Fisheries ecology of spiny lobsters Panulirus argus (Latreille) and Panulirus guttatus (Latreille) on the Bermuda Platform: estimates of sustainable yields and observations on trends in abundance

ELSEVIER

Fisheries Research 24 (1995) 113-128

Fisheries ecology of spiny lobsters Pam&-us argus (Latreille) and Panulirus guttatus (Latreille) on the Bermuda Platform: estimates of sustainable yields and observations on trends in abundance Christopher R. Evans”,‘, Arthur J. Evans Dorser Natural Resource and Environment Consultants, 2 Edgehill Road. Winton, Bournemouth, BH9 2PQ, UK

Accepted 23 November 1994

Abstract The maximum sustainable yield (MSY) of legal-size spiny lobsters Punulirus argus (Latreille) was estimated at 37 ( f SE 7.7) X lo3 animals by regression of catch per unit effort (CPUE) on total effort. A graphical plot of P. argus fishery catch and effort data for lobster seasons from 1975/76 to 1986/87 resulted in a second estimate of the P. argus MSY of 30X lo3 lobsters per season by a quadratic regression of catch on total effort. A graphical plot of catch on total fishing effort data for the Guinea chick Lobster Panulirus guttatus (Latreille), with a quadratic curve fit (R2=0.612), resulted in an estimate of MSY of approximately 33 X 10’ lobsters per calendar year. The 1985 and 1986 fishery catches of Guinea chick lobsters were close to the MSY. There was an overall increase in the CPUE of P. argus lobsters ( 1975/76-1986/87), whereas there was an overall decline in the CPUE of P. guttatus spiny lobsters over the same time period. The two species interannual patterns of change of CPUE followed that of sea temperature change on the Bermuda Platform; superimposed was a 5-year cycle of CPUE, which was particularly noticeable with P. guttutus lobsters, suggesting a causal relationship with a 5-year cycle in the annual mean temperature of sea surface waters, averaged from six temperature stations on the Platform. Annual yield of P. argus lobsters was correlated with Platform average sea temperature at the 10% level (Spearman rank test). Both P. guttutus and P. argus yields on Platform sea temperature were parabolic in form. Platform CPUE of Guinea chicks on Hamilton Harbour temperature also suggested a parabolic relationship in the range 22-26°C (coefficient of determination R2 = 0.43). There was clearly a parabolic association between P. urgus CPUE and Hamilton Harbour temperature over the same temperature range (R* = 0.93). It is suggested that the Ellis and Russell quadratic model for survival and growth of P. urgus postlarvae may be extended to juvenile P. urgus to explain these findings. * Corresponding author. ’Present address: Department of Fisheries and Marine Resources, Kanudi Fisheries Research Station, P.O. Box 165, Konedobu,

Port Moresby, Papua New Guinea.

0165-7836/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIOl65-7836(94)00367-X

114

C. R. Evans, A.J. Evans /Fisheries Research 24 (1995) 113-128

Keywords: Panulirus argus; Pam&-us gutfatus; Sustainable

yield; Temperature;

