Population Biology of the Swimming Crab Portunus sanguinolentus in the Waters off Northern Taiwan

Population Biology of the Swimming Crab Portunus sanguinolentus in the Waters off

Northern Taiwan

Author(s): Hui-Hua Lee and Chien-Chung Hsu

Source: Journal of Crustacean Biology, Vol. 23, No. 3 (Aug., 2003), pp. 691-699

Published by: The Crustacean Society

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Crustacean Biology.

POPULATION BIOLOGY OF THE SWIMMING CRAB PORTUNUS

SANGUINOLENTUS IN THE WATERS OFF NORTHERN TAIWAN

Hui-Hua Lee and Chien-Chung Hsu

(HHL) (CCH, corresponding author) Institute of Oceanography, National Taiwan University, P.O. Box 23-13, Taipei, Taiwan 106 (hsucc@ccms.ntu.edu.tw)

ABSTRACT

The growth, mortality, and reproduction of Portunus sanguinolentus were studied using size-frequency data obtained from crabs collected in pots in the waters off northem Taiwan from October 2000 to March 2001, and October 2001 to January 2002. The Bhattacharya's method and seasonal von Bertalanffy growth curve were used to estimate growth parameters. The growth curve for males was Lt= 204.75 x {1 - e-[087t+0.4(0.87/2n)sin2n(t)]} _{and the curve for females was Lt = 194.25 X} {1- e-[0.97t+0.4(0.97/2n)sin27n(t)]}. A size-converted catch curve was used to estimate the instantaneous total mortality rate (Z), and Pauly's empirical equation was used to estimate the instantaneous natural mortality rate (M). For males, Z = 3.16/year and M = 1.65/year. For females, Z = 3.37/year and M = 1.8/ year. The instantaneous fishing mortality rate (F) was 1.51/year and 1.57/year, and the exploitation rate (E) was 0.48 and 0.47 for males and females, respectively. The exponential relationships were presented for relationships of fecundity in number and weight of egg mass in terms of carapace width and body weight. Those relationships were statistically significant (P < 0.01), indicating that the fecundity increased with the size from 4.05 X 105 to 2.44 X 106 eggs.

The swimming crab Portunus sanguinolentus

(Herbst, 1783) is widely distributed in ocean

waters from East Africa, through the Indo-

Pacific region, to the Hawaiian Islands (Ste-

phenson and Campbell, 1959). Juveniles and

adult males typically inhabit sandy and muddy

bottoms in nearshore waters, about 10-30 m

deep (Chapgar, 1957; Sumpton et al., 1989). In

contrast, females are abundant in 40-80 m

depths (Wenner, 1972; Campbell and Fielder,

1986). In Taiwan, P. sanguinolentus only oc-

curs in the waters around the north and south-

west parts of the island.

There have been numerous studies of

P. sanguinolentus taxonomy (Chapgar, 1957;

Stephenson and Campbell, 1959; Dai and Yang,

1991), maturation

(Sumpton et al., 1989; Jacob

et al., 1990; Reeby et al., 1990), and re-

production (Ryan, 1967; Campbell and Fielder,

1986; Sukumaran and Neelakantan, 1997).

However, there has been only little information

about P. sanguinolentus in the waters of Taiwan

(Huang, 1993; Hsu et al., 2000). The yield of

this economically important and productive

species has declined substantially in recent

years (Anonymous, 2000), and the monthly

relative abundance also shows a declining trend

(unpublished data). To understand the popula-

tion dynamics of P. sanguinolentus around

Taiwan is urgent, and locally collected data

used to study its growth, mortality, and re-

production are needed.

In this paper, growth study was performed to

estimate growth parameters and to understand

the life span. Then, mortality was estimated to

understand causes of population reduction.

Finally, fecundity was estimated based on

carapace width and body weight to determine

recruitment. Therefore, the objective of this

study was to estimate growth, mortality, and

reproduction of P. sanguinolentus living in the

waters off northern Taiwan.

