny lobsters
15
Figure 9. Number of transits among acoustic receiver stations in San Diego Bay
17
iii
3. List of appendices
Appendix 1. Information for all spiny lobsters fitted with acoustic transmitters
22
Appendix 2. Selected movement maps for spiny lobsters in San Diego Bay
24
iv
4. Executive summary
We studied the California spiny lobster population within San Diego Bay (SDB), California,
an urbanized estuary in southern California in which spiny lobsters are known to inhabit seagrass
and artificial rocky reef habitat. The goals of the study were to (1) estimate the size of the
California spiny lobster population in SDB, and (2) monitor lobster movement patterns and site
fidelity to determine whether lobsters move out of and into SDB from adjacent kelp forest habitat
where commercial lobster fishing takes place. To estimate lobster population size, we conducted a
mark-recapture study in which lobsters were captured in baited mesh traps and individually marked
with t-bar tags. Traps were set at 16 sites in the northern and north-central ecoregions of SDB and
checked twice weekly for new and previously tagged lobsters from May to September 2009. The
size, sex, shell condition, and reproductive condition (for females) were recorded for each lobster.
Lobster movement patterns were assessed using acoustic transmitters attached to 79 lobsters.
Transmitters were detected by hydrophones anchored to the bottom of SDB at 18 sites, including
three sites in adjacent kelp forest habitat, between June 2009 and August 2010.
We captured a total of 5,681 lobsters and tagged 4,904 of them with t-bar tags. The mean
carapace length (CL) of all lobsters in SDB was 72.3 mm (+ 11 mm) with 15.6% of lobsters at or
above the legal size of 82 mm CL (3.25 inches). Male lobsters dominated the catch in early
summer (May and June), after which time females were captured in higher proportions than males.
Most female lobsters were plastered (i.e. had a spermatophore present on the thorax) or berried (i.e.
carried eggs on the abdomen) in May and June, but most had released eggs by July. We found
berried female lobsters as small as 62 mm CL. We estimated the population size of lobsters in SDB
to be between approximately 94,000 to 108,000 lobsters. This is a conservative estimate since not
all areas of SDB could be sampled. Approximately 75% of these lobsters reside at the bay mouth,
along Zuniga Jetty and across the bay near Ballast Point. Most of the remaining 25% reside within
the submerged rocky structures and seagrass beds along North Island and Shelter Island.
Movement patterns of acoustically tagged lobsters varied greatly, and there was no overall
pattern of movement toward the mouth or toward the interior of SDB. Only a few lobsters entered
the bay from the kelp forest, and no lobsters exited the bay, though some traveled from the central
eco-region to the bay mouth. Most lobsters remained in the vicinity of one or two stations for the
duration of the study, though lobsters at the bay mouth (in the vicinity of Zuniga Jetty and Ballast
Point) appeared to frequently cross the bay and to move along Zuniga jetty. We also observed that
female lobsters were more likely to travel among multiple sites than were male lobsters.
1
5. Introduction
The California spiny lobster (Panulirus interruptus Randall) is an economically and ecologically
important species in southern California waters. This species has constituted a commercial fishery
in southern California since 1872. In San Diego County since 2001, commercial lobster landings
average approximately 250,000 pounds per season with a subsequent value of ca. $2.5 million,
accounting for approximately 34% of the total state landings. Additionally, there is a large, active
recreational fishery for lobsters in southern California. California spiny lobsters are ecologically
valuable as well as economically valuable. They are major predators of benthic invertebrates, and
as such they have strong potential to dictate community structure and ecosystem function in the
habitats that they occupy. In kelp forests, lobster predation on purple and red sea urchins may help
to prevent urchin outbreaks that result in kelp barrens and a dramatic change in structural
complexity, community structure, and kelp ecosystem function (Tegner and Dayton 1981, Lafferty
2004). Shifts in lobster population size, due to fishing or to biotic and abiotic factors, may be
responsible for shifts in urchin grazing and kelp abundance over the last century (Tegner and
Dayton 2000). Spiny lobsters also may strongly influence intertidal community structure in
southern California. On Santa Catalina Island, lobsters forage on California mussels Mytilus
californianus, which are the dominant competitors for space, thereby enhancing biodiversity of
rocky shore communities (Robles et al. 1990). A study conducted in the seagrass beds of Mission
Bay, California, also suggested that spiny lobster predation is the primary controlling factor for the
invasive Asian mussel Musculista senhousia (Cheng and Hovel 2010).
Lobster fishery landings have fluctuated significantly over the last century (Figure 1), and there is
increasing interest in assessing the patterns and processes associated with lobster population
dynamics. Little information presently is available on processes that dictate lobster population size,
including egg production, settlement and recruitment, growth, movement, habitat use, and fishing
mortality. The California Ocean Protection Council has designated lobsters as a “tier 1” species,
meaning that basic information on biology and ecology are critically needed. Moreover, lobsters
are targeted for protection in marine protected areas (MPAs) currently being implemented under the
California Marine Life Protection Act. To assess how MPAs may affect lobster populations, basic
information on stock size, habitat use, and behavior (e.g. movement) must be acquired.
2
1
0.9
Catch (millions of lbs)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1920
1930
1940
1950
1960 1970
Year
1980
1990
2000
2010
Figure 1. Landings for California spiny lobsters in the commercial fishery from
the early 20th century to the present. Data from the California Department of
Fish and Game.
3
Whereas lobsters are principally thought of as open coast organisms, a significant portion of San
Diego County’s lobster population resides in subtidal rip-rap and eelgrass habitat adjacent to and
inside of San Diego Bay (SDB). SDB is a heavily developed urban estuary that supports shipping,
military operations, and recreational boating and fishing. Freshwater inflow is very low to SDB and
hydrodynamic conditions are largely tidally driven. Exchange of water with the open coast is
extensive and variation in salinity and temperature are slight near the mouth of SDB, whereas in the
back of SDB currents are slow and variation in salinity and temperature are great.