Catch per unit effort

1. Introduction 1.1. Background The Bermuda Platform has supported a stable commercial P. argus lobster fishery with net annual landings of approximately 24 000 kg (30 000 lobsters) over the last 15 years (Hateley and Sleeter, 1993). A yield of 30 X lo3 P. argus lobsters would appear to be sustainable from these figures, especially as over the last 5 years there has been fairly constant postlarval recruitment (Hateley and Sleeter, 1993). The principal stock conservation measures have included protection of P. argus juveniles in inshore spiny lobster nursery areas by prohibition of trapping, a minimum size limit of 92 mm carapace length, a 5-month closed season covering the breeding season, and the protection of gravid females. Declining stocks of reef fish necessitated an indefinite ban on the traditional Bermudan arrowhead fish trap in 1990. An experimental lobster fishery was reintroduced in 1991 employing lobster-specific traps with escape gaps for fish. The refugium in space afforded by the prohibition of trapping in the inshore lagoons, sounds and harbours at Bermuda may have been the most important lobster conservation measure (Evans, 1988). Natural refugia can be valuable to spiny lobster survival, growth and propagation. They can be refugia in time, i.e. size-related (Prof. W. Herrnkind, personal communication, 1992) or space (Lozano-Alvarez et al., 1993). The anthropogenic refugium in time afforded by the 92 mm size-limit at Bermuda may have been equally important, though mortality was also associated with the handling and return of the undersized lobsters to the sea (Evans, 1988, 1989). Yields of P. guttatus lobsters have been much less steady than those of P. argus (Tables 1 and 2). Over the period 1975-1987, P. guttatus yields have been in the range 13 Om2 000 (Table 2). Management measures for the P. guttatus stock over this period chiefly comprised protection of ovigerous females, and a 1.5 inch mesh size (of mean retention length 50 mm carapace length (CL) ) . 1.2. Previous research The Bermuda Spiny Lobster Panulirus argus was studied in the 1950s (Sutcliffe, 1952, 1953, 1957) and a minimum size limit of 92 mm CL was established by the Bermudan fisheries authorities based on the Sutcliffe ( 1952/53) studies. Larval and postlarval recruitment and ecology were studied by Farmer et al. ( 1989)) and continuing studies have shown that postlarval recruitment has been stable over the period 1987-1992 (Hateley and Sleeter, 1993). Stock replenishment processes have been studied by Hateley and Sleeter ( 1993). Evans and Lockwood ( 1995) reported the results of 1986/87 field studies on the abundance, catchability and field behaviour of P. guttatus and the results of an Overseas Development Administration (ODA) research project on the population dynamics were reported by Evans (1988, 1989).

CR. Evans, A.J. Evans/Fisheries

Research 24 (1995) 113-128

Table I Statistics of catch and effort for the Bermuda spiny lobster (Panulirus arps) 1986/87

115

at Bermuda by season, 1975/76

Season

Industry catch (no. landed)

Effort (trap-hauls)

CPUE (lobsters per trap-haul)

1975/1976 197611977 197711978 1978/ 1979 1979/ 1980 1980/1981 1981/1982 1982/ 1983 19X3/ 1984 l984/ 1985 19X6/ 1987

26920 29744 28794 20618 32317 28145 25723 22841 36327 32117 29209

70813 60907 53889 39774 54065 47971 5 1885 56768 61940 58547 44760

0.38 0.49 0.53 0.52 0.60 0.59 0.50 0.40” 059 0.55 0.65

Data courtesy of Jack Ward, Bermuda Division of Fisheries. “The 1982/83 winter had a relatively high incidence of storms and storm surge, with reduced catches as a result (D. Farias, President, Bermuda United Fishermen’s Association, personal communication, 1987)

The primary objective of the present study was to provide data on the maximum sustainable yield of both P. argus and P. guttutus on the Bermuda Platform. In this regard, it was recognised that it is essential first to delineate the stock (Beverton and Holt, 1957). Recent field studies by Farmer et al. ( 1989) and genetic analyses by Hateley and Sleeter ( 1993), at Bermuda, were aimed at determining this. Table 2 Statistics of catch and effort for the Guinea chick lobster (P. gurtarus)

at Bermuda by calendar year, 1975-1987

Calender yen,

Industry catch (No. landed)

Total effort (trap-hauls)

CPUE (chicks per trap-haul)

I975 I976 I971 1978 I979 1980 1981 1982 19x3 19x4 19x5 19X6 1987

41824 30849 21936 19311 3254 I 28342 1399s 13072 25410 27886 31952 3382 1 26017

114343 I17150 100693 81198 94439 8S269 72024 78338 10260 I 103963 123346 129060 116719

0.37 0.26 0.22 0.24 0.34 0.33 0.19 0.17” 0.25 0.27 0.26 0.26 0.22

Data courtesy of Jack Ward, Bermuda Division of Fisheries, “The 1982/83 winter had a relatively high incidence of storms and storm surge, with low catches as a result (D. Fanas, President, Bermuda United Fishermen’s Association, personal communication, 1987).

