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Fisheries Research 108 (2011) 299–306

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Fisheries Research

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iological information and population features of mud crab, genus Scylla fromangrove areas of Sarawak, Malaysia

. Ikhwanuddina,∗, G. Azmiea, H.M. Juariaha, M.Z. Zakariab, M.A. Ambakb

Institute of Tropical Aquaculture, Universiti Malaysia Terengganu, Mengabang Telipot, 21030 Kuala Terengganu, Terengganu, MalaysiaFaculty of Agrotechnology and Food Science, Universiti Malaysia Terengganu, Kuala Terengganu, Terengganu, Malaysia

r t i c l e i n f o

rticle history:eceived 19 July 2010eceived in revised form 2 January 2011ccepted 5 January 2011

eywords:iological informationopulation featuresud crabenus Scylla

a b s t r a c t

This study was conducted to determine the size at maturity, size at mating, sex ratio and carapace width(CW) to body weight (BW) ratio of the most abundant mud crab species sampled in mangrove areas ofSarawak, Malaysia. Additionally, the taxonomic statuses of Scylla spp. mud crabs from this area wereassessed. A total of 14,000 mud crabs representing different sizes, sexes and species were examined todetermine taxonomic status and sex ratio for each. In addition, 2000 mud crabs were randomly measuredto determine the CW–BW relationship. A total of 1724 female crabs and 1230 male crabs of various sizeswere randomly sampled to determine the size at maturity and size at mating. The results indicate that thesize at maturity of female S. olivacea and S. tranquebarica were 8.6 cm CW and 9.2 cm CW, respectively.The size at maturity of male crabs is very similar but could not be confirmed because all S. olivacea andS. tranquebarica individuals sampled were already mature. Size at mating for female and male S. olivaceawas 9.6 cm CW and 11.3 cm CW, respectively. For female and male S. tranquebarica, size at mating was10.8 cm CW and 13.1 cm CW, respectively. S. olivacea was the most common mud crab species in this area,comprising 77.64% of total species, followed by S. tranquebarica (21.91%) and S. paramamosain (0.45%).S. serrata were not found during this study. The sex ratios (male:female) for S. olivacea and S. tranque-barica were 1:0.89 and 1:0.75, respectively. The BW increment rate for male crabs of S. olivacea and S.

tranquebarica were higher relative to that for female crabs. The CW–BW relationship shows that S. tran-quebarica was heavier than S. olivacea. The study also indicates that the selective fishing of mature crabsat a particular site where S. olivacea and S. tranquebarica occur together can cause negative impacts on thepopulations of these species by affecting size at maturity, size at mating and population dynamics. Thebiological information and population features from this study can be used in sustainable management

rab r

and exploitation of mud c

. Introduction

The mud crab of the genus Scylla is common throughout thendo-Pacific region and ranges from the mud flats of littoral zonesnd parts of the supralittoral and intertidal zones of mangroveorests (Keenan et al., 1998). Scylla spp. is also a valuable componentf artisanal fisheries in most countries in the region (Le Vay, 1998).hese crabs are highly valued and provide an important source ofncome for small-scale fishers throughout this region (Yalin andingsheng, 1994). Mud crab fisheries in Malaysia are limited to

stuaries and coastal areas that contain mangroves. Traditionally,he mud crab was exploited by local individuals for immediateonsumption. However, since the first commercial mud crab aqua-ulture operations began in Malaysia in 1991, the mud crab has

∗ Corresponding author. Tel.: +60 9 668 3638; fax: +60 9 668 3390.E-mail address: ikhwanuddin@umt.edu.my (M. Ikhwanuddin).

165-7836/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.fishres.2011.01.001

esources in Malaysian coastal waters and the South China Sea.© 2011 Elsevier B.V. All rights reserved.

gradually entered local markets and become a main componentof the local crab fishery. The crab fishery in Malaysia is expected tocontinue to grow in the future because of increasing demand for thecrabs. However, the expansion of the mud crab fishery should pro-ceed cautiously to ensure the sustainability of the fishery resourcesand prevent overexploitation.