Mark-recapture surveys performed on California spiny lobsters over thirty years ago estimated that
SDB held ca. 17,000 – 21,000 legal-sized lobsters (Goforth and U’Ren 1980). More recently,
SCUBA-based surveys indicated that lobsters are locally dense along the edges of some eelgrass
beds, particularly along the northern edge of Coronado Island and in beds near the mouth of SDB
(Hovel, unpublished data). Not only were lobsters more dense in these eelgrass/rocky reef habitat
patches than in adjacent coastal (e.g. kelp forest) habitats, their average size was larger. This
suggests that SDB may house a substantial fraction of San Diego County’s legal size and
reproductively viable lobsters. These lobsters support a substantial recreational (hoop net) fishery
in SDB. Though commercial fishing is not allowed, SDB may have a major role to play in the
commercial fishery if lobsters spillover from SDB into the adjacent Pt. Loma kelp forest. SDB may
buffer coastal (= fished) lobster populations from fishing pressure via movement of lobsters
between SDB and coastal habitats, and eelgrass and rocky habitat may serve as nursery areas that
support the survival and growth of juvenile lobsters. Spiny lobsters are thought to make inshoreoffshore migrations, and sometimes display “nomadic” behavior in which they occasionally change
home range. If spillover from the Bay to the open coast is significant, SDB may contribute
substantially to the fishery, as the Point Loma fishing grounds are some of the most heavily fished
in the state. The number and behavior of SDB lobsters also may influence the way in which marine
protected areas (MPAs) are implemented and function in southern California.
Thus, determining the size of SDB’s lobster population, and the movement behavior of lobsters
inhabiting SDB are important undertakings if we are to understand how the Bay and its capacity to
house spiny lobsters affect San Diego County’s lobster population, its commercial and recreational
fishing, and efforts to conserve lobster populations. We conducted this study to evaluate the
4
importance of SDB to California spiny lobsters by measuring SDB lobster population size and by
assessing lobster movement within SDB and between SDB and the Point Loma kelp forest.
Objectives
Our specific objectives were to:
1. Estimate the size of the spiny lobster population in San Diego Bay using tag-recapture
techniques and SCUBA-based surveys, and determine which specific localities in San Diego
Bay (a) have the highest lobster densities, (b) have the largest lobsters, and (c) act as corridors
for movement of lobsters between Bay and coastal locations.
2. Determine movement patterns of lobsters in San Diego Bay, and in particular rates of Bay-open
coast exchange using state-of-the-art acoustic tagging techniques and conventional tag-recapture
techniques.
6. Methods
Objective 1: Estimate the size of the spiny lobster population in San Diego Bay
Field methodology. To estimate population size, we conducted a tag-recapture study using
customized lobster traps (Figure 2) placed at 20 stations around SDB (Figure 3). Traps were 28 x
36 x 16 inches with 13 twist main funnel and an 11 twist funnel into the inside chamber, with a 1 x
1 inch mesh size and no escape ports. The small mesh size allowed us to capture lobsters as small
as 40 mm carapace length (CL). Traps were baited with one or two white sea bass heads donated
by Hubbs Sea World Research Institute. A single trap was deployed at each station. Traps were
initially deployed on 8 May 2009, and were checked approximately twice weekly for the presence
of lobsters until 20 July 2009 (= 18 trap pulls). Upon
retrieval, all lobsters found in traps were measured
(CL, in mm), sexed, and assessed for reproductive
condition and shell condition (see below). Each
newly-captured lobster then was tagged with a plastic
t-bar crustacean tag (Hallprint Tag Co., Hindmarsh
Valley, South Australia; Figure 4) rapidly inserted
into the dorsal musculature with a tagging gun. Tags
were individually numbered and labeled with contact
information so that if lobsters later were captured in
5
Figure 2. A custom lobster trap used in
the study.
Station 1 2 3 4 5 6 7 8 9 10 Latitude 32 40.315 32 40.016 32 39.631 32 40.368 32 40.693 32 41.092 32 40.950 32 41.860 32 42.453 32 42.258 Longitude 117 15.318 117 15.020 117 14.467 117 13.983 117 13.482 117 13.538 117 14.025 117 13.655 117 14.023 117 13.483 Activity A T*, A A T, A T, A T, A, D T, A, D
T T, A T, A, D Station 11 12 13 14 15 16 17
18 19 20 Latitude 32 43.123 32 43.430 32 42.798 32 43.434 32 43.528 32 42.900 32 42.123
32 42.130 32 41.665 32 41.200 Longitude 117 13.138 117 12.303 117 12.729 117 10.658 117 11.110 117 11.522 117 9.694 117 9.833 117 9.823 117 9.743 Activity T, A T, A T, A, D T, A T, A T, A, D T, A, D T, A, D T, A, D A Figure 3. Location for stations in San Diego Bay. RS = release site for acoustically
tagged spiny lobsters. Activity: A = acoustic receiver, T = trapping, D = dive surveys.
* denotes that trapping here took place on only 3 days before traps were stolen.
6
the commercial or recreational fishery, the identity and location of the lobster upon capture could be
provided. The tag numbers of any recaptured lobsters in traps were recorded. We also recorded the
number of individuals of any other species found within traps. All tagged lobsters were returned to
the water immediately after tagging.
The shell condition and reproductive condition for each lobster was assessed following the methods
of Mitchell (1969) and Goforth and U’Ren (1980). Females were recorded as being plastered (i.e. a
spermatophore was present), unplastered, or berried (= ovigerous). For plastered females, the color
(white, gray, or black, with darker colors signifying more time since mating) was noted, as was the
color of the eggs for berried females. For males and females, shell condition was classified as (i)
old hard shell, (ii) old soft shell, (iii) new hard shell, or (iv) new soft shell. This was assessed by
visually inspecting the carapace for the presence of fouling organisms (e.g. barnacles, algae) and
pressing on the carapace to qualitatively assess hardness.