116

C. R. Evans, A. J. Evans / Fisheries Research 24 (I 995) I1 3-l 28

The P. urgus population has a single breeding season in the summer months (Sutcliffe, 1952, 1953), but postlarval recruitment occurs throughout the year with peak influx also occurring in the summer (Hateley and Sleeter, 1993). It has been hypothesised that peak settlement coincided with the arrival of locally derived lobsters, whereas the low-level incidence during winter was either the result of foreign recruitment, or of locally derived larvae exhibiting plasticity in development times (Farmer et al., 1989). In their 2-year study of phyllosoma dispersal and puerulus recruitment at Bermuda, Farmer et al. ( 1989) showed that Stages I-V phyllosoma larvae were found in near-shore waters during June-September and that Stages VI-XI dominated in sequence through the winter. They also found that puerulus recruitment peaked in August/September although it occurred throughout most of the year. Farmer et al. (1989) concluded that the period between peak concentrations of Stage I and peak recruitment, together with estimated development times in the plankton, suggest a planktonic duration of about 1 year for Panulirusphyllosoma larvae in the Sargasso Sea. Marchal ( 1967) reported observations of the capture on 15 September and 27 December ( 1964) of two large living adult specimens of P. argus lobsters (one male and one female) found on the Ivory Coast continental shelf, Africa. This suggests that it is also possible that some P. argus phyllosoma larvae may drift completely around the North Atlantic gyre before returning to the Western Atlantic to settle. Indeed, a hypothesis of long larval drift in Palinurid lobsters of the Indo-Pacific was proposed by Pollock ( 1992)) who postulated complete circuits of the Pacific gyres, and also shorter drift in sub-gyres such as the Kuroshio Recirculation near Japan. Menzies ( 1981) and Menzies and Kerrigan ( 1979) found considerable heterogeneity in the populations of spiny lobster Punulirus argus among the island shelves of the Caribbean, and postulated that this resulted from natural selection rather than from genetic drift arising from isolation. However, preliminary results of a more recent study at the University of Miami suggested that lobster populations in the Caribbean are not significantly different in genetic composition (Hateley and Sleeter, 1993). There is thus conflicting evidence upon which to base delineation of lobster stocks in the Gulf and Caribbean. Hateley and Sleeter ( 1993) concluded that: ( 1) delayed larval development cannot be dismissed as a factor in P. argus larval development; (2) genetic analyses of benthic and postlarval individuals are consistent with a hypothesis of local replenishment; (3) there is no significant heterogeneity between spiny lobsters in Florida and Bermuda; (4) the Bermuda population may comprise part of a Florida/Western Atlantic stock. Since spiny lobster populations of Florida, the Bahamas and Bermuda all have annual breeding seasons, and populations in the southern Caribbean region breed throughout the year, the annual pulse of postlarval recruitment observed in mid-summer at Bermuda may comprise the offspring of these northern populations, whereas that which occurs throughout the year is perhaps derived from the populations of the Caribbean islands. Farmer et al. ( 1989) pointed out that there was a sequence of larval Stages VI-XI during the winter, and that the length of time that each larval stage was dominant was longer than that predicted for solely local replenishment. This suggested that there had been admixture of larvae from sources which have spiny lobster populations with longer annual breeding seasons than those of Bermuda (but not all year round), such as the spiny lobster populations of Florida and the Bahamas. These observations are consistent with the finding of Hateley and Sleeter

C. R. Evans, A. .I. Evans / Fisheries Research 24 (I 995) 113-128

( 1993) that there is no significant muda spiny lobster populations.

heterogeneity

between contemporary

I17

Florida and Ber-

1.3. Assumptions The assumptions underlying the analyses of the present were: (1) there is chiefly local replenishment of P. argus and P. guttutus spiny lobsters on the Bermuda Platform, although a significant proportion of recruitment probably originates from Florida, the Bahamas and the southern Caribbean; (2) these Bermudan sub-stocks each form part of a larger northern (regional) stock, comprised of sub-stocks of the northern rim which reproduce annually in a summer breeding season, viz. Florida, Bahamas and Bermuda; (3) the northern region is in turn a sub-set of a single Pan-Caribbean stock of each species. Implicit in these assumptions is an hypothesis that the southern rim of the Gulf and Caribbean region, comprising lobster sub-stocks which reproduce all-year round, is also a regional stock. A fourth assumption, relating to CPUE and temperature, was that industry CPUE is an approximate guide to abundance. 1.4. Sea temperature and the abundance

of lobsters

The abundance of legal-size clawed lobsters Homarus americanus and H. gammarus is correlated with both current and lagged annual mean sea surface temperature in Maine, and predictive equations/correlations have been determined for both species (Dow, 1969,1977, 1978, 1980). The second objective of this study was to investigate evidence suggesting the possibility of a similar correlation in Palinurid spiny lobsters. This evidence was the 5-year cycles in CPUE observed in the catch and effort record for P. guttatus. The population dynamics and fisheries ecology of Panulirus argus and P. g&tutus at Bermuda were targeted for research by the Natural Resources Division of the ODA, London, and the resulting research programmes (including the present study) were undertaken in the Department of Oceanography, University of Southampton, from May 1986 to August 1988, Professor A.P.M. Lockwood supervising. Results of parallel studies of the population dynamics were reported to the ODA (Evans, 1988,1989).

