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The mud crab Scylla serrata (Forskål) in Maputo Bay, Mozambique.
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Adriano Afonso Macia Jose Paula Eduardo Mondlane University University of Lisbon
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Adriano Macia, Paula Santana Afonso, José Paula and Rui Paula e Silva
The mud crab in Mozambique is locally known as “Hala”. The Bay is one of the The mud crab Scylla serrata is a portunid most important economic areas in Mozambique brachyuran crab with a complex life cycle. It for commercial harvesting of this species due to inhabits Indo-Pacific shores and recruits to man- their relative abundance and easy access (Piatek, grove forests where it resides during the juve- 1981). nile phase, making burrows in the muddier parts Basic biological information on S. serrata in of the forest, among the roots of Rhizophora Mozambique is scarce. Existing information is mucronata. When it matures the crabs move off- known from a few studies and restricted to catch shore where reproduction takes place (Robert- estimates and size distribution (see Morais, 1972; son and Kruger, 1994). No ovigerous females are Piatek, 1981) and the assessment of catches usually seen within the mangrove, and the larg- within mangrove forests, generally referred to as est crabs are collected in the subtidal zone and the nurseries for this species (Peatek, 1981; Hill, mostly outside the estuarine environment. 1994; Robertson and Piper, 1991; Barnes et al., Exploitation of the mud crab in Mozam- 2002). Halare (1999) estimated catch per unit bique is a traditional artisanal activity involving effort by means of baited traps at Inhaca Island, local fishermen. Due to the relatively small while Barnes et al. (2002) estimated density and investment required for this fishery and to its biomass based on the mangrove forest area at the demand as a delicacy for the tourism industry Quirimbas Archipelago. These studies represent and local consumers, mud crab harvesting has modest contributions to the knowledge on Scylla great potential for expansion (Piatek, 1981). The serrata in Mozambique. Population data such as total annual catch of mud crabs in Mozambique size structure, sex balance and reproductive in general is not known, and Piatek (1981) esti- parameters, are vital information in that they cre- mated the annual production potential of this ate a foundation for the study of population crab to be around 5,700 t yr-1 taking into account dynamics (Robertson and Kruger, 1994), provid- a total mangrove forest area of 850 km2 through- ing the basis for managing the sustainable utiliza- out Mozambique. In Maputo Bay, the mud crab tion of this resource.
The Maputo Bay Ecossystem 1 II . Main Habitats and Ecological Functioning
A B
Figure 1. (A) The mud crab Scylla serrata, (B) woman digging for mud crabs in Ponta Rasa mangrove at Inhaca Island. Photographs by José Paula.
Mud crab biological data from the Incomati season males outnumbered females, suggesting that Estuary, Maputo Bay offshore migration of mature females for spawning This report presents some aspects of the biology of occurs during this season. Scylla serrata (body size, sex ratio, size at maturity as There was a strong relationship between mating well as gonad development), obtained from bi- season and the moult stages over the study period. monthly sampling of artisanal catches using baited For both sexes, soft-shelled individuals were most traps from the Incomati Estuary during spring tides, common from July to November, suggesting that over a period of 23 months (February 1997 to July mating occurs more frequently during this period of 1999). the year (Figure 5). In December at Inhaca Island, Results reveal that mud crab carapace width Paula et al. (2003) collected recruiting megalopae lar- (CW) varies from 70-200 mm, with the majority of vae of Scylla serrata settling in the creek of Ponta male and female crabs (74%) exhibiting a width of Rasa mangrove; this indicates that fertilization was 90-120 mm. High percentages of juveniles (CW<80 active a few weeks before. During the study, no mm) were recorded from catches in May, July and ovigerous female crabs were encountered in the sam- November (Figure 2) indicating a major recruitment pled catches. Fishermen from Inhaca Island reported peak during the cold season (May to July) with a the occurrence of large crabs, including ovigerous smaller peak around the start of the warm season. females, in the shallow subtidal vegetated banks in Male and female carapace width to wet weight the eastern part of Maputo Bay and seldom caught (W) relationships were analysed using least square them with gillnets (J. Paula, pers. obs.). regression. The relationships suggest that males The analysis of gonad developmental stages increase in weight faster than females (W=0.0013.L (based on three colour stages) revealed that a higher 2.561 for females and W=0.0001.L 3.060 for males) as percentage of mature females were present in the seen in Figure 3. Male crabs attain a larger size than mud crab population of the Incomati Estuary around females, which suggest a longer growth period or a January (Figure 6), but mature females were always higher growth rate. The sex ratio was 53% to 47% present in the samples which indicates a potential (males to females) for the entire period of study, with continuous reproductive capacity. The intermediate ratios alternating slightly (Figure 4). During the hot stages were found dominant throughout the year