Ad-hoc dive surveys were conducted using SCUBA during the course of the study. The purposes of
these surveys were to visit a select number of sites to (i) provide an additional relative measure of
lobster abundance among sites to compare with estimates from traps, and (ii) assess lobster use of
eelgrass habitat during the daytime. Surveys were conducted by two divers swimming multiple
side-by-side 30 m x 4 m transects within eelgrass habitat on which all lobsters encountered were
counted. We also surveyed submerged rocky habitat at selected sites that were previously known to
contain a high abundance of lobsters (i.e., the North Island fish structures at site 10, and in front of
the Marriot Hotel at site 19). The reef jacks adjacent to the submerged rocky habitat at site 19,
deployed by the Port of San Diego, also were surveyed for lobsters.
Data analysis. We estimated population size of SDB spiny lobsters using the Schnabel equation
(Schnabel 1938) and the modification to this equation derived by Schumacher and Eschmeyer
(Rounsefell and Everhart 1953, Odemar et al. 1975, Goforth and U’Ren 1980). Both methods were
used to estimate spiny lobster population size in San Diego Bay by Goforth and U’Ren in 1980,
with the Schumacher and Eschmeyer method generally providing slightly higher estimates of
population size. These two models accounts for multiple mark-recapture events, as used in our
study. For both models, as in all mark-recapture analyses, certain assumptions must be met for the
analysis to be valid. These assumptions include (1) marked animals are not subject to increased
7
Figure 4. A California spiny lobster tagged with a plastic
t-bar tag in summer 2009.
Figure 5. California spiny lobsters acoustically tagged
with Sonotronics CT-82-2-E transmitters.
8
mortality or emigration, (2) marks are not overlooked, and (3) marked and unmarked animals have
the same chance of being captured in traps, and (4) marked animals mix freely into the population
Assumptions (1) and (2) were met by virtue of the fact that lobster mortality likely is low in SDB,
particularly since our study took place before the fishing season, and because t-bar tag retention
generally is very high (ca. 95%) for spiny lobsters over short time spans (months) (GonzalezVicente et al. 2009). For assumption (3), we assumed that any trap avoidance by tagged-andreleased lobsters would become negligible within the course of the study. Whether the data meet
assumption (4) is best determined by studies on lobster movement in which the degree of movement
and the likelihood of lobsters moving among trapping locations can be estimated. We therefore
used results from our acoustic tagging study (see below) to identify distinct zones within SDB in
which we assumed that lobsters freely intermixed, and performed estimates of population size for
each zone. The total estimated population size for each zone was summed to yield an estimate for
SDB.
Objective 2: Determine movement patterns of lobsters in San Diego Bay, and in particular rates of
Bay-open coast lobster exchange
Field methodology. In part two of our study we used acoustic tagging and passive tracking to
determine patterns of lobster movement within SDB and at the mouth of SDB within the Point
Loma kelp forest. Concurrent with our trap-based tag-recapture study, we deployed 18 Sonotronics
SUR-2 automated acoustic receivers on the substratum at various locations in SDB and in adjacent
kelp forest habitat (Figure 3). Receivers are designed to detect signals from coded acoustic
transmitters which are placed on individual lobsters. In shallow water within SDB, transmitters can
be detected up to ca. 150-200 m from receivers, depending on the surrounding conditions. Each
transmitter emits a coded pulse signal that identifies the individual tagged organism. When a
lobster wearing a transmitter comes into range of a receiver, the signal is detected by the receiver
and the time and date of detection is stored in memory.
Receivers were deployed at all stations on 19 - 20 May, 2009. Each receiver was attached to a 31
kg poured concrete anchor using two meter length of rope and plastic-coated steel wire. A small
float was attached to the upper end of the rope, which allowed receivers to hang vertically
approximately 1.5 m above the bottom. Receivers were not marked at the water’s surface in order
to prevent theft and fouling within propellers. Beginning the following week, we attached acoustic
transmitters (Sonotronics model CT-82-2-E, 54 x 15.6mm, weight 9.5g, with a battery life of 14
9
mo; Figure 5) to the carapace of 79 lobsters that were captured in traps used in part 1 of the study.
Due to the size of acoustic transmitters, only lobsters greater than 65 mm CL were acoustically
tagged. Transmitters were prepared by inserting them into pieces of clear vinyl tubing (length = 4
cm) which were attached to plastic cable ties (a.k.a. “zip ties”). Aboard the boat, lobsters
designated for acoustic tagging were sexed and measured, and the cable tie then was wrapped
around the ventral side of the lobster and fastened tight. Super glue was used to provide a bond
between the carapace and the vinyl tube encasing the transmitter. Lobsters were held on the boat
for ca. 10 minutes as the glue dried, and then were gently returned to the water.
Acoustic receivers were retrieved using SCUBA every 2 – 3 months to download data to a handheld
field computer. Each receiver was brought to the surface, connected to the handheld computer to
transfer data, cleaned, and returned to the same location. This procedure took ca. 20 min for each
receiver. Receivers remained in the water until August 2010, for a total deployment time of ca. 14
months for each receiver.
Data analysis. Detections for each receiver were downloaded and organized using Microsoft Excel
and Matlab. To reduce the number of spurious detections, which may be attributed to extraneous
noise within SDB, we assumed detections for individual lobsters that occurred < 5 times on a given
day were false, and these were deleted from the data set. This was a reasonable threshold because
lobsters generally move slowly and would be expected to remain within range of a receiver for at
least several minutes. For each lobster we calculated the total number of stations at which the
lobster was detected, the proportion of detections at the station of release, the overall distance and
direction of movement, and whether the lobster exited or entered SDB.