2. Materials and methods Fishery catch and effort data for spiny lobsters P. argus (by lobster season) and P. year) on the Bermuda Island shelf for the period 1975-1987 were obtained courtesy of the Bermuda Division of Fisheries (Tables 1 and 2)) and analyzed by quadratic regression of catch upon effort and by linear regression of CPUE on effort. The standard errors of the slope and intercept of this linear regression were used to obtain the standard error of the MSY estimate of P. argus. For P. guttutus the MSY determination was not possible using this method as P. guttutus CPUE on effort was not significantly correlated. A single-tail test for negative linear correlation was used for P. argus (Till, 1974, pp. 87-88). Statistical analysis by the ‘C-Stat for Windows’ Correlation/ Regression proguttutus (by calendar

118

Table 3 Mean sea temperature at six locations

C.R. Evans, A.J. Evans/ Fisheries Research 24 (1995) 113-128

(“C at sea surface) for the Bermuda Platform compiled from annual average sea temperatures

PartA: 1975-1980 Location

1975

1976

1977

1978

1979

1980

North Lagoon Gt. Sound Hamilton Harbour Harrington Sound Castle Harbour St. Georges Harbour

18.750

24.745 25.452 25.920 24.286 24.898 25.660

21.086 21.523 15.600 22.750 23.638 23.553

23.489 20.818 22.014 21.125 19.696 20.040

22.419 _

23.229 24.581 25.031 24.769 23.300 23.567

25.160 6 0.619 0.253

21.358 6 3.007 1.228

21.197 6 1.390 0.567

Platform average Sample (n) S.D. S.E.

_

_ _

18.750”

I _ _

24.433

23.426 2 1.424 1.007

24.080 6 0.803 0.328

Part B: 1981-1987 Location

1981

1982

1983

1984

1985

1986

1987

North Lagoon Cit. Sound Hamilton Harbour Harrington Sound Castle Harbour St. Georges Harbour

21.892 21.775 22.142 21.825 21.675 2 1.708

22.392 22.275 22.558 22.492 22.142 22.258

22.450 22.620 22.680 21.610 22.540 22.410

22.150 22.150 22.367 22.317 22.050 22.000

23.027 22.982 23.245 22.973 22.627 22.700

23.408 23.283 23.650 23.408 23.167 22.800

22.071 21.929 22.086 21.971 21.883 22.533

Platform average Sample(n) S.D. SE.

21.836 6 0.169

22.353 6 0.157

22.385 6 0.393 0.160

22.172 6 0.145 0.059

22.926 6 0.227 0.093

23.286 6 0.287 0.117

22.079 6 0.236 0.096

- indicates data not available. Data courtesy of D. Connelly, Bermuda Biological Station. “It should be noted that this is a single point, not au average.

gramme was used to derive the standard error (SE) of the slope (B) and the y-intercept (A), and from Schaeffer ( 1954)) the MSY was estimated from the equation MSY = A */4B lobsters per season. The upper SE limit of the MSY was estimated using the lower SE limits of the slope and the intercept (irrespective of sign). Similarly, the lower SE limit of the MSY was estimated using the upper SE limits of the slope and the intercept. The MSY was taken to be the median of the SE limits of the MSY so derived. 2.1. Calculation of the coejjicient of determination R* and the reduced ,y*factor R for the parabolic curves The coefficient of determination R2 was calculated for selected parabolic (quadratic) curves from the sum of the squares of the residuals (I) and the sum of the squares of the

CR. Evans. A.J. Evans/Fisheries

30000

40000

Research 24 (1995) 113-128

60000

50000

70000

60000

TOTAL EFFORT PER LOBSTER FISHING SEASON (TRAP-HAULS)

Fig. I. Linear regression of CPUE for the P. clrgu.7 fishery at Bermuda, providing an estimate of the MSY. The values of the slope (B) and intercept (A) give an estimate of the MSY from (Schaeffer, 1954): MSY =A’/4B.