Despite the increasing interest in mud crab farming, little infor-mation exists on mud crab biology and the crab fishery in general.Previous studies indicate that no assessments exist concerning mudcrab catches in the coastal waters of Malaysia. Therefore, this studywas conducted to further the understanding of the basic biology ofmud crabs and to assist in the management of mud crab resourcesfor capture fishery and aquaculture operations. The most important

biological and fishery information that was lacking was determinedto be the taxonomic status of mud crabs of the genus Scylla inMalaysian coastal waters of the South China Sea, size at maturity,mating size, sex ratio and the CW–BW relationship of the mostabundant mud crab species.

300 M. Ikhwanuddin et al. / Fisheries Re

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ig. 1. The study site at the Sematan mangrove forest, Sarawak, Malaysia coastalater of South China Sea.

. Materials and methods

.1. Crab sampling

The study was conducted from June 2000 to July 2001 in theematan mangrove forest, Sarawak, Malaysia, within a total area ofpproximately 1735 ha (Fig. 1). All crab samples were taken fromhis location, and the crabs were obtained from local fisherment the market place. A total of 14,000 mud crabs were measuredhroughout the study period and used to determine taxonomictatus and sex ratio. From the total crabs sampled, 2000 crabsf various species, sexes and sizes were randomly measured toetermine the CW–BW relationship. The CW–BW relationship wasalculated separately for both sexes and species. A total of 1724emale crabs and 1230 male crabs of various sizes were randomlyampled from two species, S. olivacea and S. tranquebarica, to deter-ine the female and male size at maturity and size at mating.

.2. Crab taxonomic status, morphometrics and sexes

The species identity of each crab from the genus Scylla wasetermined using the taxonomic guide by Keenan et al. (1998).he morphological characteristics used to determine the speciesdentity of adult mud crabs are described in Table 1. Crab size

as measured using the carapace width (CW), or the distance

etween the tips of the ninth antero-lateral spines of the carapaceFig. 2). The carapace width was measured to the nearest 0.1 cmith Vernier callipers. The live weight of the crabs was measured to

he nearest gram using a digital electronic balance with sensitivityo 0.1 g. The crabs were separated by sex based on the shape of the

able 1orphological characteristics useful in determining species identity of adult mud crab, g

Species Frontal lobe spines

Shape Height

S. serrata Blunted point HighS. tranquebarica Blunted ModerateS. paramamosain Triangular Moderately highS. olivacea Rounded Low

search 108 (2011) 299–306

abdomen. The female crab has a wider and more globular abdomenwhile the male crab has a narrow and straight abdomen (Fig. 3). Inyounger females, the abdomens were triangular (Poovachiranon,1992).

2.3. Size at maturity and mating

All crabs samples were measured and sexed. Female crabs thathad undergone the pubertal (or maturity) moult that causes awidening and darkening of the abdomen were classified as maturefemales (Arriola, 1940; Heasman, 1980) (Fig. 3). All other femaleswere recorded as immature. Male maturity could not be deter-mined from external characteristics, so male crabs were dissectedand examined in the laboratory. A section of the anterior vas defer-ens (AVD) of male crabs was teased out in a drop of water on a glassslide and examined under a microscope (magnification 100×) forthe presence of spermatophores containing sperm (Robertson andKruger, 1994). The presence of spermatophores indicates that thespecimen could be classified as a mature male.

The female crab samples were examined for external evidenceof mating in the form of two groove-like depressions immediatelyposterior to the genital openings on the ventral surface of the tho-rax (Robertson and Kruger, 1994). These groove-like depressionswere produced by the pressure of the male pleopods on the softexoskeleton of the female during copulation. In addition, the firstpair of walking legs of all male crabs were examined for the matingscars that occur only in mature males (Perrine, 1978). These scarsare formed by the rubbing of the female carapace on the legs of themale during the pre-copulatory embrace.

3. Results

3.1. Taxonomic status and sex ratio

The results from the fishery catch show that the species com-position of mud crabs was dominated by S. olivacea, representing77.64% of the total catch. The sex ratio (male:female) of this specieswas 1:0.89, or 41.06% male and 36.58% female. S. tranquebarica wasthe second most common mud crab species, representing 21.91%of the total catch and having a sex ratio of 1:0.75, or 12.54% maleand 9.38% female. S. paramamosain was the most uncommon mudcrab species in this area, representing only 0.45% of the total catch(Table 2). The sex ratio for S. paramamosain was not determineddue to insufficient data.

