Animal Sciences Group Aquaculture and Fisheries Group De Elst 1 6708 WD Wageningen The Netherlands Tel: +31 (0) 317 483307 Fax: +31 (0) 317 483962 Name: Imke van Gerwen Reg.nr. 880410259040 MSc Thesis nr. T 1914 THESIS December 2013 AQUACULTURE AND FISHERIES GROUP LEERSTOELGROEP AQUACULTUUR EN VISSERIJ The effects of trap fisheries on the populations of Caribbean spiny lobster and reef fish species at the Saba Bank
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Animal Sciences Group
Aquaculture and Fisheries Group De Elst 1 6708 WD Wageningen
and the long larval stage of P. argus the distance between the
spawning and settlement area of larvae has been estimated to reach up to 400 km (Butler et al.,
2011). The dispersal model also showed a maximum dispersal of approximately 1,000 km. This
indicates that a part of the larvae (~60%) settle close to the adult population; the so called “self –
recruitment” (Butler et al., 2011). The other part (~20%) contributes to long distance dispersal of P.
argus (Butler et al., 2011).
Fig. 4. Phyllosomata of P. argus.(1.6-27mm) (UNC)
9
Post- larval stage (puerulus)
When the planktonic larvae undergo
their last metamorphosis they
transform in a shape that is more
recognisable as an adult spiny lobster
(fig. 5). This stage is called “puerulus”
which means “little boy” with a
carapace length of 5-9 mm (Cox et al.,
2008). The actual metamorphosis
occurs far offshore close to the
continental shelf (Phillips & Williams,
2008). This stage is called puerulus and
only lasts three to four weeks. In this
stage the puerulus does not feed (Cox
et al 2008) and swims towards the shore, mostly on the surface at night during new moon flood tides
to decrease predation pressure (Acosta et al., 1997; Acosta & Butler, 1999). Upon arrival, the pueruli
settle in structurally complex hard-bottom habitats that have abundant, preferably red, macroalgae
vegetation (Marx and Herrnkind, 1985; Herrnkind & Butler, 1986; Field and Butler, 1994). The
physical and chemical cues that the pueruli respond to for the migration into nursery areas are
poorly understood. It is suggested that the metabolites of red algae in nurseries attract the pueruli
and enhance their settlement into coastal habitats (Goldstein & Butler, 2009).
2.1.5 Juvenile stage (10 - ± 80 mm CL)
Approximately 15 days (Goldstein et al., 2008)
after the settlement in the nurseries, the
transparent pueruli of P. argus get more
pigmented and their flattened body shape
transforms into more cylindrical shaped juvenile
stage (fig. 6). Juveniles are solitary living and their
camouflaged bodies blend well in the algal
substrate of the nursery habitat where they stay a
couple of months until they reach 17 mm CL
(Herrnkind & Butler, 1986, Marx & Herrnkind,
1985). Although the juveniles in the nurseries are
sheltered from predators and food is abundant
(Marx & Herrnkind, 1985) mark and recapture
studies show that in the first couple of months only 2-4 % of the settled lobsters survive (Butler et al.,
1997).The mortality rate of the juveniles is negatively related to the number of shelter crevices in the
nursery habitat (Butler et al., 2001). When the juveniles reach a carapace length of 15 mm, they
migrate from the algal vegetation towards the crevices that are mainly provided by sponges where
they reside during daytime (Forcucci et al., 1994, Butler et al., 1995, Herrnkind et al., 1997). During
nighttime these juveniles leave their shelters, where the distance they cover increases with
Fig. 5. Transparent P.argus puerulus. (M. Butler et al., 2010). Size: 5-9 mm CL
Fig. 6. Juvenile of P. argus 12 mm CL (I. van Gerwen 2012)
10
maturation. After one or two years the juveniles leave the nurseries and move towards deeper lying
areas further offshore (Butler et al., 2011).
2.1.6 Adult stage
When the lobsters reach their adult
stage (fig. 7) in 2-3 years (Maxwell et
al., 2013) they change from solitary to
social animals. They more often
aggregate in shelters and with
increasing length natural mortality
declines (Eggleston et al., 1990; Smith
& Herrnkind, 1992), because they have
more effective anti-predator responses
(Briones-Fourzán, 2006). Also the
larger the lobsters grow the less
predators can physically prey upon
them, because the lobsters then do not
fit in their mouths (Nilsson &
Bronmark, 2000). On the other hand
fishing mortality increases with body size (Phillips & Kittaka, 2001). The growth rate of the lobsters
decreases with carapace length (Erhardt, 2008). It is estimated that the lobsters can reach 20 years of
age (Maxwell et al., 2007).