J’- ylllcanvalues (II), where R’ is 1 - (I/II) (Eusyplat Technical Support Officer, Cherwell Scientific, Oxford Science Park, Oxford, UK, personal communication, 1994). The reduced x2 factor R was produced by the Easyplot programme as a measure of the goodness of fit of the regression curves: R = sum of (ye, - y) */ (n - 3)) where II is the number of points in the scattergram; it is a measure of the variance of the data from the = -2 OOE-5x’ +241x’

-4 20E4. R 4014, max dev 732

40000

s x .83/84

.? 6

35000.

P 5

79180 .

6

3

.84/85

30000

s 3 t 0 5

25000.

!: 2

.

82/83

_ ~.--~

20000 30000

40000 TOTAL

50000 EFFORT

PER LOBSTER

60000 FISHING

SEASON

~_..~_ 70000

80000

(POT-LIFTS)

Fig. 2. Parabola of catch upon effort showing an estimate of the MSY of P. argus on the Bermuda Platform. The curve is that of the following (quadratic) expression: y = ( - 2.00 X 1O-5)x’ + 2.41x- (4.20 X 104), where v is the industry catch per season, and x is the total effort per lobster season in pot-lifts (trap-hauls). The MSY is estimated at approximately 30 X I@’lobsters per season.

120

C.R. Evans, A.J. Evans/ Fisheries Research 24 (1995) 113-128

b =-6.23E-52 6

+1.58x’ -5 78E4. R5535. max dev:l,OZE4/

5ocQa

0” 2 5 g 5

.1975 40000.

-I 0 7x10-

0 9x10’

1.1x10’

1 3xio5

TOTAL EFFORT PER CALENDAR YEAR (NO OF POT-LIFTS)

Fig. 3. Parabola of catch upon effort showing an estimate of the MSY of P. guttatus on the Bermuda Platform. For the period 1975-1987, the curve for the relationship between industry catch per calendar year (y) and total effort per calendar year (x) is quadratic (coefficient of determination, R’=0.612): y= (-6.23X IO-‘)I’+ 1.58x- (6.78X 104). The MSY is estimated at approximately 33X lo3 Guinea chick lobsters per calendar year.

fitted curve (Easyplot Technical communication, 1994).

Support Officer, Cherwell

Scientific,

Oxford, personal

2.2. Sea temperature data and temperature analyses Annual mean sea surface temperature data were obtained courtesy of D.P. Connelly, of Bermuda Biological Station, for six locations on the Platform. A Bermuda Platform Average Sea Temperature Record was calculated from these data (Table 3). Spiny lobster and Guinea chick yield and CPUE was graphed on annual average sea temperature to investigate if there was a possible relationship. The Bermuda Platform Average and Hamilton Harbour temperature data sets were used in the analyses. Hamilton Harbour in Great Sound was chosen as it is located near the water density centre of the Bermuda Platform, and is open to the ocean.

3. Results 3.1. Maximum sustainable yield of P. argus The results suggested an MSY of P. argus lobsters of 37 ( f SE 7.7) X lo3 lobsters per season (derived by a linear regression of catch per unit effort on total effort: Fig. 1; Table 1).

CR. Evans, A.J. Evans/Fisheries

1974

1970

Research 24 (1995) 113-128

1992

121

1986

(a) YEAR AT BEGINNING OF LOBSTER FISHING SEASON

0.25 2 4 !f

0.20

5 &I?

0

(b)

0.15 1974

1978

1982

1986

CALENDAR YEAR

Fig. 4. Fluctuation and trend in the CPUE of P. argus and P. guftarus and in the annual average sea temperature of the Bermuda Platform, 1975-1987. (a) P. argus CPUE: based on Table 1 (data courtesy of J. Ward, Bermuda Division of Fisheries). (b) P. guttafus CPUE: based on Table 2 (data courtesy of J. Ward, Bermuda Division of Fisheries), (c) annual mean sea temperature on the Bermuda Platform (averaged from annual mean sea temperatures at six locations: based on Table 3), (data courtesy of D. Connelly, Bermuda Biological Station for Research, Ferry Reach l-15, Bermuda).

The intercept (A) and slope (B) of correlation/regression of catch per unit effort on effort were as follows: A = 0.8082; B = - 5.14 X 10m6. The correlation coefficient (r) was - 0.5349. Total effort and CPUE are negatively correlated (P=O.O5, one-tail test for negative correlation, Murdoch and Barnes, 1970).