3.2. The carapace width (CW)–body weight (BW) relationship

The relationship between body weight and carapace width ofmud crab species sampled shows that S. tranquebarica are gener-ally heavier than S. olivacea for both sexes (Table 3). The CW–BWrelationship for females and males of S. olivacea was estimated

as: y = 11.24e0.2639x (R2 = 0.735; n = 751) and y = 4.7959e0.3777x

(R2 = 0.8608; n = 665), respectively (Fig. 4). The CW–BW relation-ship for females and males of S. tranquebarica was estimatedas: y = 11.563e0.2588x (R2 = 0.8892; n = 270) and y = 5.767e0.3426x

(R2 = 0.8743; n = 312), respectively (Fig. 4). The increment rate in

enus Scylla (Keenan et al., 1998).

Cheliped

Carpus spines Propodus spines

Both obvious ObviousBoth obvious ObviousInner absent, outer reduced ObviousInner absent, outer reduced Reduced

M. Ikhwanuddin et al. / Fisheries Research 108 (2011) 299–306 301

Fig. 2. Diagram of carapace width (CW), which is the distance between tips of the ninth antero-lateral spines of the carapace.

Fig. 3. Sex differentiation of mud crab. (a) Matured male; (b) immature female; (c) matured female.

Table 2Percentage of composition and sex ratio of mud crab species sampled from Sematan mangrove forest, Sarawak, Malaysia.

Species/sex Total % % by species Sex ratio (male:female)

S. olivacea (male) 5748 41.06 77.64 1:0.89S. olivacea (female) 5121 36.58S. paramamosain (female) 27 0.19 0.45S. paramamosain (male) 36 0.26S. tranquebarica (female) 1755 12.54 21.91 1:0.75S. tranquebarica (male) 1313 9.38

Total 14,000 100.00 100.00

Table 3Relationship between body weight and carapace width of mud crab species sampled from Sematan mangrove forest, Sarawak, Malaysia.

S. olivacea S. tranquebarica

Female Male Female Male

CW (cm) BW (g) CW (cm) BW (g) CW (cm) BW (g) CW (cm) BW (g)

Mean 9.41 145.78 9.24 177.86 10.48 191.47 9.74 190.53Max. 15.30 575.00 13.00 619.60 15.70 447.70 14.40 702.30Min. 5.30 20.10 6.20 47.50 6.70 47.54 6.10 39.70sd 1.31 58.64 1.20 95.07 1.62 83.17 1.52 121.02n 751.00 751.00 665.00 665.00 270.00 270.00 312.00 312.00

N 1416 582

302 M. Ikhwanuddin et al. / Fisheries Research 108 (2011) 299–306

apace

tSpd

3

Sa

TS

TL

Fig. 4. Morphometric relationship between body weight and car

he BW of male crabs was larger than for female crabs for both. olivacea and S. tranquebarica. The CW–BW relationship for S.aramamosain was not determined because there was insufficientata.

.3. Size at maturity

Only 593 female crabs of S. olivacea and 416 female crabs of. tranquebarica were considered to be mature based on theirbdomens (Table 4). The largest immature S. olivacea female

able 4ize at maturity and mating in female crabs and size at maturity in male crabs (cm).

Species S. olivacea

Female Male

Maturedabdomen

Immatureabdomen

Matingmarks

Matureabdom

Mean 10.2 8.4 10.2 8.5Max. 15.3 12.7 12.9 11.5Min. 6.2 6.8 8.0 6.7n 593 536 108

Total sample 1129 153

able 5ength frequency of female crab size at maturity and size at mating.