Reproduction
When female and male P. argus reach their adult phase they look for a suitable mating partner. In
pristine ecosystems, the mating system of the Caribbean spiny lobster resembles a lek system, where
one big male has a harem of females and defends his territory (George, 2005). During courting the
males caresses the body of the females with its long legs which enables them to assess the size of the
females (George, 2005). Then the male places a sperm package on the abdomen of the female, which
is called a “tar spot” (George, 2005). The size of the spermatophoric mass (tar spot) that a male
deposits, depends both on the size of the male as well as the size of the female (MacDiarmid &
Butler, 1999). The female produces rows of eggs and keeps them on the ventral side of the abdomen.
She then scratches the sperm package open with her hind legs and transfers the sperm to the eggs.
After the eggs have been fertilized they are retained until they are all ready to hatch. The eggs are all
simultaneously released upon hatching and a cloud of phyllosoma larvae is released into the water
column. Spawning peaks in the spring from March to June (Bertelsen & Mathews, 2001). In areas
closer to the equator it has been found that part of the adult population spawns throughout the year
(Butler et al., 2009). An overview of size at maturity, fecundity and spawning season is found in table
1.)
Fig. 7. An adult male P.argus finds shelter in a crevice (>80 mm CL). (I. van Gerwen
2012)
11
Reproductive characteristic
Reference
Size at maturity –
CL50%
- Female: 86.0±5.1 SD mm CL;
Male: 97.4±5.0 SD mm CL
(Bermuda, UK)
- Female: 81 mm CL (Cuba)
- Female: 93 mm CL (Turks &
Caicos Islands)
- Female: 92 mm CL (Colombia)
- Evans et al. 1995
- Evans et al. 1995
-
- Cruz & Léon, 1991
- Medley & Ninnes, 1997
- Gallo et al., 1998
Fecundity – Clutch size - 0.3 - 0.8 million eggs/clutch
(Florida Keys, FL, USA)
- 147,00 – 1,952,000 eggs/clutch
- Bertelsen & Matthews
2001
- Cox et al. 1997
Spawning season - March – June (Florida, US)
- March-May (Cuba)
- Throughout the year
- Bertelsen & Matthews
2001
- Cruz & Léon, 1991
- Butler et al., 2009
Table 1. Overview of size at maturity, fecundity and spawning season of P.argus in the Caribbean
12
2.2 Caribbean P. argus fisheries and management
2.2.1 Caribbean Fisheries
The P. argus fishery is one of the commercially most important fisheries in the Caribbean.
Throughout the Caribbean P. argus is fished upon with several fishing techniques, ranging from
catching the lobsters by hand by SCUBA divers to hundreds of baited traps hauled by boats that are
equipped with state–of the art devices, such as fish finders, GPS, depth sounder etc. Over 456 million
US dollars are generated with the fisheries on P. argus annually (Ehrhardt, 2005; CRFM, 2011).
Approximately 50,000 lobster fishers are active in the industry, and another 200,000 people that are
working in the lobster fisheries related jobs (FAO, 2003). Brazil, the Bahamas and Cuba are the three
top countries landing most lobsters annually (fig. 8).
The lobsters are sold alive, as a whole, or only the abdomen is frozen or canned (FAO, 2013). With
the industrialization of the fisheries the total landings of the lobsters increased steeply in the
Caribbean from 2,957t in 1950 to 42,519 t in 1996 but then levelled off, and in the past 5-10 years
the landings show a declining trend (FAO, 2013) (fig. 1). This decline has been consistent throughout
the Caribbean and because of the economic importance of this species, it has been stressed that
monitoring and enforcement of regulation is necessary to protect the stocks from overexploitation or
even collapse.