MATERIALS AND METHODS

From October 2000 to March 2001, and October 2001 to January 2002, monthly samples of P. sanguinolentus were collected using circular crab pots in the offshore waters of northern Taiwan (25020'-25050'N and 120?40'-121?20'E). Pot is made of a rigid frame (diameter is 550 mm and height is 240 mm) with meshes (upper mesh size is 35 mm and lower mesh size is 40 mm) and three entrances are inserted on the side (mesh size of entrance is 15 mm). The entrances are designed to prevent crabs escaping. Before setting, pots are baited with frozen mackerels in the center of the pot.

Basic hydrographic data were obtained from the National Center for Oceanic Research database (National Taiwan Uni- versity, Taipei, Taiwan). In the study area, the water depth was around 80 m, and the substrate was sandy. Bottom temper- atures ranged from 20 ? 0.29?C in winter to 25 t 1.53?C in summer. The annual average temperature was 23 ? 3.2?C, and salinity averaged 34 + 0.39 psu (practical salinity units). Nearly all crabs captured in each pot were identified, sexed, and measured for carapace width (CW). Precision

JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 23, NO. 3, 2003

vernier calipers were used in the field to measure CW (the distance from the left tip to the right tip of the posterior margin of the carapace) to the nearest 0.01 mm (Mitutoyo digimatic caliper). Each month, from October 2000 to March 2001, 20 gravid females were selected and brought to the laboratory to study reproduction. During the main spawning season, from October 2001 to January 2002, the number of gravid females in each sample was counted. Altogether, 3298 females and 1504 males were measured, and 117 gravid females with extruded eggs were collected and frozen.

Prior to measuring, each frozen, gravid female was thawed for about 4 h in the laboratory. For each crab, the CW was measured as described above. The wet weight of body, with eggs, was determined to the nearest 0.1 gram with an electronic balance (Snowrex digital scale KF-600). Then the eggs were carefully removed from the pleopods, wiped with tissue paper, and weighed. To maintain tonicity (Wenner et al., 1987) for measurements, eggs were preserved in a mixture of 30% water, 30% ethanol, 30% acetone, and 10% glycerol.

Growth

First, the ELEFAN I subroutine of the FiSAT software package (Gayanilo and Pauly, 1997) was used to estimate a seasonalized version of the von Bertalanffy growth parameters. The growth model with a seasonal fluctuation is:

L, = Lo { 1 -e [KX(t-to)+CX(K/2nt)Xsin27r(t-ts)] _{}, ( )} where Lt (mm) is the predicted length at time t, Loo (mm) is the asymptotic length, t is a given instant age, to is the theoretical age when carapace width is zero, ts is the starting point of the growth oscillation, K (1/year) is the intrinsic growth rate, C is the amplitude of the growth oscillation and ranges from 0 to 1. The growth performance index ((p') was used to compare von Bertalanffy growth of P. sanguino- lentus with that of other crab species, in which (p' = log K + log Lo (Pauly and Munro, 1984). Then, Bhattacharya's method was used to determine the number of age groups in monthly samples (Bhattacharya, 1967).

Hotelling's T2, calculated with the SAS/IML module, was used to compare male and female growth curves based on the parameters Lo, K, C, ts, and to (Bernard, 1981; Quinn and Deriso, 1999). When the T2 statistic was significant, indicating the growth of males and females was significantly different, simultaneous Roy-Bose confidence intervals were computed to determine the most important parameter causing the difference between the sexes (Bernard, 1981).

Mortality

The instantaneous total mortality rate (Z, 1/year) was estimated based on the size-converted catch curve (King, 1995) using seasonal von Bertalanffy growth parameters:

ln( sAt ) fa ,, (2) where N, is the number of individuals of size class i, At is the time needed to grow through size class i, ti is the relative age of size class i, cx and P3 are parameters to be estimated. Thus, the instantaneous total mortality rate is Z = -13.