Species Size range (cm)

6.0–6.9 7.0–7.9 8.0–8.9 9.0–9.9 10.0–10.9 11.0

S. olivaceaSize at maturity

Freq. 1 7 50 175 232 107% 0.2 1.2 8.4 29.5 39.1 18.0

Size at matingFreq. 5 41 42 17% 4.6 38.0 38.9 15.7

S. tranquebaricaSize at maturity

Freq. 0 1 27 49 78 95% 0.0 0.2 6.5 11.8 18.7 22.8

Size at matingFreq. 0 0 2 9% 0.0 0.0 5.6 25.0

width of S. olivacea and S. tranquebarica (a) female and (b) male.

encountered was 12.7 cm CW. The largest immature S. tranque-barica female was 12.8 cm CW. The smallest mature femaleswere 6.2 cm CW and 7.7 cm CW for S. olivacea and S. tran-quebarica, respectively. Size at maturity for females based onmature abdomens was most frequently found for the size range

of 10.0–10.9 cm CW for S. olivacea and 12.0–12.9 cm CW for S.tranquebarica (Table 5). Using the widening and darkening of thefemale’s abdomen as an indicator of crab maturity, the percentageof mature crabs was calculated for each 1.0 mm CW interval. Thesize when 50% of crabs were sexually mature was determined to

S. tranquebarica

Female Male

den

Maturedabdomen

Immatureabdomen

Matingmarks

Maturedabdomen

11.4 9.1 12.2 9.015.7 12.8 13.8 13.9

7.7 6.8 10.5 6.9416 179 36

595 98

Total

–11.9 12.0–12.9 13.0–13.9 14.0–14.9 15.0–15.9

17 3 0 1 5932.9 0.5 0.0 0.2 100.0

3 0 1082.8 0.0 100.0

109 48 8 2 41726.1 11.5 1.9 0.5 100.0

18 7 3650.0 19.4 100.0

M. Ikhwanuddin et al. / Fisheries Research 108 (2011) 299–306 303

Table 6Length frequency of male crab size at maturity and size at mating.

Species Size range (cm) Total

6.0–6.9 7.0–7.9 8.0–8.9 9.0–9.9 10.0–10.9 11.0–11.9 12.0–12.9 13.0–13.9 14.0–14.9

S. olivaceaSize at maturity

Freq. 3 60 35 29 23 3 0 0 153% 2.0 39.2 22.9 19.0 15.0 2.0 0.0 0.0 100.0

Size at matingFreq. 11 25 24 12 6 0 0 78% 14.1 32.1 30.8 15.4 7.7 0.0 0.0 100

S. tranquebaricaSize at maturity

Freq. 2 27 32 16 10 8 1 2 98% 2.0 27.6 32.7 16.3 10.2 8.2 1.0 2.0 100.0

Size at matingFreq . 1 2 7 3 6 5 6 30

% 3.3 6.7 23.3 10.0 20.0 16.7 20.0 100.0

Table 7Size at mating in male crabs (cm).

Species S. olivacea S. tranquebarica

Mating scars present Mating scars absent Mating scars present Mating scars absent

Mean 10.2 8.8 12.1 9.6Max. 12.9 12.9 14.6 13.5Min. 8.1 6.4 8.6 7.1

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Total sample 895

e an appropriate measure of the size at maturity in females (SMy-50) (Quinn and Kojis, 1987). SMy-F50 occurred around 8.6 cm CWor S. olivacea females and 9.2 cm CW for S. tranquebarica femalesFig. 4).

All male crabs examined were mature based on the pres-nce of spermatophores in the anterior vas deferens (AVD). Themallest mature male sampled was 6.7 cm CW for S. olivaceand 6.9 cm CW for S. tranquebarica (Table 4). Size at maturityn males was most frequently found within the size range of.0–7.9 cm CW for S. olivacea, and 8.0–8.9 cm CW for S. tranque-arica (Table 6). The size when 50% of male crabs were sexuallyature was not determined because all male crabs examined wereature.