C.R. Evans, A.J. Evans /Fisheries Research 24 (1995) 113-128

122

24

1974

(4

1962

1970

1966

CALENDAR YEAR

Fig. 4

(continued).

Statistical analysis by the ‘C-Stat for Windows’ Correlation/Regression programme resulted in the derivation of the SE of the slope and the SE of the y-intercept: slope standard error=2.71 X lo-“; intercept standard error = 0.1496. From Schaeffer ( 1954) : MSY = A2/ 4B lobsters per season, Using the lower limits (SE) of the value of the slope and intercept (irrespective of sign), the upper SE limit of the MSY was 44.6 X lo3 lobsters per season, and similarly the lower SE limit was 29.2 X IO3 lobsters per season. The MSY was estimated by the median of the limits 36.9 ( + SE 7.7) X lo3 lobsters. A quadratic regression of P. argus catch on effort for lobster seasons from 1975176 to 1986/87 resulted in a second estimate of the P. argus MSY at approximately 30.3 X lo3 lobsters per season (Fig. 2). This second estimate is in agreement with the first from the linearregression analyses (i.e. it falls within the standard error margins of the first estimate). 3.2. Maximum sustainable yield of P. guttatus Quadratic regression of catch upon effort (a scattergram of 13 points) for P. guttutus resulted in an estimate of the MSY of 33 X lo3 lobsters per season (Fig. 3; Table 2). The coefficient of determination R2 was calculated at 0.612. The form of the curve and the fact that the 1985 and 1986 fishery catches of Guinea chicks were close to the MSY suggests that the fishery is moving towards, or has reached, equilibrium stock level. 3.3. CPUE of P. argus Linear regression of CPUE on year at the start of the season in regard to P. argus legalsize lobsters indicated an overall increase in abundance from 1975176 to 1986/87 (Fig. 4(a)) (P = 0.05, one-tail test for positive correlation). The fluctuation in CPUE of P. argus

C.R. Evans, A.J. Evans/Fisheries

Research 24 (1995) 113-128

123

??

PG 1975

8

(4 AVERAGE SEA TEMPERATURE

IN CALENDAR YEAR, BERMUDA PLATFORM (“C)

k = -2931x2 +1.40E5x1 -1.61E6.

R:6801, max dev:i.l5d

.

I 60000

21 (b)

22

23

24

25

26

AVERAGE SEA TEMP. IN CALENDAR YEAR, BERMUDA PLATFORM (“C)

Fig. 5. Association of annual yield of Palimnid lobsters with Bermuda Platform average sea temperature. (a) P. argus and P. guttam yields each versus Platform average sea temperature, 1976/77-1986187 seasons (P. argus), 1976-1987 calendar years (P. gutrafus). The 1975/76 lobster season and the 1975 calendar year have been excluded from the quadratic regression. (b) Total catch of Panulirid lobsters on the Bermuda Platform (P. qr+s plus P. guffutus) versus Platform average sea temperature, for the temperature range 22-26°C. Data for the 1982183 lobster season and the 1982 calendar year have been excluded from the regression because heavy storms resulted in a decreased total catch.

lobsters, underlying (Fig. 4(a)).

this pattern, was perhaps one of 5-year cycles, especially

after 1978

C.R. Evans, A.J. Evans/Fisheries

124

Research 24 (1995) 113-128

b = JI.0353 +1.72x' -19.9, R:0.0185.maxdev:0.023?j

o

0.4 21

23

25

ANNUAL MEAN SEA TEMP., HAMILTON HARBOUR (“C)

3

F

= -O.O142z? +0.698x' -8.24, R:0.0311, max dev:O.O34 0.35 ??

B t Y 0 ;

.

0.30,

I 6

. 0.25.

? % 4

0.20'

??

!Y 8 !i 8

a

0.1521

(b)

23 ANNUAL

MEAN

/I

25

SEA TEMP, HAMILTON HARBOUR (“C)

Fig. 6. Association of CPUE of Palinurid lobsters with Hamilton Harbour temperature, over the temperature range 22-26°C. (a) Pant&us argus. Data for the 1982-83 lobster season are excluded from the regression because winter storms were more severe than usual and the total catch was relatively low. (b) Punulirus gurtatus. Data for the 1982 calendar year were excluded from the regression.