The instantaneous natural mortality rate (M, 1/year) was estimated with Pauly's empirical equation (Pauly's, 1980):

ln(M) = -0.0152 - 0.279 ln(Loo) + 0.6543 ln(K)

+ 0.463 ln(T), (3)

where In is the natural logarithm operator; Lo (cm) and K (1/ year) are growth parameters (described above) and T (?C) is

the mean annual habitat temperature. Thus, the instanta- neous rate of fishing mortality (F, 1/year) is F = Z-M, and the exploitation rate (E) is E = F/Z (Quinn and Deriso, 1999).

Reproduction

The proportion of gravid females, which were used as mature females, was fitted to a logistic equation as described by Quinn and Deriso (1999):

Pmax

P(L) = _{+ e-KX(L-y)'} 1 (4) where P(L) is the cumulative proportion of gravid females in CW upper class limit (L) and K, _{y, and Pmax} are parameters estimated by the log-linear least square method (Zar, 1995). Pmax is the asymptotic cumulative proportion of gravid females as L -p _oo, K _{is the curvature, and y is the CW at the} inflection point.

Before pooling all the monthly data to fit equation (4), a Kruskal-Wallis' one-way analysis of variance (SAS, Version 8.02) was used to examine the randomness of the monthly data, which were assumed to be from one population. The proportion of gravid females did not vary significantly from month to month (X2 (o.o5,3) = 0.547, P > 0.05). Therefore, we assumed all samples were randomly collected from the same population, and all monthly samples were combined by size intervals to fit the logistic curve (equation 4).

The gravimetric method was used to estimate number of newly deposited eggs extruded by gravid females. The diameters of 30, randomly selected eggs were measured. The relationships between the number of eggs or the weight of the extruded eggs and female carapace width or weight can be expressed by:

Y = a .X, (5)

where Y denotes either the number of eggs or the weight of the eggs; X is either carapace width or the body weight of the female crab from whence the eggs came; and ot and 13 are estimated parameters.

RESULTS

Growth

The von Bertalanffy growth equations with

seasonal fluctuations (Fig. 1) were:

(1) Males: Lt= 204.75

}

((p' =2.25);

(2) Females: L,

_194.25

(6)

}

((p' =2.28),

(7)

Loo

and K were the only parameters

that differed

between sexes. Female growth rate (K = 0.97/

year) was greater than male growth rate (K=

0.87/year), but males reached a larger asymp-

totic size (204.75 mm) than females (194.25

mm). Loo and K were significantly negatively

correlated (males: r = -0.826, P < 0.01;

females: r - -0.787, P < 0.01). Thus, females

reach maximum size in less time than males.

Furthermore, the results obtained by Bhatta-

charya's method indicated almost all monthly

samples might include two age-classes, and the

longevities of male and female P. sanguinolen-

tus are asymptotically over 11.4 and 10.2 years,

respectively, corresponding to the asymptotic

carapace widths (Lxo) in growth equations (6)

and (7), respectively.

Hence, to determine whether the growth rates

and asymptotic sizes of male and female P.

sanguinolentus differed, the test statistic, Hotel-

ling's T2 (Bernard, 1981), was computed.

Female and male Loo and K were significantly

different (reject Ho:

0.01; Table 1). However, the 99% Roy-Bose

confidence interval for _K(female)- K(male) in-

dicated that K is not significantly different

between sexes, but the 99% confidence interval

for Loo(female) - Loo(male) is different between

sexes. In conclusion, females do not grow

significantly faster than males, but they do

achieve their maximum size, which is signifi-

cantly smaller than the maximum size of males,

in significantly less time (CW range: males 90-

193 mm and females 68-182 mm).