.4. Size at mating

Mating depressions in females were most frequently found onrabs ranging in size from 9.0 to 10.9 cm CW for S. olivacea and2 to 12.9 cm CW for S. tranquebarica (Table 5). Mating depres-

Fig. 5. Female crab (a) size at maturity and (b) size at mating for S. olivacea and S. t

30 305

335

sions were found on 108 of the 593 mature S. olivacea femalesand 36 of the 416 mature S. tranquebarica females (Table 4).The size range for females bearing mating scars was between8.1–12.9 cm CW for S. olivacea and 8.6–14.6 cm CW for S. tran-quebarica (Table 7). The CW size at which 50% of crabs were ableto mate was used in determining size at maturity. The size atfirst mating in females (SMg-F50) was determined to be approxi-mately 9.6 cm CW for S. olivacea and 10.8 cm CW for S. tranquebarica(Fig. 5).

The smallest crab size for males at mating was 8.1 cm CW for S.olivacea and 8.6 cm CW for S. tranquebarica. The largest unmatedmale was 12.9 cm CW for S. olivacea and 13.5 cm CW for S. tran-quebarica (Table 7). Mating scars in males were most frequentlyfound on crabs in the size range of 9.0–9.9 cm CW for S. olivacea and

10.0–10.9 cm CW for S. tranquebarica (Table 6). The size at mating inmales (SMg-M50) was determined using the aforementioned CWsize at which 50% of crabs were mating (Fig. 6). SMg-M50 occurredat approximately 11.3 cm CW for S. olivacea and 13.1 cm CW for S.tranquebarica (Fig. 6).

ranquebarica sampled from Malaysian coastal waters of the South China Sea.

304 M. Ikhwanuddin et al. / Fisheries Re

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ig. 6. Male crab size at mating for S. olivacea and S. tranquebarica sampled fromalaysian coastal waters of the South China Sea.

.5. Discussion

According to (Keenan et al., 1998), both S. olivacea and S. tranque-arica are common mud crab species in the South China Sea and inther locations around the Indo-Pacific. In addition, the distributionf these two species overlaps (Keenan et al., 1998), and results fromhe present study confirm this pattern. Keenan et al. (1998) alsouggest that S. paramamosain is the most abundant species of Scyllan locations where this species occurs. However, the most common

ud crab species in the Sematan mangrove forest were S. olivaceand S. tranquebarica, while S. paramamosain was the most uncom-on, a result that differs from Keenan et al. (1998). The species

ad been positively identified on the continental coast of the Southhina Sea, which includes the coast of South-eastern China (Mat al., 2006) and coastal waters of Mekong, Vietnam (Macintosht al., 2002). S. paramamosain was also identified in coastal watersther than those of the continental coast of the South China Sea,uch as the Java Sea (Keenan et al., 1998).

No individuals of the mud crab species S. serrata were founduring this study. Evidence from this study shows that S. serrata

s not normally found on the continental coast of the South Chinaea. Other than the South China Sea, the species has been positivelydentified at a wide range of locations in the Indo-Pacific (Keenant al., 1998). The crab occurs naturally from South Africa to Tahiti,orth to Okinawa, and south to Port Hacking in Australia and theay of Islands, New Zealand (Keenan et al., 1998). The exploitationf mud crab populations has been increasing. The sustainability ofhe fishery and its affect on mud crab abundance must be deter-

ined (Le Vay, 1998). Results from this study suggest that removalr overexploitation of one species may affect other species thatccur in this area. Therefore, an effective management plan muste created to maintain the mud crab fishery and ensure its sustain-bility.

The sex ratio (male:female) of the mud crabs sampled were:0.89 for S. olivacea and 1:0.75 for S. tranquebarica, and more malerabs were captured than female crabs. This pattern likely emergedecause the study was carried out during the monsoon season. Sea-onal peaks in offshore migration by females have been recordedn many Scylla populations, and the timing of spawning appears toe linked to environmental factors such as temperature and salin-

ty fluctuations (Le Vay, 1998). The low occurrence of female crabsuring the monsoon season is explained by the fact that female

igrate from inshore areas to offshore areas to spawn, as noted byill (1994) and Knuckey et al. (1995). This result suggests that S.

ranquebarica do migrate offshore during spawning season, in theame manner as S. serrata, but S. olivacea females do not migrate.he number of S. tranquebarica females was reduced, resulting in a

search 108 (2011) 299–306

sex ratio (male:female) of 1:0.75, as compared to S. olivacea, whichhad a sex ratio (male:female) of 1:0.89. This study also suggeststhat S. tranquebarica ranges freely while S. olivacea is restricted tointertidal areas.