3.4. CPUE of P. guttatus

There were marked S-year cycles of change in the CPUE of P. guttutus lobsters at Bermuda during the period 1975-87, and there was an overall decrease in the CPUE of P. guttutus lobsters over 1975-87 (Fig. 4(b) ) .

C. R. Evans, A.J. Evans /Fisheries Research 24 (I 995) 113-128

125

3.5. P. argus and P. guttatus CPUE and sea temperature

Sea temperature change on the Bermuda Platform occurred with a periodicity of around 5 years, showing a fluctuation and trend of change which largely coincided with the fluctuation and trend in P. guttatus CPUE (Figs. 4( a-c) ) 3.6. Annual lobster yield on platform sea temperature Yields of P. argus by lobster season and of P. guttatus temperature suggested a parabolic association (Fig. 5 (a) tively) . The total catch of Palinurid lobsters on Platform parabolic association, with a peak catch at approximately

by calendar year on Platform sea ; R = 4557 and R = 5768, respecsea temperature also suggested a 23.8”C (Fig. 5(b); R = 6801).

3.7. CPUE on Hamilton Harbour sea temperature CPUE

of P. argus

Harbour temperature was parabolic (Fig. 6(a); Harbour temperature was similarly associated (Fig. 6(b) ; R’ = 0.43). The best catch rates occurred at an optimum annual average sea surface temperature in Hamilton Harbour of around 24°C for P. argus and 24.5”C for P.

R’ = 0.93).

on Hamilton

P. guttatus CPUE on Hamilton

guttatus.

4. Discussion Information on potential yield was obtained by Smith and Van-Nierop (1986) in regard to the P. argus spiny lobster populations nearest to Bermuda, at Little and Great Bahama Banks, which indicates the feasibility of such research for P. argus on the Bermuda Platform. The results of research in a parallel study of population dynamics in 1986/87 indicated that exploitation rates for P. argus and P. guttatus at Bermuda were close to the optimum level for the maximum sustainable yield, viz approx. 0.6 for each species (Evans, 1988, 1989). The estimate of the exploitation rate of P. argus supports the estimates of MSY because the total catches of P. argus in the 1985/86 and 1986/87 lobster seasons were 29 209 and 32 117, respectively, which is within the standard error margin of the estimate of MSY, viz. 37 ( + SE 7.7) X 10’ lobsters per season. The industry CPUE is not strictly an index of abundance since CPUE is subject to change in the catchability coefficient (4) from year to year. Catchability in Panulirus Cygnus (George) is positively correlated with temperature and salinity, and negatively correlated with the percentage of animals in a premoult condition (Morgan, 1974). However, annual mean sea temperature averaged for the Bermuda Platform only fluctuated in a relatively narrow band of 4°C (21-25°C) in 197687 and CPUE may therefore give an approximate guide to population fluctuation and change in P. argus and P. guttatus at Bermuda. The overall increase in the population of P. argus lobsters from the 1975/76 to 1986/87 seasons suggests a positive and successful conservation programme, which is perhaps