Mortality

Based on the carapace width-frequency

distribution (Table 2), the size-converted catch

250 - 200 - --- Females Males ,. 150 -S H 100 50 0 1 2 3 4 Age (years)

Fig. 1. Seasonal von Bertalanffy growth curves based on carapace width frequency data for male (-) and female (- - -) P. sanguinolentus.

curve was used to estimate instantaneous

total mortality for both sexes (Fig. 2). The

instantaneous total mortality rate (Z) was 3.16/

year for males and 3.37/year for females. Using

Pauly's empirical equation and an average

annual habitat temperature

of 23?C, the natural

mortality rate (M) was 1.65/year for males and

1.8/year for females. The fishing mortality rate

(F) was 1.51/year for males and 1.57/year for

females, and the exploitation rate (E) was 0.48

Table 1. Hotelling's T2 calculation to test for equality of parameters Lxo and K for male and female P. sanguinolentus in the waters off northern Taiwan. HO: O(female) = ((male) versus HI: )(female) #7 {(male)-

Parameters Female Male

Loc 194.25 204.75

K 0.97 0.87

Correlation matrix [ -0.787 [ 0826

-0.787 1 -0.826 1

?Variancecovariance Varlance-covaance matrix matrlx 8887.949 -179.5791 [15623.171 -192.126

L[-179.579 5.861 -192.126 3.462

F

-183.508 5.110 A * [ 1 [Kim [-10.5] [ K female [ K male - [ 00 Fo.01,2,4799 = 6.604 > Fo.01,2,o = 4.605

To0.ol,2,47992 13.210 > To.o01,2,o2 = 5.991 Conclusion: reject Ho Critical F

-20.403 < Lo(female) - Lo(male) < -0.597 3.554 Roy-Bose intervals (99% CI)

JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 23, NO. 3, 2003

Table 2. Size-converted catch curves for male (upper panel) and female (lower panel) P. sanguinolentus based on carapace width frequency data.

Carapace width Age (t) at Age (t) change Age (t) at mid

CWI-CW2 (mm) Mid CW (mm) Number CWl (years) (years) CW (years) In(N/At) Males 85-95 90 2 0.673 0.107 0.726 2.932 95-105 100 29 0.779 0.093 0.826 5.741 105-115 110 180 0.873 0.090 0.918 7.596 115-125 120 307 0.963 0.098 1.012 8.055 125-135 130 235 1.060 0.120 1.120 7.582 135-145 140 236 1.180 0.189 1.275 7.132 145-155 150 278 1.369 0.316 1.527 6.781 155-165 160 162 1.684 0.232 1.800 6.548 165-175 170 61 1.917 0.246 2.040 5.512 175-185 180 10 2.163 0.659 2.513 2.659 185-195 190 4 2.752 0.743 3.102 1.743 Females 65-75 70 4 0.375 0.130 0.440 3.426 75-85 80 23 0.505 0.138 0.574 5.115 85-95 90 23 0.643 0.113 0.699 5.317 95-105 100 111 0.756 0.097 0.804 7.044 105-115 110 496 0.853 0.093 0.899 8.582 115-125 120 609 0.946 0.100 0.996 8.716 125-135 130 650 1.046 0.123 1.107 8.572 135-145 140 660 1.169 0.199 1.268 8.108 145-155 150 500 1.367 0.343 1.539 7.285 155-165 160 178 1.710 0.256 1.838 6.546 165-175 170 39 1.966 0.361 2.146 4.684 175-185 180 5 2.326 -2.326 1.163 1.864

for males and 0.47 for females. To determine

total mortality rate (Z) of male and female P.

sanguinolentus differed, the analysis of co-

variance (ANCOVA) using age as the covariate

revealed that there is a statistically significant

difference (P < 0.0001). Accordingly, total

mortality rates were higher for females than

males.