Many P. argus stocks (40%) throughout the Caribbean are considered to be overexploited, 7 out of
18 countries (CRFM, 2011). Therefore, countries have formed management plans and rules and
regulations are enforced locally (appendix A). One common regulatory measure is the closing of the
fishing area for a predetermined amount of months, often coinciding with spawning season. Other
Bahamas 24%
Brazil 21%
Cuba 19%
Nicaragua 12% U.S.A
6% Dominican Republic 4%
Honduras 3%
Mexico 2%
Haiti 2%
Venezuela 2%
Belize 2% Jamaica
1% Colombia 1%
Turks and Caicos Islands
1%
Puerto Rico 0%
Saba 0%
Fig. 8. Top 15 countries harvesting P. argus, as measured by average annual landings from 2000-2007 inclusive and Saba, and the
percentage of the total average landings (34, 664 t) by all countries over the same period of time (adapted from CRFM, 2011)
13
measures are the size limits and restrictions on the landing of moulting or berried females (carrying
eggs). The previously mentioned measures are also in place in the management of the P. argus stock
on the Saba Bank, an area that has not been covered in the CRFM review. However, due to the
aforementioned connectivity of the Caribbean spiny lobster population, management of the stocks is
an international problem. This makes the effectiveness of the management measures difficult to
quantify.
2.2.2 Population monitoring
With artificial collectors (fig. 9), the supply of post-
larvae of several spiny lobster species, have been
monitored (Butler et al. 2010; Gonzalez & Wehrtmann,
2011). By monitoring the recruitment over a longer
period of time (several years) studies have shown that
it is possible to predict stock size and/or catch of the
adult population a couple years later (Hancock, 1981;
Phillips, 1986; Lozano et al.; Cruz et al., 1993). To
predict the stock size of P. argus on the Saba Bank a
recruitment monitoring program was set up at the
coast of the Island Saba, a protocol for the monitoring
is provided in Appendix B.
Fig. 9. Recruitment collector deployed near Saba (I. van
Gerwen, 2012) (60x40x40 cm)
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2.3 Biodiversity on the Saba Bank
Recently assessments on biodiversity of marine organisms have been performed on the Saba Bank
(Hoetjes & Carpenter, 2010). The Saba Bank is a 2,200 km2 submerged plateau. At the fringes of the
plateau, coral growth is more pronounced. Together with other rocky substrate it forms a habitat
that provides food and shelter for a great variety of marine organisms, including fish and
crustaceans. On this “fore-reef”, the habitat complexity is high, resulting in highest fish biodiversity
(Toller et al., 2010). Further towards the middle of the Bank, the habitat complexity decreases and
with it the biodiversity. In the more shallow parts of the Bank the Caribbean Spiny lobster finds its
habitat. Also large predators are found on the bank, for example nurse and reef sharks, groupers and
barracudas (Williams et al., 2010). During the winter months marine mammals like baleen whales
and several dolphin species are spotted at the edges of the Bank (Lundvall, 2008).
It is suggested that the lack of nursery areas, i.e. sea grass beds or mangroves, and the isolated
location are limiting factors to the fish biodiversity on the Saba Bank (Toller et al., 2010).
Nevertheless, 270 fish species have been recorded on the bank, which is an intermediate number
compared to other studies on reef fish biodiversity (123-517 fish species) (Williams et al., 2010). Also,
45 sponge species, 48 Gorgonian octocoral species and 320 macroalgal species have been
documented (Littler et al., 2010; Williams et al., 2010; Etnoyer et al., 2010; Thacker et al., 2010).
Because the species count did not reach its asymptotic phase, it is expected that the number of these
species on the Saba Bank will even be higher.
Because of this species richness the International Maritime Organization designated the Saba Bank as
the world’s 13th Particular Sensitive Sea Area in 2012. When, in 2010, Saba became part of the Dutch
territory the Saba Bank received the status of an official National Marine Park.