126

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Research 24 (1995) 113-128

attributable to the four principal conservation measures. These are the 92 mm carapace length size-limit based upon Sutcliffe ( 1952), the closed season for P. argus lobsters ( 1 April-3 1 August) covering the breeding season, the protection of gravid females and, perhaps most importantly, protection of P. argus juveniles from trap-fishing in the nursery grounds along the North Shore and in the inshore waters of the Sounds and Harbours. The assumption that industry CPUE is an approximate guide to abundance is supported by the observed negative linear correlation of seasonal CPUE on total effort for P. argus. A similar negative correlation was not found for calendar year P. guttutus data. This probably reflects the fact that, whereas there is a lobster (P. urgus) fishing season, there is no Guinea chick (P. guttutus) fishing season, and the record of fishing effort for P. guttutus comprises ( 1) trapping effort primarily directed at catching fish, (2) trapping effort primarily directed at catching Guinea chicks, and (3) trapping effort primarily directed at catching both fish and Guinea chicks. This Guinea chick ‘fishing effort’ occurs throughout the calendar year. However, the accuracy of the Guinea chick calendar year record of trapping effort may also be affected by the seasonality of occurrence of P. guttutus, which is throughout the year, but chiefly in the months June-December. Despite all this an, index of effort so defined appears to be associated with yield in a parabolic way (Fig. 3)) and the graph gives a first approximation of long-term sustainable yield of P. guttutus at Bermuda. The 5-year cycles (and trends) in CPUE of P. urgus and P. guttutus lobsters seem to coincide with the cycles and trends in annual mean sea temperature at Bermuda (Figure 4( a-c) ), suggesting that the abundance of Palinurid lobsters at Bermuda is affected by sea temperature. This is inferred by aquarium-based studies on postlarval P. urgus lobsters in Florida (Lellis and Russell, 1990), which resulted in the determination of an optimum temperature for the growth and survival of the postlarvae, estimated at 29-30°C; these parameters each increased over temperatures up to 29-3O”C, then decreased at higher temperatures. Lellis and Russell (1990) found that survival, moult frequency and moult increment of postlarval lobsters showed significant quadratic correlations with water temperature. The metabolic rate of postlarvae at 33°C was so high that it became physically impossible to take in sufficient food to cope with searching, metabolism and growth. The Lellis and Russel model for survival of P. urgus postlarvae therefore relates an increase in the basal metabolic rate with increasing temperature and the inability of the postlarvae to increase their food uptake (to maintain body growth) at relatively high temperatures. If survival and growth of juvenile/sub-adult Palinurid lobsters at Bermuda each has a quadratic response to water temperature, like postlarval P. urgus, it would offer an explanation of the observations of the present study. It is suggested that the magnitude and timing of juvenile recruitment into the lobster and Guinea chick fishery stocks is controlled by sea temperature, i.e. that the Lellis and Russel model may be extended to juvenile and subadult P. urgus and P. guttutus lobsters. The dependence of rock lobster Punulirus Cygnus catchability upon environmental variables is best described by a linear combination of temperature, salinity and percentage in premoult condition (Morgan, 1974). This may perhaps be extrapolated to fit the situation at Bermuda, but would not account for the quadratic association observed in Figs. 5 and 6. Analyses of more up-to-date and complete catch and effort data (including P. urgus catch and CPUE by calendar year, recently obtained) may help to further detail the associations of lobster and Guinea chick CPUE and yield on annual mean sea temperature, and investigate

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the roles of catchability currently in progress.

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and fishing effort in these associations.

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This research work is

5. Conclusions ( 1) The MSY of P. argus at Bermuda is estimated at 37 ( + SE 7.7) X lo3 lobsters per season. (2) The MSY of P. guttutus at Bermuda is approximately 33 X lo3 Guinea chicks per calendar year. (3) The long-term trend of change in the CPUE of P. guttutus spiny lobsters was one of decrease. (4) The long-term change in the CPUE of P. argus spiny lobsters was one of increase. (5) The P. argus MSY estimate (37 000)) average yields of 30 000 P. argus lobsters over the last 15 years and the trend of change in the relative abundance of P. argus from the 1975/76 to the 1986/87 season all point to the efficacy of the conservation measures for P. urgus spiny lobsters at Bermuda. (6) There were 5-year cycles in the relative abundance of Palinurid lobsters at Bermuda which coincide with 5-year cycles of change in the annual mean temperature of the surface water of the Platform. (7) Seasonal mean CPUE of P. argus lobsters caught on the Bermuda Platform from 1975 was correlated with annual mean sea temperature at Hamilton Harbour in a quadratic way (R 2 = 0.93) over the temperature range 22-26”C, with the greatest catch rates occurring at about 24°C. A similar, though somewhat less noticeable, association between Guinea chick CPUE and the Hamilton Harbour sea temperature was also inferred ( R2 = 0.43)) with the greatest catch rates occurring at around 24.5”C. (8) It is suggested that the magnitude and timing of juvenile recruitment into the lobster and Guinea chick fishery stocks at Bermuda is controlled by sea temperature, i.e. that the Lellis and Russell ( 1990) model may be extended to juvenile and sub-adult Punulirus urgus and P. guttutus.

Acknowledgements

We would like to thank Professor A. Peter M. Lockwood of the Department of Oceanography, University of Southampton, for editing the manuscript, the Overseas Development Administration for funding the study, Jack Ward, Dr. James Burnett-Herkes and Dr. Brian Luckhurst of the Bermuda Division of Fisheries for the statistics of catch and effort contributed for the ODA project, Douglas Connelly of BermudaBiological Station for Research for the temperature data, Dr. Colin Bannister at the Fisheries Laboratory, Lowestoft, for technical advice, and Emma Free of the Department of Oceanography for information on the association of temperature and abundance in clawed lobsters.

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