Reproduction

Based on the female proportion in the

population (the number of females in total

catch) estimated from monthly samples, nearly

all crabs caught in February and March were

female (Fig. 3). Carapace width ranged from 90

mm to 193 mm for males and from 68 mm to

182 mm for females (Fig. 4). From 20% to 30%

of the females caught each month were gravid

and had extruded eggs (Fig. 4). The monthly sex

ratio (the number of females divided by the

number of males) was tested using X2 test, and

the value was highly significant

(X

= 1300.54 >

X o.ooo1,o1

= 35.557) to reject the null hypothesis

that the sex ratio is 1:1.

The proportion of gravid females among

monthly samples was not significantly different

(Kruskal-Wallis,

= 0.547, P > 0.05).

Therefore, the samples were pooled and a logis-

tic curve (Fig. 5) was:

1

P(L)- 1 +

(8)

Further, the mean carapace width for ovigerous

females

was obtained as 135.27 mm

from equation (8) assuming P(L) = 0.5.

The extruded egg mass was detached from

the abdomen of each female, and the eggs were

counted. The number of eggs ranged from

405,375 to 2,438,645 (average 1,075,857). Egg

diameter ranged from 234 gtm to 297 itm, with

a mode of 270 jtm (n -3420). Additionally, the

relationship between the number and weight of

the eggs in each extruded egg mass were

exponentially related to female carapace width

and weight (Table 3). Egg number and weight

were significantly positively correlated with

body weight and carapace width (n =- 117, each

P < 0.01).

DISCUSSION

Growth

In this study, seasonal fluctuation was in-

corporated into estimates of P. sanguinolentus

Males

growth

_in

the waters off

_{northern Taiwan.}

Asymptotic carapace widths for males and

females were estimated as 204.75 mm and

194.25 mm, respectively. These estimates are

?-

a*

greater than those for P. sanguinolentus (163

mm and 173 mm for males and females,

respectively) from waters off the southern

o

_{Kanara Coast, India (Sukumaran et al., 1986).}

However, estimated growth rates (males: 0.87/

0.5 1...

year; females: 0.97/year) in this study were

Relative age (year-t) 5

much lower than those (3.54/year for both

sexes) estimated by Sukumaran et al. (1986).

Females

_{The growth}

performance

indexes for

y=-3.3657x+ 12.307

population ((p'

2.76 for males and 2.79 for

-0

present study, but both were located at the

-

_{reasonable range (2 < (p' < 3) within the same}

^-

\""~

?

R2=0'9634family

(Pauly and Munro, 1984). Defeo and

0-

? ?

Cardoso (2002) indicated that environmental

o-

Dfactors,

such as water temperature, may affect

crab growth rate. Crab growth may be faster in

?

_{warm water than in cool water (Leffler, 1972).}

As it is so, environmental

differences may result

0 0.5 1 1.5 2 2.5 3 in the discrepancies of growth of P. sanguino- Relative age (year-to)

lentus from those two waters indicated above.

Growth rate differences between males and

'. Size-converted catch curves of male (upper _{panel) females result mainly from the greater repro-} female (lower panel) P. sanguinolentus. The total _{ductive output of females. When crabs become} taneous mortality rates (Z) were estimated from the _{ual mtu o o en cras H}

of the regression line (solid squares); data points

sexually mature, growth often decreases (Hart-

led in the regression line because of data from mean

noll, 1982) because of the significant amount of

with very small sample size (less than 10) and the

_{energy used for reproduction.}

_{In this study, the}

_{for females is 135.27 mm}

,

Z = 3.155/ear and for females, and for females, 3.3657/year Z .

carapace width (about 1.17 years).

females grew slowly (Fig. 1) and seemed

coincident with Hartnoll's points. With greater

100

80

60

40

20

0

Nov. Dec. Jan. Feb. Mar.

2001

Oct. Nov. Dec. Jan.

2001

2002

Fig. 3. Monthly change in the P. sanguinolentus female proportion in the population, October 2000 to January 2002. 9 8 7 6

1

Oct.