Trap fisheries in the Caribbean have been found to cause over-fishing, biodiversity loss and alteration
of ecosystem structure (Hawkins et al., 2007). Although lobster fisheries mainly target lobsters, with
the traps also shallow water reef fish are caught (Dilrosun, 2000; Toller & Lundvall 2008). Lobster
fisheries have been suggested to affect the biomass distribution of reef fish on the Bank (Toller et al.,
2010). Fish abundance and assemblage on different parts of the Saba Bank was compared based on
reef structure. Biomass and density of reef fish was lower on the fore reef than on reef flat habitat
(Toller et al., 2010). This was not expected because the fore reef was of higher habitat complexity
than the reef flat habitats. Species richness on the Saba Bank, on the other hand, was highest in fore
reef areas (Toller et al., 2010). Fish abundance and diversity has found to be positively correlated
with habitat complexity (Dominici-Arosemana & Wolff, 2005). Other factors that have been found to
affect or have the potential to affect the biodiversity and abundance of organisms on the Saba Bank
are coral bleaching, invasive species i.e. Lion fish; oil spills from neighbouring islands; the loss of traps
due to hurricanes or strong tropical storms; and anchoring of large ships on the Saba Bank (Lundvall,
2008).
15
3. Material and Methods
3.1 Study site
The Saba bank (17025' N, 63030' W) is one of the largest submerged atolls in the world with an
estimated surface area of more than 2,200 km2. It is located 3 – 5 km southwest of the island of Saba
and 25 km West of St. Eustatius (Hoetjes & Carpenter, 2010) (fig. 10). The average depth of the Bank
is 25m, but the shallowest parts in the north and northeast are only 11-13 m deep, from where the
bottom slowly slopes down towards the west. The edges of the bank are quite steep, dropping in
some areas from 11-20m to >500m depth. The term atoll is given to the Saba Bank because of the
presence of corals on the Banks edges creating a reef structure that forms a circle around a lagoon
zone where corals are virtually absent (Toller et al., 2010). The Saba Bank is likely to have a volcanic
core.
3.2 Study design
To determine the status of the P. argus stock on the Saba Bank, catches were monitored over a
fieldwork period of five months (July-November 2012). Basic fisheries catch and effort data was
obtained through standardized short interviews. The catch and effort data was compared to the
assessments of 1999 and 2007. Also, biological data was obtained through sampling of both landed
and discarded catches. This data was collected to estimate the size at maturity and length frequency
of P. argus on the Saba Bank as well as to determine fish species composition and length frequency
Fig. 10. Location of Saba Bank in the Caribbean. (Hoetjes & Carpenter 2010) doi:10.1371/journal.pone.0010769.g001
16
distribution. Monitoring landings alone creates a skewed view on fisheries effects, because in case of
the Saba Bank fishery, a size limit is set for the lobsters landed at the harbour. This will result in a
biased estimate of the selectivity of the traps. Therefore, we decided to quantify discards because
this could give us valuable information on the effect of fisheries on both P. argus and reef fish
populations. With the results of the on-board trips we wanted to validate the importance of discard
monitoring, because the measuring of discards is labour intensive (you have to go on-board, this
takes a day per trip).
The research consisted of both port sampling and on-board sampling of both P. argus and mixed fish
species using the following procedures:
1. Fishing trip log (every day) to collect effort data.
2. Short interviews (66 % of fishing trips) to collect catch and effort data.
3. Long interviews (17.2 % of lobster trips) to collect biological data
4. On-board sampling (9 trips in total) to collect biological data
Lobster
Port sampling On board sampling
Carapace length (CL)
Sex
Tar spot
Merus length (of some male lobsters)
Carapace length (CL)
Sex
Tar spot
Berried
Moulting
Merus length (all discarded male lobsters)
Mixed Fish
Of 27 landed catches and all the discarded catches (9) species composition was recorded. Fish was
identified on the species level. Length data was measured to the nearest cm.
3.2.1 Fishing trip log
In order to obtain a good indication on the fishing intensity and effort on the Saba Bank, every fishing
trip was logged (fig. 11). This meant that every morning the presence or absence of the fishing boats
was monitored. Sometimes fishermen used their boats for purposes other than fishing (i.e. buying
bait or visiting neighbouring Islands). These trips were not logged. On rare occasions fishermen hired
boats (and crew) from other fishermen to haul their traps, because their own boat was out of the
water for repair. In this case the boat that belonged to the owner of the traps was logged instead of
the boat that was hired, this way the trip frequency per boat can be estimated more accurately. An
Table 2. Biological data obtained with “long interviews” at the harbor and with on board sampling
17
average number of trips per day,
for the five months the trips
were logged, was calculated and
extrapolated to average fishing
trips per year, so this could be
compared with previous
assessments. With the short
interviews the percentage of
fishing types (lobster, redfish,
long lining etc.) of the total trips
has been calculated. With the
estimated total fishing trips per
year and the percentage that is
accounted per fishing type the
number of lobster fishing trips
has been calculated.