Difficulties may arise when measuring spiny crab species likethe mud crab because the crab’s extremities can easily be broken.Therefore, it is useful to convert BW into CW when only the BWis known, or to use CW–BW regression to estimate CW from BWbecause of the difficulties in handling these crabs and the amountof time required to record CW. CW and BW are the most frequentlyused dimensions in the study of crustaceans. The CW–BW rela-tionship can be used in condition indices and ontogenetic analysesand in the study of fish or crustacean population dynamics. TheCW–BW relationship also can be used to indicate the condition orestimate the recovery of edible meat from crabs of various sizes, orto calculate crab biomass. Because CW–BW relationship has practi-cal value, it is possible to convert length into weight and vice versa.Crab populations can also be managed so that fisherman returnsthe undersize crabs to their habitats.

Sexual maturity is defined as the ability to successful mateand extrude of fertilised eggs (Elner and Beninger, 1989 cited byRobertson and Kruger, 1994). The pubertal moult stage in Scyllaspp. is said to be a reliable indicator of the ability of female crab tomate (Robertson and Kruger, 1994). Studies have also shown thatthe females of Scylla spp. will moult again after the pubertal moult(Heasman, 1980; Ong, 1966; Quinn and Kojis, 1987; Robertsonand Kruger, 1994). A detailed reproductive study was not possi-ble within the scope of this investigation for all four species ofScylla. However, it has been suggested that some of these femalecrabs pass through at least two mature instars. Previous studieshave found no comparable data on the reproductive biology of S.olivacea and S. tranquebarica, and very little data on S. paramamo-sain. Studies on the reproductive and larval culture biology of S.paramamosain are also rather limited (Hoang, 1999; Li et al., 1999).Table 8 summarizes the available data on the size at which 50% ofScylla spp. crabs reach maturity.

According to Table 8, the SMy-female50 for unidentified Scyllaspp. is larger than the other two Scylla spp. (S. olivacea and S. tran-quebarica) in the study. The study also shows that the size at mating(SMg-50) was larger than the size at maturity (SMy-50) for bothmales and females of S. olivacea and S. tranquebarica. The SMg-50 was estimated based on the mating marks. Mating scars wereformed after the crabs had completed the mating process or copula-tory embrace. The existence of these scars indicates that the femalecrabs had undergone a true pubertal moult, of which the SMg-50is larger than SMy-50. The results from this study show that malesmature at a smaller size than female crabs. However, the maleswere larger than females during mating for both species.

As seen in Table 8, the SMy-female50 recorded in Papua NewGuinea was smaller (10.5 cm CW; Quinn and Kojis, 1987) than inSouth Africa (12.3 cm CW; Robertson and Kruger, 1994). This dif-ference in size could be related to the different criteria on whichsize at maturity was based and also being influenced by geograph-ical differences. In Papua New Guinea, Quinn and Kojis (1987)used colour and size as the indicator for gonad maturity to deter-mine SMy-female50. In South Africa, Robertson and Kruger (1994)used a method similar to that used in this study to determineSMy-female50. Unfortunately, the size at which 50% of crabs weremature was mostly unknown by Robertson and Kruger (1994), andwas therefore based on minimum size at maturity. The smallestmature female in Malaysia was recorded by Ong (1966) to be 9.9 cm

CW for Scylla spp. Poovachiranon (1992) and Quinn and Kojis (1987)suggested that the size at maturity for Scylla spp. is larger in higherlatitudes than lower latitudes. Robertson and Kruger (1994), how-ever, did not agree with Quinn and Kojis’s hypothesis because of thelarge difference in the minimum size at which females attained a

M. Ikhwanuddin et al. / Fisheries Research 108 (2011) 299–306 305

Table 8Size CW (cm) when 50% of crab at maturity based on two different criteria in Scylla sp. from various localities.