7. Results
Objective 1: spiny lobster population size in San Diego Bay
We captured a total of 5,681 lobsters in our traps over the course of the study, of which 4,904 were
tagged with t-bar tags. The number and proportion of lobsters captured and recaptured at each
trapping station is shown in Table 1, and the size distribution of captured lobsters is shown in
Figure 6. The carapace length (CL) of lobsters in our traps ranged from 41 to 154 mm. The mean
CL for male lobsters was 73.6 mm (+ 10.6 mm SD) and for females was 71.8 mm (+ 11.1 mm SD),
and the mean CL for all lobsters combined was 72.3 mm (+ 11.0 mm SD). Only 15.6% of captured
lobsters were at or above the legal size of 3.25 inches CL. The vast majority of lobsters captured
10
Table 1. Number of female and male lobsters captured at each trapping station in San Diego Bay, and the percent of captures and recaptures that occurred at each station. Note that trapping at sites 2 and 12 took place for only ca. two weeks before traps were stolen. Site F M Total % captured % recaptured 2 8 9 17 0.3 0.0 4 260 186 446 7.8 0.6 5 188 487 675 11.9 7.3 6 340 678 1018 17.9 2.9 7 268 233 501 8.8 0.0 8 106 392 498 8.8 3.8 9 38 56 94 1.7 0.6 10 135 757 892 15.7 6.0 11 25 69 94 1.7 1.0 12 0 1 1 0.0 0.0 13 52 367 419 7.4 21.9 14 18 65 83 1.5 0.6 15 14 175 189 3.3 24.1 16 52 245 297 5.2 4.8 17 24 71 95 1.7 1.3 18 12 75 87 1.5 3.2 19 25 250 275 4.8 21.9 Total 1565 4116 5681 100.0 100.0 Table 2. Number of lobsters captured in each survey month, percent of the catch for each month composed of males vs. females, and the percent of captured females in each reproductive condition in 2009 trap surveys. May June July August September Total caught 2177 1847 947 605 105 Males (%) 95.2 79.9 34.6 35.0 27.6 Females (%) 4.8 20.1 65.4 65.0 72.4 Unplastered (%) Plastered (%) Berried (%) Female lobster reproductive condition 28.6 27.4 73.7 97.2 52.4 26.6 4.2 0.3 17.1 45.4 8.1 2.5 11
94.7 1.3 3.9 had hard shells; only 10 trapped
Size distribution of California spiny lobsters in San Diego Bay
lobsters were soft shelled. Sixty
percent of lobsters had new shells, and
1200
40 percent of lobsters had old shells.
1000
Overall, 72% of captured lobsters were
captured was particularly high in May
and June (Table 2), when very few
Frequency
males. The proportion of males
800
600
400
females were captured. The proportion
of captured lobsters that were females
200
increased from May to September.
Female reproductive condition changed
0
throughout the study period (Table 2).
Carapace length (mm)
In May, slightly over 50% of females
were plastered (i.e., had a
spermatophore), and 17% were berried
(i.e., carried eggs). This reversed in
Figure 6. Size distribution of captured spiny lobsters in San
Diego Bay from May – September 2009. Solid vertical line
indicates mean size (72.3 mm CL), and dotted vertical line
indicates minimum legal size (83 mm).
June, when only 26.6% of females were
plastered, and 45% were berried. Thereafter, the majority of females were unplastered, and many
were seen with degraded spermatophores on the carapace, indicating that they had recently released
eggs.
We divided SDB into five discrete zones to calculate population estimates. Zones were defined by
lobster movement data, and represented areas in which acoustic tagging data suggested that tagged
lobsters mixed thoroughly into the population (i.e., the assumptions of the mark-recapture procedure
were met). Population size estimates for each zone are shown in Table 3. Using the Schnabel
equation, our analysis indicates that at the time of our study, SDB housed approximately 94,093
lobsters. The estimate was somewhat higher (108,841) using the Schumacher-Eschmeyer equation.
This estimate does not include any lobsters smaller than 40 mm CL which would not be captured in
our traps. These estimates should be considered conservative since we could not sample all areas of
SDB.
12
Table 3. Population size estimates for California spiny lobsters within five zones identified in San Diego Bay. No lobster recaptures occurred in zone 1 outside the bay, where our trapping efforts were minimal. "S‐E equation" refers to the Schumacher‐Eschmeyer mark‐recapture equation. Numbers in parentheses are stations included in each zone. Zone 1 2 3 4 5 Description Outside SDB (1, 2, 3) Bay mouth (4, 5, 6, 7) Northern ecoregion (8, 9, 10, 11, 12, 13, 16) Northeast SDB (14, 15) Central ecoregion (17, 18, 19, 20) TOTAL Schnabel equation NA 71,644 20,613 543 1,293 94,093 S‐E equation NA 80,955 25,634 758 1,494 108,841 Dive surveys corroborated our estimates of population size for each zone (Figure 7). Sites with the
highest density of spiny lobsters were at the bay mouth (near sites 6 and 10), where lobsters were
relatively dense within both seagrass and rocky habitat. We found no lobsters near Ballast Point,
even though this area appears to be frequently used by lobsters (see objective 2 results below).
Interestingly, the seagrass bed at Ballast Point, which in previous years was found to house spiny
lobsters (Hovel, unpublished data) was not present during the August 2010 dive survey. Lobsters
also were not seen in the narrow seagrass beds adjacent to North Island near site 13, even though
these habitats appear to be a corridor for lobster movement (see objective 2 results below). It is
possible that fewer lobsters were seen here due to the lack of adjacent rocky habitat. Finally, we
found relatively dense populations of lobsters in seagrass near the North Island hangars (near site
16) and in rocky and seagrass habitat at site 19 (Marriott Hotel). On average approximately two
lobsters were seen in each reef jack deployed near the rocky habitat at site 19.