A total of 377 trips were logged in July-November 2012. Data obtained:
- Number of fishing trips per day
3.2.2 Short interview
The duration of short interviews was approximately 5 min in which fishermen were asked for data
about their trip to obtain basic effort and catch data which we then compared to data from 1999 and
2007. Data included (appendix C):
- number of traps hauled during the trip: effort
- soaking time in days: how long the traps were in the water: effort
- fishing area (quadrant) (fig. 12)
- number of traps lost
- How many lobsters were caught: catch
- if they returned (discarded) any berried females: biological data
- if they returned (discarded) any undersized lobsters: biological data
- how much fish was caught (in lbs. later converted to kg): catch
Additional information for other research
- If they had seen whales or dolphins (not used in this thesis)
- If they discarded lionfish
Fig.11. Typical fishing boat (I. van Gerwen 2012)
18
3.2.3 Long interview
Long interviews were a combination of the short interviews and measuring sessions. First the short
interview was done to obtain basic CPUE data. Then the long measuring sessions were conducted to
obtain biological data to estimate length frequency, size at maturity, reproductive season, species
composition and sex ratio (table 2). Lobster was measured to the nearest mm carapace length, fish
was measured to the nearest cm fork length or total length. Approximately once a week the catch of
each boat was measured during a long interview. In the July-November 2012 a total 44 “long
interviews” were done (incl. 27 fish species composition, 31 lobster (length-frequency/sex/tar spot)
and 13 fish length-frequency data)
3.2.4 On board sampling trips
The biological data obtained with long interviews of the landed catch during port sampling was
expected to be skewed, because legislation prohibits the landing of P. argus with a carapace length
smaller than 95 mm, females that have eggs and individuals that are moulting (ecdysis). Therefore
nine on-board sampling trips were conducted in which the carapace length and sex of discarded
lobsters was determined. Also, females were examined for the presence of a tar spot and/or eggs,
and the merus length of discarded males was measured.
Before the start of the trip the fishermen were asked to divide the catch into discarded and landed
catch during the trip. Because of time constraints, most of the time only the discarded catch was
measured.
Fig. 12. Map of the Saba Bank divided in quadrats.
19
Because mixed fishes attributed a considerable part of the catch, species composition and length
frequency data was also obtained in all on-board sampling (except for one trip) trips and during port
sampling with long interviews.
Biological data collection
Lobster
3.2.5 Carapace length (CL)
The carapace length (CL) of P.argus was measured with a calliper in mm. The outside jaw of the
calliper is placed between the eyes and the inside jaw is placed where carapace ends and the tail
begins (fig. 13a+b).
3.2.6 Sex
The sex of the lobsters can be determined by the hind legs. The ends of the last hind legs of males
Fig. 13b Measuring the carapace length of a male P. argus with a caliper (I. van Gerwen
2012).
Fig. 13a Drawing of carapace length (CL) in detail
(CFR).
Fig. 14. Morphological difference between sexes in P. argus in last walking legs. Left = male (blue),
right = female (pink) (I. van Gerwen, 2012)
♂ ♀
20
are pointed (fig. 14). The ends of the legs of females have extra claws. Other sex specific features
include the presence of a tar spot and/or eggs for (adult) females and the length of the second leg in
(adult) males, which commonly longer than the rest of the legs.
3.2.7 Tar spot & Berried (females)
A tar spot is a sperm package placed by a male on the cephalothorax on the ventral side between the
walking legs of the female (fig. 15) and is an indicator of the maturity of the female. The tar spot is
scratched open to release the sperm and fertilize the eggs. Berried females are females carrying
eggs. To determine the presence of a tar spot and or eggs the female is examined from the ventral
side.
3.2.8 Merus length (males)
In the several crustacean species, i.e. Panulirus cygnus, allometric growth of the second walking leg
(pereiopod) of males has been found to be an indicator for their maturity (Evans et al., 1994;
Fig. 16. Measuring merus length of left walking leg of male P. argus with caliper (I. van Gerwen, 2012)
Fig. 15. Adult female with tar spot and eggs. (I. van Gerwen, 2012)
Tar spot
Eggs
21
Melville-Smith & de Lestang, 2006). Larger males typically have large pereiopods. We measured the
merus of the left side of the male with a calliper to the nearest mm. The calliper was placed behind
the pointed end of the merus and at the beginning of the merus where the isohium ends (fig. 16).