Locality Lat. Species SMy-50 SMg-50 Source

Female Male Female Male

Sarawak, Malaysia 1◦N S. olivacea 8.6 – 9.6 11.0 Present studySarawak, Malaysia 1◦N S. tranquebarica 9.2 – 10.8 13.0 Present studyPapua New Guinea 6◦S Scylla spp. 10.5 – – – Quinn and Kojis (1987)South Africa 29◦S Scylla spp. 12.3 9.2 – – Robertson and Kruger (1994)

SMy-50 = size when 50% of crab at maturity based on mature abdomen or colour and size of gonad for female and presence of spermatophores in AVD for male. SMg-50 = sizewhen 50% of crab at maturity based on mating depressions for female and mating scars for male.

Table 9Size at maturity based on 3 different criteria in female Scylla spp. from various localities.

Locality Lat. Species Matured abdomen Mated Ovigerous Source

n CW range (cm) n CW range (cm) n CW range (cm)

Sarawak, Malaysia 1◦N S. oliv. 593 6.2–15.3 108 8.0–12.9 94 7.2–12.3 Present studySarawak, Malaysia 1◦N S. tranq. 416 7.7–15.7 36 10.5–13.8 6 10.2–13.4 Present studyPenang, Malaysia 5◦N S. spp. 6 9.9–11.4 6 9.9–12.0 46 9.2–15.2 Ong (1966)South Vietnam 12◦N S. para. 54 9.6–16.9 Hoang (1999)Thailand 12◦N S. spp. 8.9 Poovachiranon (1992)Thailand 12◦N S. spp. 12 9.4–12.7 Varikul et al. (1972)India 13◦N S. spp. 12.0–? Pillai and Nair (1968)India 13◦N S. spp. 9.0–11.0a 20 11.7–14.0 Marichamy and Rajapackiam

(1992)Philippines 15◦N S. spp. 24 6.2–12.6 Lavina (1980)Papua New Guinea 6◦S S. spp. 8.5–19.0 Quinn and Kojis (1987)Australia 28◦S S. spp. 339 13.8–20.4 15 14.5–17.9 10 14.8–20.3 Heasman (1980)South Africa 29◦S S. spp. 1269 10.4–20.0 202 11.6–12.0b 9 12.9–17.0 Robertson and Kruger (1994)South Africa 34◦S S. spp. 3 14.3–14.8 21 13.7–16.1 Hill (1975)

S olivac

mSl

pdsybccoCsS1nm1g

TS

A

. spp. = Scylla spp.; S. tranq. = S. tranquebarica; S. para. = S. paramamosain; S. oliv. = S.a Basis for classification as mature not known.b Sperm present in spermathecae.

ature abdomen form in Australia (13.8 cm CW at 28◦S) and inouth Africa (10.4 cm CW at 29◦S), despite the almost identicalatitudes at which they occur (Table 9).

The common criterion for maturity in the Brachyuran is theresence of spermatozoa enclosed in spermatophores in the vaseferens (Hartnoll, 1969). However, in Scylla spp., as in otherspecies of crab, sperm production occurred in crabs that were notet capable of mating. 50% of male Scylla spp. were producing spermy the time they attained a CW of 9.2 cm, whereas the smallest malerab in which mating was evident was in the 11.5–11.9 cm CW sizelass (Robertson and Kruger, 1994). Robertson and Kruger’s (1994)bservation is similar to that found in this study, where the smallestW of S. olivacea and S. tranquebarica that were found to produceperm attained a CW of 6.7 cm and 6.9 cm, respectively. 50% of male. olivacea and S. tranquebarica that had mated were 11.0 cm and3.0 cm CW, respectively. The absence of mating scars does not

ecessarily mean that the male crabs have never mated becauseating scars can be lost during a moult (Robertson and Kruger,

994). This study and work by Robertson and Kruger (1994) sug-ested that crabs smaller than 6.4 cm CW (S. olivacea), 8.6 cm CW (S.

able 10ize at maturity based on 3 different criteria in male Scylla spp. from various localities.