Objective 2: spiny lobster movement in SDB
Information on the 79 spiny lobsters tagged with acoustic transmitters is in Appendix 1. Forty two
acoustically tagged lobsters were males and 37 were females. The mean CL of acoustically tagged
lobsters was 85.7 mm (+ 10.8 mm SD) and CLs ranged from 66 mm to 127 mm. Acoustically
tagged female lobsters included unplastered, plastered, and berried individuals.
13
Lobsters generally were detected at a
Avg. lobster density (per 100 m2)
small number of receivers, with most
Lobster density
hits occurring at the station of release
(Figure 8). This trend varied with sex
but not with region of the bay. The
number of stations visited by females
was significantly greater than the
20
15
10
5
0
0
0
0
number visited by males (t-test: df =
77, t = 3.7, P < 0.001). On average,
female lobsters were detected at nearly
Site
twice the number of stations (3.11
stations + 2.04 SD), than were males
(1.66 station + 1.35 SD; Figure 8).
There was a trend for plastered females
to be detected at more stations than
Figure 7. Density of spiny lobsters in seagrass habitat
at selected sites around San Diego Bay. Error bars = 1
SE.
unplastered and berried females, but this was not significant (ANOVA: df = 2, 33, F = 1.4, P =
0.26) and was mostly due to two plastered females who were detected at a large number of stations.
Though some areas
of SDB appeared to have more movement than others, the proportion of stations visited by lobsters
in each zone of the Bay did not differ (ANOVA: df = 4, 73, F = 0.5, P = 0.6). Station use for a
subset of acoustically tagged lobsters is shown in Appendix 2.
Table 4 shows the number and proportion of transmitter signals received at each station and the
number of transits among stations is shown in Figure 9. The most transits among stations were
made by lobsters at the bay mouth, where lobsters frequently moved along Zuniga Jetty, as well as
across the bay from Zuniga jetty to Ballast Point and vice-versa. Other frequently traveled routes
also crossed SDB, for instance, between stations 11 and 13, and stations 17 and 18. There was no
evidence for any collective directionality of movement in acoustically tagged lobsters, whether
transits occurred along the bay or across the bay. Movement toward the bay mouth occurred with
approximately the same frequency as movement up-bay, and the same was true for crossings of bay
between the western and eastern shorelines.
14
Males ‐ no. stations visited
No. stations visited ‐ all lobsters
30
40
35
25
25
Frequency
Frequency
30
20
15
20
15
10
10
5
5
0
0
No. stations
No. stations
Females ‐ no. stations visited
Figure 8. Histogram for the number of
acoustic receiver stations visited by all
lobsters, male lobsters, and female
lobsters from May 2009 to August 2010.
Numbers on the x axis are not station IDs
but rather refer to the total number of
stations at which lobsters were detected.
The data indicate that most lobsters,
particularly males, were detected at only
one or two stations during their time at
large.
12
Frequency
10
8
6
4
2
0
No. stations
We found very little evidence that spiny lobsters enter or exit San Diego Bay. No acoustically
tagged lobsters that were tagged with transmitters inside the bay were detected outside the bay over
the course of the study, and only one acoustically tagged lobster tagged outside the bay was
detected inside SDB.
The capture of 37 t-bar tagged lobsters was reported to us from recreational and commercial fishers
in and outside of SDB. Of these, 33 included the tag number and approximate location of capture.
Seven of these lobsters were captured at the same station of release. Of the remaining 26, four had
made movements from stations close to the mouth of SDB to stations outside SDB at Pt. Loma, and
16 made movements from within the bay towards the bay mouth (but did not go outside the bay),
with most moving one or two stations. Three lobsters were reported captured adjacent to the
sewage treatment plant off of Pt. Loma.
15
8. Discussion
The last published study of spiny lobster population size in San Diego Bay was conducted by
Goforth and U’Ren (1980) who estimated that there were between 17,650 and 21,666 lobsters in
SDB (with 7,484 [42%] legal size or above), as compared to our estimates of 94,093 to 108,841
lobsters (with 14,679 [15.6%] legal size or above). Two factors likely account for the difference in
the estimates. First, they used standard lobster traps in their study, and suggested that their
population estimates did not include lobsters smaller than 65 mm CL. We used small mesh traps to
sample lobsters as small as 40 mm CL. Second, their study region corresponded closely with ours
but did not include sites near the Embarcadero and the Coronado Bay Bridge, nor (more
importantly) sites along the length of Zuniga Jetty, in which we found a large fraction of our
lobsters. Due to these differences and to the amount of time that has elapsed since their study, it is
impossible to determine whether the number of lobsters in SDB has increased, or whether the
discrepancy can be accounted for by methodological differences. We found some trends in our
data that were similar to Goforth and
Table 4. Number and proportion of detections by acoustic receivers at each station in San Diego Bay. U’Ren (1980), particularly the large
proportion of male lobsters caught in
traps and the timing of the reproductive
cycle for females. Sixty nine percent of
Station 1 2 3 4 5 6 7 9 10 11 12 13 14 15 16 17 18 19 20 TOTAL lobsters were male in Goforth and
U’Ren’s study, as compared with 72%
in our study. In both studies, males
dominated the catch in early summer,
and females became an increasing
proportion of the catch in mid to late
summer. It is unclear whether female
lobsters are less abundant than males in
early summer, or whether female
lobsters may be avoiding lobster traps in
early summer. However, given the lack
of evidence that lobsters move into SDB
from coastal areas, a plausible
explanation for low female lobster catch
16
# detections 95 5766 2666 10370 10556 23518 11946 23900 32574 18452 21949 21888 637 8741 47621 102687 2668 53121 6 399161 % detections 0.02 1.44 0.67 2.60 2.64 5.89 2.99 5.99 8.16 4.62 5.50 5.48 0.16 2.19 11.93 25.73 0.67 13.31 0.002 100 Figure 9. Total number of transits between all stations for acoustically tagged lobsters in
San Diego Bay. Thickness and color of line denote relative number of transits. Lines do
not necessarily indicate actual routes of travel, but rather the number of times lobsters
moved from one station and were next detected at another station.
in spring and early summer is that plastered and berried females (which outnumbered unplastered
lobsters in May and June) avoid lobster traps filled with male lobsters, or are not attracted to baited
traps.