2000

JOURNAL OF CRUSTACEAN BIOLOGY. VOL. 23. NO. 3. 2003 Female, 2000 October n=50 Male n=278 November n=120 _-g n=130 December n=398 >, n=161 Female, 2001 January n=274 Male n=238 February n=284 _ L i n=12 March - n=751 2 4 n=24 March _n=304 n=3 October n=341 n=142 November n=1 18 n=242 December n=313 n=177 Female, 2002 January n=345 Male n=97 65 75 85 95 105 115 125 135 145 155 165 175 185 65 75 85 95 105 115 125 135 145 155 165 175 185

Carapace width (mm)

Fig. 4. The frequency distribution of carapace widths of male and female P. sanguinolentus. Solid squares denote gravid females and numbers indicate samples sizes of gravid females.

110 0 60

6

z

>-t

Q

(D

O

cr

0

1

0.8

40 80 120 160 200

Carapace

width

(mm)

Fig. 5. A logistic curve of gravid female P. sanguinolentus and dotted line indicate the carapace width of proportion 0.5 corresponding to L50%.

investment in reproduction, females may be

smaller than males at maturity. Reproductively

active females typically postpone growth, and

their growth rates often lag behind those of

males (Cobb and Caddy, 1989). In this study,

the asymptotic size of female P. sanguinolentus

was smaller than that of males, and, overall,

their growth was significantly slower. This

supports the hypothesis that differences in

reproductive

output may account for differences

in male and female growth (Cobby and Caddy,

1989; Hartnoll, 1982). However, the asymptotic

size of females was larger than that of males in

Indian population. This difference depends on

the observed maximum carapace width due to

the different sample location between Indian

waters and the waters off North Taiwan.

Gayanilo and Pauly (1997) proposed that the

maximum predicted size may be equivalent to

95% of the estimated asymptotic size. Accord-

ing to this viewpoint, the maximum predicted

carapace width in the present analysis should

be 194.5 mm and 184.5 mm for males and

females, respectively, compared with the ob-

served maximum carapace widths that were 193

mm and 182 mm CW for males and females,

respectively, indicating that the present growth

estimations and the corresponding predicted

longevities are reasonable.

Mortality

Total mortality rates for P. sanguinolentus in

the waters off northern Taiwan (males: Z =

3.16/year; females: 3.37/year) were much great-

er than those obtained for P. sanguinolentus in

the Indian Ocean (males: Z = 0.78/year;

females: 0.79/year; Sukumaran et al., 1986).

Part of these differences may be attributed to

different methods of estimation and temper-

atures (Leffler, 1972). The exploitation rates

(males: E = 0.48; females: E = 0.47) indicated

that natural and fishing losses contributed

equally to the decrease in the P. sanguinolentus

population off northern Taiwan. During this

study, P. sanguinolentus natural mortality was

high. This is typical of r-selected species

(Gunderson, 1980), which mature early and

have high fecundity, short life spans, and small

body size. However, these population traits may

be affected by temperature and latitudinal

variation (Defeo and Cardoso, 2002).

The size-converted catch curve is strongly

influenced by population structure such as size

at recruitment

and sample size, which affect the

goodness of fit and the slope of the regression

line (cf. equation 2). There is no information

about P. sanguinolentus recruitment to the crab

pot fishery in the waters off Taiwan. Thus, the

32 C4. 0 0 04 0.6 - 0.4- 0.2- 0 0

JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 23, NO. 3, 2003

Table 3. The percentage of gravid female P. sanguinolentus from October 2001 to January 2002 by carapace width class. October, 2001 November, 2001 December, 2001 January, 2001 Pooled Carapace width

class (mm) No. Percent (%) No. Percent (%) No. Percent (%) No. Percent (%) No. Percent (%)