Locality Lat. Species Sperm in AVD Mating o

n CW range (cm) n

Sarawak, Malaysia 1◦N S. oliv. 153 ?–11.5Sarawak, Malaysia 1◦N S. tranq. 98 ?–13.9Penang, Malaysia 5◦N S. spp. 3Philippines 15◦N S. spp. 11Australia 28◦S S. spp. 48 10.8–18.7 30South Africa 29◦S S. spp. 93 8.3–15.6 57South Africa 34◦S S. spp. 24

VD = anterior vas deferens; S. spp. = Scylla spp.; S. tranq. = S. tranquebarica; S. oliv. = S. oliv

ea.

tranquebarica) and 11.6 cm CW (Scylla spp.) are incapable of mating.Table 10 depicts the size at maturity of male Scylla spp. from variousstudies and localities and also indicates that the size at maturity ofthe undetermined Scylla spp. was larger than both S. olivacea andS. tranquebarica.

S. serrata is the most widespread Scylla spp. within the Indo-Pacific with S. olivacea and S. tranquebarica are commonly foundin the South-China Sea (Keenan et al., 1998). This fact led to theidea that both S. olivacea and S. tranquebarica from different loca-tions will show fewer differences in size at maturity because theyare commonly found throughout the South China Sea. There wereinsufficient data to determine the size at maturity of S. para-mamosain in the study. Quinn and Kojis (1987) suggest that S.paramamosain can undergo their maturity moult at a larger CW(12.0 cm) than Scylla spp. Hoang (1999), showed that the smallestovigerous female’s size of S. paramamosain (9.6 cm) was larger than

Scylla spp. (9.4 cm) as shown in Table 9. Therefore, according to thelatter study, it is suggested that the size at maturity in Scylla spp.is larger than that for S. tranquebarica, and that for S. tranquebaricathe size at maturity is larger than for S. olivacea.

bserved Mating scars Source

CW range (cm) n CW range (cm)

817 6.4–12.9 Present study30 8.6–14.6 Present study

Mean = 100 Ong (1966)6.8–9.8 Lavina (1980)14.5–18.4 Heasman (1980)12.4–17.3 431 11.6–18.1 Robertson and Kruger (1994)14.1–16.6 Hill (1975)

acea.

3 ries Re

tf1mcceilhlvliirwcrfmp

4

tstioi

A

mSa

R

A

HH

H

Varikul, V., Phumiphol, S., Hongpromyart, M., 1972. Preliminary experiment on pond

06 M. Ikhwanuddin et al. / Fishe

The increasing exploitation of the mud crab population, such ashe targeting of juveniles for pond culture, adult and sub-adult crabsor fattening and ovigerous females for premium markets (Le Vay,998) can be sustained through the implementation of effectiveanagement strategies for the crab fishery. Effective management

an be attained by establishing new guidelines on capturing maturerabs with sizes smaller than the minimum size at maturity forach species. For example, if selective fishing is to be carried outn this study area, the removal of specific mature crabs, regard-ess of species, will result in the larger mature S. olivacea beingeavily fished. As noted by Robertson and Kruger (1994), in the

ong-term, this selective removal of larger mature crabs (e.g., S. oli-acea in this study) will result in a decrease in the size at maturity. Ifarger males were removed from the population (for example dur-ng fishing), the frequency of mating by smaller crabs would likelyncrease (Robertson and Kruger, 1994). In addition, the selectiveemoval of smaller mature crabs (e.g. S. tranquebarica in this study)ill not result in an increase in the size at maturity of these smaller

rabs, whereas large mature S. tranquebarica were also caught andemoved. Therefore, fishers should avoid removing mature crabsrom sites where S. olivacea and S. tranquebarica occur together to

inimise the changes in size at maturity, size at mating and theopulation dynamics of each species.

. Conclusion

The biological information and population features gathered inhis study, including size at maturity, size at mating, taxonomictatus, CW–BW relationship and sex ratio, could potentially informhe sustainable management of mud crab resources, particularlyn Malaysia and the South China Sea. New guidelines on the sizesf crabs that are appropriate for commercial capture should bemplemented to avoid overexploitation of mud crab resources.

cknowledgements

We would like to thank the Inland Fisheries Agriculture Depart-ent, Sarawak for their financial support and the staff of the

ematan Fisheries Centre, Agriculture Department, Sarawak forssisting in the laboratory and field studies.

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