3.2.9 Size at maturity males
The males of P. argus were considered morphometrically mature based on changes in the
relationship between the length of the merus (ML) and the carapace length (CL) as determined by a
regression analysis (Melville-Smith & de Lestang, 2006). First, to test whether the merus length data
actually was allometric, two lines were fitted to the regression ML - CL. With a likelihood ratio test
the appropriateness of using two lines instead of one was tested. When this was determined each
data point was assigned either a 0 (immature) or a 1 (mature) based on their distance from one of
the lines. So if a point was closer to the left hand line then the point was assigned a 0, if it was closer
to the right hand line it was assigned a 1. The two lines were then refitted to these points, so the left
line to all the zeroes and the right line to the ones. Next, all the points were assigned a 0 or 1
according to their distance to either line. This process was iterated 100 times, but stabilized already
after 8 times .The lobsters were then allocated a maturity stage. To determine length when 50% of
the male P.argus is mature a logistic curve was fitted to the data.
3.2.10 Ecdysis
Ecdysis is the moulting of the exoskeleton of crustaceans in order to grow. Moulting animals have a
soft shell and when soft animals were observed they were documented.
Mixed Fish
3.2.11 Mixed fish species composition
Identification of the caught fish species was done based upon the reef fish identification book by
Humann & Deloach (2003). Either all fish species were counted or in some cases the lengths of the
whole catch was measured.
3.2.12 Fork length (FL) and Total Length (TL)
Depending on the fish species either the fork length (FL) was measured or the total length (TL) to the
nearest cm. The species with forked tails like H. melanurum (cottonwick) or Holocentrus rufus
(Longspine squirrelfish) FL was measured as opposed to species like Acanthostracion polygonia
(Honeycomb cowfish) and Chaetodon striatus (Banded butterflyfish) of which the TL was measured.
For the whole list see (appendix D).
22
3.3 Statistical analysis
3.3.1 Standardization of CPUE
Initially the CPUE was expressed as number of lobsters per trip, but because there was a difference in
trap hauls per trip this could influence the variation in CPUE separate from the abundance of
lobsters. The influence of trap hauls per trip could be taken into account by simply expressing the
CPUE as number of lobsters per trip per trap-haul. A simple regression graph where the number of
lobsters per trip was plotted against the number of traps showed a saturation effect of the number
of trap hauls on the number of lobsters being caught per trip (fig. 17). Simply put: the chance of
catching a certain number of lobster per trap-haul diminishes with increased trap-hauls per trip.
Therefore the trips were standardized to take out the “trap-haul” effect. The standardization of the
CPUE was used as described in of Tsehaye et al. (2007).
Other factors that could affect the CPUE, for example boat type or soaking time, were checked for
correlation with a descriptive regression analysis. In case of boat type there was no relation found
with catch per trip, and was therefore not used in the statistical analyses. There was no significant
relationship between soaking time and catch per trip and was not used in the analyses.
Standardisation involved the log-transformation of both the catch data and the effort data. Then the
regression coefficient β was calculated and the standardised CPUE was calculated with the following
formula:
(Tsehaye et al., 2007)
= standardized CPUE
= the mean value for in this case numbers of trap hauled per trip
= the number of traps hauled on trip
Catch data was available for the years 1999, 2000, 2007, 2011 and 2012. Because the data was
obtained in different months and to rule out the possibility of seasonal effects, a GLM was used on
y = 3.534x0.6436 R² = 0.1613
0
50
100
150
200
250
300
0 50 100 150 200 250
No
. Lo
bst
er
pe
r tr
ip
No. Trap-hauls
Fig. 17a +b. Power regression of catch per trip of 1999, 2007 and 2012 in the months July, August, September, October and November
against effort (no. trap hauls). (a) Catch: no. lobster per trip, (b) Catch: No. lobster per standardized trip.
y = 114.44x-0.162 R² = 0.0121
0
50
100
150
200
250
300
0 50 100 150 200 250
No
. Lo
bst
er
pe
r st
and
ard
ize
d
trip
No. Trap-hauls
23
data of the months July, August, September, October and November of the years 1999, 2007 and
2012. Those months were covered in both Dilrosun’s data and data of Toller & Lundvall 2008. Trips
are standardized on 75 trap-hauls per trip.