91-100 1 0.98 0 0 0 0 1 1.52 2 0.71 101-110 0 0 0 0 2 2.15 6 9.09 8 2.83 111-120 0 0 1 4.55 6 6.45 19 28.79 26 9.19 121-130 5 4.9 3 13.64 19 20.43 21 31.82 48 16.96 131-140 39 38.24 5 22.73 22 23.66 12 18.18 78 27.56 141-150 40 39.22 8 36.36 21 22.58 5 7.58 74 26.15 151-160 14 13.73 5 22.73 15 16.13 2 3.03 36 12.72 161-170 3 2.94 0 0 7 7.53 0 0 10 3.53 171-180 0 0 0 0 1 1.08 0 0 1 0.35

A Kruskal-Wallis test was used to assess the homogeneity of the percentage of gravid females across monthly samples: x2 (0.05, 3) =0.5467, (P >0.05, not significant)

total mortality rate was estimated for males and

females separately based on carapace width

frequency data (Table 2; Fig. 2), which were

measured from all crabs caught during each trip

to increase satisfactorily the sample size avail-

able in the present analysis.

Reproduction

In the monthly samples, female P. sanguino-

lentus outnumbered males. The inequality of

female proportion in the population or sex ratio

may result from the depth at which most

samples were collected. Females were relatively

more abundant around 80 m in depth, while

males were more abundant from 40 m to 60 m.

In addition, the proportion of females varied

from year to year. This also was observed in the

waters of Queensland, Australia (Sumpton et al.,

1989). Because the proportion of mature

females increases significantly with increased

depth (Wenner, 1972), offshore samples contain

larger proportions of gravid females than

samples from coastal areas.

For the Queensland population, Campbell

and Fielder (1986) found that the female

matured at 75 mm carapace width. Sumpton

et al. (1989) found the smallest mature male and

female were 83 mm and 74 mm carapace width,

respectively, and the smallest female with

recently implanted spermatophores

was 94 mm

CW. In this study, the observed smallest

ovigerous female was 96 mm CW, which is

similar to the smallest female with recently

implanted spermatophores in the Queensland

population.

Egg number and size are significantly

correlated

with female crab weight and carapace

width. In the waters off northern

Taiwan, gravid

female P. sanguinolentus had from 4.1 X 105 to

2.44 X 106 eggs. The mode diameter was 270

ptm. These numbers were very similar to those

for gravid female P. sanguinolentus in Indian

waters, which had from 9.6 X 105 to 2.25 X 106

eggs (Ryan, 1967), and greater than estimates

of 4.4 X 104 to 1.19

106 eggs for P.

sanguinolentus in the waters off Kamataka,

India (Sukumaran

and Neelakantan, 1997). The

discrepancy may be affected by environmental

factors, including predation, parasitization, and

temperature, which may affect the balance

between the optimal number and size of eggs

(Smith and Fretwell, 1974; Lawlor, 1976).

This study is the first on the growth,

mortality, and reproduction of P. sanguinolen-

tus in Taiwan. However, validation of growth

estimates has not been undertaken

because hard-

parts are lost during molting. As noted, age

determination

is absolutely necessary for nearly

all studies of population dynamics (Wolff and

Soto, 1992; Marques et al., 1994). Therefore,

studies of the growth of cultivated crabs and

tagged wild crabs are needed to develop and

validate methods of age determination. In

mortality analysis, biased estimates of growth

parameters

seriously affect estimates of popula-

tion parameters, such as the instantaneous

natural mortality rate (Lai and Gunderson,

1987; Lai et al., 1996). Hence, more reliable

estimates of natural mortality rate are still

required. Further research is urgently needed

on other population dynamics studies such as

yield per recruit model and length-based models

analyses for estimating crab recruitment and

abundance.

ACKNOWLEDGEMENTS

We appreciate the National Science Council, Taiwan, for financially supporting this project with grant NSC89-2313- B-002-167 to C.-C. Hsu, and Ms. Stacy Kao, Research Assistant at the Institute of Oceanography, National Taiwan 698

University, for collecting and measuring crabs. We also thank the anonymous reviewers for their very constructive and critical comments.

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