In both our study and in Goforth and U’Ren (1980), the proportion of plastered females decreased
from May to July; however, berried females peaked in July in their study (47% of females
captured), whereas we found the highest proportion of berried females in June. One major
difference between our study and other censuses of California spiny lobsters is the size at which
females were berried. We found lobsters as small as 62 mm CL and as large as 113 mm CL to have
eggs (mean CL for berried females = 76.7 mm + 7.2 mm SD). In contrast, Goforth and U’Ren
(1980) reported no lobsters smaller than 70.8 mm CL or larger than 106.8 mm CL to have eggs.
17
Their minimum size corresponded to other studies of California spiny lobsters suggesting
approximately 70 mm CL as the minimum size for reproductive viability (Lindberg 1955, Odemar
1975).
Our data suggest that about 75% of the lobsters in SDB live at the bay mouth. This zone includes
much of Zuniga jetty to the east, and Ballast Point and the bayward shoreline of Pt. Loma to the
west. A major factor contributing to the high proportion of lobsters in this area is Zuniga Jetty,
which provides abundant shelter for lobsters. The jetty is a popular area for hoop netting. Lobsters
in this area appear to regularly cross the bay and may also be inhabiting seagrass beds that line the
margins of the bay mouth. Interestingly, the large seagrass bed adjacent to Ballast Point, in which
lobsters historically have been found, was not present in 2010 during our SCUBA surveys. Nearly
all lobsters that were not found at the bay mouth were found adjacent to the main channel in the
northern ecoregion of SDB (ca. 22.5% of the population), between the base of Zuniga Jetty and the
aircraft carriers along North Island, and across the bay to Shelter Island. Most movement in this
zone occurred along North Island, though lobsters also regularly crossed the bay in this zone as
well. Our SCUBA surveys suggested that many lobsters are found within the seagrass beds that
fringe North Island, particularly those that are adjacent to submerged rocky structures (for example,
the fish structures at site 10, directly across the bay from Shelter Island).
Fewer lobsters were found to live in the central bay zone, between the aircraft carriers to the north
and the Coronado Bay Bridge to the south, and the Embarcadero to the east. A substantial number
of the lobsters in this zone appear to reside within the submerged rocky structure and fish structures
fringing the seagrass beds along the western shoreline of the city of Coronado (particularly in front
of the Marriott Hotel just to the north of the Coronado Bay Bridge). Due to logistical and safety
considerations, we could not place traps along the eastern side of SDB near Tuna Harbor and the
Cruise Ship Terminal, and we therefore likely undersampled this area. Our movement data also
identified the northeast corner of SDB, adjacent to the Coast Guard station and Maritime Museum,
as a discrete zone in which lobsters reside within the seagrass bed and fish structures at site 15.
This area appears to house a relatively small, but consistent population of spiny lobsters.
Regarding patterns of movement, the major conclusions from study are that (i) lobsters generally
remained close to the station at which they were tagged and released, and (ii) very few lobsters
18
entered or exited SDB. The majority of acoustically tagged lobsters (72%) were detected at only
one or two stations. However, female lobsters were detected at nearly twice the number of stations,
on average, than males. Goforth and U’Ren (1980) also found evidence from their trapping study
that females moved farther than males in SDB. They suggested that transitions from the plastered
to the berried state may induce females to move over longer distances. In an acoustic tracking
study in the Pt. Loma kelp forest, female lobsters were more likely to switch shelters and undergo
nomadic behaviors than were males (Hovel and Lowe 2007).
Though it is commonly believed that California spiny lobsters make seasonal inshore-offshore
migrations, there is little experimental evidence to support this. We found no directionality of
movement in our study; lobsters were just as likely to move seaward as bayward, or across the bay.
Long-term acoustic tagging studies conducted in the La Jolla Ecological Reserve also provided no
evidence for a seasonal trend in lobster movement (Withy-Allen 2010). Actively tracked lobsters in
Withy-Allen’s (2010) study exhibited small home ranges, and moved an average of only 50 – 100 m
per night in generally circular paths. Stull (1991) tracked California spiny lobsters along the coast
of Santa Catalina Island, CA, and found small home ranges and limited movement for lobsters after
an initial tendency to move long distances following the tagging procedure. In contrast, Hovel and
Lowe (unpublished data) found that tagged lobsters in the Pt. Loma kelp forest moved an average of
600 m per night. One factor that may in-part dictate the scale of lobster movement is habitat
availability. In SDB and La Jolla, for instance, the nearshore habitat includes seagrass and many
numerous rocky crevices that provide shelter, whereas the floor of the Pt. Loma kelp forest
generally is flat, with crevices that are farther apart.
We found that most lobster movement between stations occurred near the mouth of SDB. Lobsters
appeared to readily move along Zuniga Jetty and across the bay between Ballast Point and Zuniga
Jetty. We found similar numbers of transits by lobsters to the north vs. to the south along Zuniga
Jetty, and to the east vs. to west across the channel. Lobsters also tended to move along the main
axis of the bay near the shoreline of North Island, across the bay between North Island and Shelter
Island, and across the bay between Coronado and the Embarcadero area. The ship channel does not
appear to be a barrier of movement to lobsters in SDB.