The calculated standardised CPUE values of the different years were analysed with a GLM with year
as a fixed factor. Since standardized CPUE-values were not normal distributed they were log-
transformed. Post hoc comparisons between years using a Bonferroni correction were performed.
3.3.2 CPUE weight per trip
The weight of male and female lobster was determined using the length weight relationship
determined by Dilrosun (2000). The following formulas were used:
Females: y = 3.3835x2.4724
Males: y =6.1318x2.2234
where x is the carapace length in cm and y the weight in grams. The total weight per trip was
calculated by summing up the individual weights of the lobsters. These estimated weights per trip
were then standardised in the same way as the CPUE no. lobster per trip per trap-haul. Trips were
standardized on 63 trap-hauls per trip.
3.3.3 Comparing of means
Means, standard errors and 95% BCa (Bias corrected and accelerated) confidence intervals were calculated with the use of bootstrapping. This non-parametric test was used to overcome large differences in sample size and the non-normal distribution of the data obtained during on board and port sampling. Bootstraps results came from 1000 bootstrap samples.
24
4. Results
4.1 Effort
During the sampling period 8 (partially) decked fishing boats were active on the Saba Bank. The crew
mainly consisted of the captain and one deckhand (sometimes 2). De boats were powered with
diesel engines; their horsepower ranging from 215 to 600hp (mean 406hp ± 41.7 S.E.). Vessel length
ranged from 30 to 39 ft. (9.14-11.89 m) with a mean of 34.4 ft. ± 1.12 S.E. (10.49m ± 0.34 S.E.). The
tonnage ranged from 5-15 tonnes with a mean of 8.6 tonnes. The fishermen owned a total of 1780
lobster traps, which they deployed on the Saba Bank and checked every 11.6 ± 0.38 S.E. days. These
traps were mainly of arrow head type but some fishermen also used square traps. The size of the
traps differed: 3-4 x 3-5 ft. (0.91-1.22m x 0.91-1.52m). Traps consisted of mesh steel wire, either
galvanized or coated. The size of the mesh in inch: 1x2, 1.5x1.5, 1.5x2, 2x2; in cm: 2.54x5.08,
3.81x3.81, 3.81x5.08, 5.08x5.08.
The fishing areas in 2012 for
lobster fisheries are mainly
situated in the North and East
parts of the Saba Bank (fig. 18).
The total number of trips doubled
from 1999 to 2007 but dropped
with 20% in 2012 based on the
months July-November (table 3).
Interestingly, the percentage of
trips that were lobster trips
decreased with more than 40%
over the time period of 13 years
(table 3). On the other hand the
average number of traps hauled
per trip in 2012 is similar to the
average of 2007. Both are
approximately 30 % more than the average in 1999 (table 3). The average number of trips per day
drops during the weekend in the months July –November in 2012 (appendix E). Due to storms (Isaac
21-28th of August and Raphael 13-16th of October), bad weather or maintenance, boats were out of
the water for several days, sometimes even longer.
Fig. 18. Lobster fishing activities in 2012 at the Saba Bank. Red= high, yellow=
medium, green=low fishing activity.
25
4.2 Catch
With the average weight per lobster in the months July-November and the average number of
lobsters per trip, the total landings of lobsters in kilo is calculated. Striking is that the total lobster
weight per year estimated in 2007 was 50% higher than the total catch in 1999. The total weight then
dropped again in 2012, even lower than 1999, due to decreased number of lobster per trip. The
mean weight per lobster (July-November) differed significantly between the years 1999, 2007 and
2012. Comparing the means with the use of bootstrapping an increase can be seen in weight per
landed lobster from 1999 (1,140.9g ± 482.6 SD, n=10420, BCa 95% CI [1132.11, 1149.42]) to 2007
(1,365g ± 547.9 SD, n=1203, BCa 95% CI [1333.75,1393.18]). The mean in weight per lobster in 2012
(1,217.9g ± 11.8 SD, n=1520, BCa 95% CI [1196.58,1240.54]) is significantly lower than 2007 but
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