19
Our data suggest that there is relatively little exchange between the SDB lobster population and that
of the open coast, at least during the time of year at which our study took place. Many lobsters in
SDB therefore likely are residents that remain at the bay mouth or in rocky and seagrass habitat that
exists along the shores of North Island and Coronado. This suggests that relatively few lobsters in
SDB exit the bay to contribute to the commercially fished population within the Pt. Loma kelp
forest. Our data did not provide many clues for how lobsters enter the bay. Lobsters may walk into
the bay as juveniles or adults, or may settle in SDB habitat (e.g. eelgrass) as pueruli (postlarvae).
However, we have observed no pueruli in eelgrass samples taken for numerous other research
projects in SDB. The primary settlement habitat for California spiny lobsters likely is surfgrass
along the open coast (Engle 1979) and most lobsters may walk into SDB as juveniles after
settlement in nearby surfgrass habitat. Rates of lobster natural mortality likely are low in SDB due
to a paucity of large predators and an abundance of food within seagrass and rocky habitat; thus,
once a lobster enters the bay, it is likely to survive unless it is captured in the recreational fishery.
Relatively low rates of movement into SDB, combined with very low rates of exit and mortality,
could account for a substantial population of lobsters within the bay.
9. Literature cited
Cheng, B.S. and K.A. Hovel. 2010. Biotic resistance to invasion along an estuarine gradient.
Oecologia 164: 1049-1059.
Engle, J.M. 1979. Ecology and growth of juvenile California spiny lobster, Panulirus interruptus
(Randall). PhD dissertation, University of Southern California. 273 pp.
Gonzalez-Vicente, L., D. Diaz, S. Mallol, and R. Goni. 2009. Double tagging experiments and tag
loss in Palinurus elephas. The Lobster Newsletter 22: 6-9.
Goforth, H., and S. U'Ren. 1980. A survey of the spiny lobster (Panulirus interruptus) population in
San Diego Bay, California. Technical report 542, Naval Ocean Systems Center, San Diego, CA.
Lafferty, K.D. 2004. Fishing for lobsters indirectly increases epidemics in sea urchins. Ecological
Applications 14: 1566-1573.
Lindberg, R.G. 1955. Growth, population dynamics, and field behavior in the spiny lobster,
Panulirus interruptus (Randall). Univ. California Publications in Zoology 59: 157-247.
Mitchell, C.T., C.H. Turner, and A.R. Strachan. 1969. Observations on the biology and behavior of
the California spiny lobster, Panulirus interruptus (Randall). California Dept. of Fish and Game
55(2): 121-131.
20
Odemar, M.W., R.R. Bell, C.W. Haugen, and R.A. Hardy. 1975. Report on California spiny lobster,
Panulirus interruptus (Randall) research with recommendations for management. Presented to
the California Fish and Game Commission, July 25, 1975, Monterey, California.
Robles, C., D. Sweetnam, and J. Eminike. 1990. Lobster predation on mussels: shore-level
differences in prey vulnerability and predator preference. Ecology 71:1564-1577.
Rounsefell, G.A. and W.H. Everhart. 1953. Fishery science. John Wiley and Sons, New York, 444
pp.
Schnabel, Z.E. 1938. The estimation of total fish population of a lake. The American mathematical
monthly 45: 348-352.
State of California. Department of Fish and Game. 2001. Appendix 15. Draft Supplement to the
draft Environmental Documentation Re: Ocean Sport Fishing Regulations concerning Spiny
Lobster (Sections 27.00-30.10, Title 14, California Code of Regulations).
Stull, A.T. 1991. Nightly foraging movements and den fidelity of the California spiny lobster
(Panulirus interruptus) at Santa Catalina Island, California. CSU Long Beach MS thesis, 86 pp.
Tegner, M., and P. K. Dayton. 1981. Population structure, recruitment and mortality of two sea
urchins (Strongylocentrotus franciscanus and S. purpuratus) in a kelp forest. Marine Ecology
Progress Series 5:255-268.
Tegner, M., and L. Levin. 1983. Spiny lobsters and sea urchins: analysis of a predator-prey
interaction. Journal of Experimental Marine Biology & Ecology 73: 125-150.
Tegner, M., and P. K. Dayton. 2000. Ecosystem effects of fishing in kelp forest communities. ICES
Journal of Marine Science 57:579-589.
Withy-Allen, K.W. 2010. California spiny lobster (Panulirus interruptus) movement behavior and
habitat use: implications for the effectiveness of marine protected areas. SDSU MS thesis, 45 pp.
21
Appendix 1. Information on 79 California spiny lobsters affixed with acoustic transmitters in San
Diego Bay in summer 2009. Site = site of release; date = date of release; repro. = reproductive
condition for females (UN = unplastered, PL = plastered (possessing a spermatophore), and B =
berried (carrying eggs)). Shell = shell condition (NH = new hard shell, OH = old hard shell).
Site Tag # Date CL Sex 2 2 2 2 2 2 2 2 2 2 2 2 2 2 4 4 4 4 5 5 5 5 6 6 6 6 6 7 7 7 7 7 7 7 9 11 11 8 10 11 19 45 55 58 80 22 37 70 71 84 96 38 7 72 94 14 115 66 68 13 26 110 95 120 59 105 111 43 52 98 118 64 28 79 06/22/2009 06/22/2009 06/22/2009 06/22/2009 06/11/2009 06/11/2009 06/11/2009 06/11/2009 06/22/2009 06/22/2009 06/22/2009 06/22/2009 06/22/2009 06/22/2009 05/29/2009 06/02/2009 05/29/2009 06/30/2009 05/29/2009 05/29/2009 06/02/2009 06/02/2009 05/29/2009 05/29/2009 05/29/2009 05/29/2009 05/29/2009 05/29/2009 06/02/2009 05/29/2009 06/02/2009 05/29/2009 06/02/2009 06/02/2009 06/30/2009 06/16/2009 06/16/2009 82 82 81 76 70 77 85 84 82 82 77 83 80 70 91 77 83 80 91 81 66 80 82 83 87 81 83 87 91 82 100 77 73 71 114 90 82 M M M M M M M M F F F F F F M F F F M M F F M M M F F M M M F F F F F F F Repro. PL UN UN PL UN UN PL B UN PL B B B B PL B UN B B PL 22
Shell Last heard Days at large NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH OH OH NH NH NH OH OH NH NH OH OH OH NH OH NH NH OH NH NH 06/06/2010 06/13/2010 05/27/2010 05/19/2010 06/07/2010 06/19/2009 05/28/2010 04/11/2010 05/30/2010 05/23/2010 06/05/2010 06/18/2010 05/29/2010 04/15/2010 08/30/2009 05/18/2010 05/17/2010 08/23/2009 04/03/2010 09/20/2009 05/27/2010 06/03/2010 06/02/2010 05/19/2010 05/09/2010 06/07/2010 05/09/2010 04/28/2010 04/14/2010 06/07/2010 05/19/2010 05/26/2010 06/03/2010 06/08/2010 05/02/2010 05/01/2010 06/03/2010 369 362 339 348 356 21 340 313 359 359 372 367 347 289 89 350 349 62 285 114 356 370 351 351 321 374 313 334 296 370 355 358 370 371 334 337 356 Site 13 13 13 13 13 13 13 13 14 15 15 15 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 Tag # 4 51 104 117 49 69 75 103 99 90 27 87 6 29 41 42 81 116 60 82 100 109 36 65 73 88 89 12 30 53 74 9 21 50 54 56 67 97 101 112 20 57 Date 06/12/2009 05/29/2009 05/29/2009 05/29/2009 06/05/2009 06/02/2009 05/29/2009 05/29/2009 05/29/2009 05/29/2009 06/05/2009 05/29/2009 06/02/2009 05/29/2009 05/29/2009 06/02/2009 05/29/2009 05/29/2009 06/16/2009 06/02/2009 06/02/2009 06/02/2009 05/29/2009 06/02/2009 05/29/2009 06/05/2009 06/05/2009 05/29/2009 05/29/2009 05/29/2009 06/05/2009 05/29/2009 06/02/2009 06/02/2009 05/29/2009 06/09/2009 05/29/2009 05/29/2009 06/23/2009 06/30/2009 06/16/2009 06/16/2009 CL 76 94 76 103 80 77 70 83 87 93 73 68 87 105 94 90 95 84 84 94 71 85 94 100 91 87 93 94 93 82 72 95 97 90 96 92 102 97 86 127 101 69 Sex M M M M F F F F F M F F M M M M M M F F F F M M M M M M F F F M M M M M M M M M F F Repro. PL PL UN PL PL B UN PL PL PL PL PL PL PL B UN 23
Shell OH NH NH OH OH NH NH NH NH NH NH NH NH NH NH OH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH NH OH NH NH NH OH NH NH NH NH Last heard 02/27/2010 05/25/2010 05/20/2010 02/21/2010 05/12/2010 05/03/2010 04/25/2010 06/06/2010 06/06/2010 06/07/2010 03/17/2010 11/28/2009 06/03/2010 05/31/2010 06/06/2010 04/28/2010 06/02/2010 06/07/2010 05/05/2010 09/21/2009 06/03/2010 04/16/2010 05/25/2010 10/03/2009 06/11/2010 05/27/2010 06/17/2010 05/22/2010 04/28/2010 05/23/2010 04/29/2010 06/16/2010 04/30/2010 02/17/2010 06/05/2010 03/05/2010 06/05/2010 06/12/2010 05/25/2010 05/05/2010 05/19/2010 06/02/2010 Days at large 270 337 356 244 324 339 331 373 349 370 292 183 370 367 373 330 356 370 341 115 357 311 348 127 374 363 384 345 334 352 328 383 336 239 372 276 372 379 329 341 355 369 Appendix 2. Movement patterns for several acoustically tagged spiny lobsters in San Diego Bay. For
each individual lobster, the top portion shows a summary of stations at which the lobster was
detected (for at least five consecutive signals, to reduce false detections), and the bottom portion
shows the time series for the duration of the life of the transmitter (or until the last time the
transmitter was detected).
24
Movement pattern for lobster 19, captured and tagged at station 2 in June of 2009.
This lobster was detected at only two stations, a pattern which was typical of lobsters
captured outside San Diego Bay.
Movement pattern for lobster 7, captured and tagged at station 4 in June 2009. This
lobster frequently moved among stations at the bay mouth, which was typical of
lobsters captured in this area.
Movement pattern for lobster 79, captured and tagged at station 11 in June 2009. This
lobster remained near station 11 for 2 months and then moved toward the bay mouth,
eventually moving back to station 11. The corridor for movement was the shoreline of
North Island.
Movement pattern for lobster 60, captured and tagged at station 16 in June 2009. This
lobster moved north to stations 14 and 15, and then was next detected at station 11 at
Shelter Island before moving across the bay to North Island. This lobster crossed the
bayy several times and made a ggeneral movement toward the bayy mouth.
Movement pattern for lobster 50, captured and tagged at station 19 in June 2009. This
lobster moved across the bay to station 18 and then across again to station 16, only to
cross the bay again to station 11 and eventually move to the bay mouth.
Movement pattern for lobster 100, captured and tagged at station 16 in June 2009.
This lobster moved along the shoreline of North Island to the south and north, and
then was found at the bay mouth. This lobster then returned to the northern ecoregion.