2 The Maputo Bay Ecossystem Case Study 7.3. The mud crab Scylla serrata (Forskål) in Maputo Bay, Mozambique
September 1997 Males (n=57) November 1997 Males (n=46) January 1998 Males (n=32) 60 Females (n=32) Females (n=49) Females (n=38)
40 Males
30
% 0
20
40
60 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200
March 1998 Males (n=48) May 1998 Males (n=42) July 1998 Males (n=7) 60 Females (n=40) Females (n=43) Females (n=9)
40
30
% 0
20
40
60 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200
September 1998 Males (n=22) November 1998 Males (n=31) January 1999 Males (n=22) 60 Females (n=20) Females (n=13) Females (n=23)
40
30
% 0
20
40
60 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200
CW (mm) CW (mm) CW (mm)
Figure 2. Monthly size frequency distribution for both sexes of Scylla serrata from the Incomati Estuary over the study period. except during the maturation January peaks. How- scopic analysis is needed to establish greater accu- ever, few females with immature gonads (first gonad racy in determining gonad development stages and development stage) were encountered throughout seasonality. Nevertheless, the findings are encourag- the study period, indicating that probably the selec- ing and suggest that gonad colour analysis can be uti- tivity of the gear used is excluding the smaller and lised for a quick appraisal of the sexual maturity of immature female crabs. mud crabs from Maputo Bay waters.
Quin and Kojis (1987) noted that enlargement Size at first [?] maturity (CW50) was estimated to and maturation of female oocytes are associated with be around 100 mm for females and 110 mm for males, a colour change from yellow to orange. In our study, showing that females attain maturity size at smaller gonads of yellow coloration were not considered sizes (Figure 7). In Natal, South Africa (approxi- “fully” mature and were classified separately from mately 300 km south of Maputo Bay) mud crabs “mature” orange gonads. A more detailed micro- attain maturity when they are only 90 mm CW (Rob-
The Maputo Bay Ecossystem 3 II . Main Habitats and Ecological Functioning ertson and Kruger, 1994), at slightly smaller sizes that reflect important trends that could form the basis for those from the Incomati Estuary. Some authors have a further, more detailed fisheries assessment. The reported that S. serrata becomes sexually mature at reproduction pattern for Incomati Estuary mud crabs smaller sizes in the tropics than in the subtropics appears to include mating inside the mangrove envi- especially in African and Australian shores (Quinn ronment, with embryonic development taking place and Rojis, 1987). These comparisons, however, do in deeper waters, usually in the estuarine environ- not apply to all geographic locations (Robertson and ment, with females eventually moving offshore to Kruger, 1994). spawn. These findings and the study methods uti- lised should be considered in further mud crab inves- Summary tigations, especially in view of the growing Mindful of the limited data obtained thus far, it is exploitation of this species in Mozambique and the evident that the biological parameters examined for need for a sound management programme for this the mud crabs at the Incomati Estuary (Muntanhana) valuable resource.
Males Females 800 700
700 y = 0.0001x3.0699 y = 0.013x2.5606 600 R2 = 0.9419 R2 = 0.9228 600 P<0.05 P<0.05 500 500 400 400
300 300 Wet weight (g) Wet weight (g)
200 200
100 100 0 60 80 100 120 140 160 80 100 120 140 160 180 Carapace width (mm) Carapace width (mm)
Figure 3. Relationship between carapace width and wet weight for both sexes of Scylla serrata from the Incomati Estuary (males=180, females=193).
0.80
0.70
0.60
0.50 Sex ratio (M:F) ratio Sex
0.40
0.30 JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAY Months (July 97 to May 99)
Figure 4. Sex ratios (male to female) for Scylla serrata from the Incomati Estuary over the study period (July 1997 to May 1999).
4 The Maputo Bay Ecossystem Case Study 7.3. The mud crab Scylla serrata (Forskål) in Maputo Bay, Mozambique
Premoult Intermoult Postmoult Males 100 90 80 70 60 50 40 30 Frequency (%) Frequency 20 10 0 White Yellow Orange
Females 37 32 43 28 40 43 8 20 12 22 16 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 Frequency (%) Frequency Frequency (%) Frequency 20 20 10 10 0 0 JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAY JUL SEP NOV JAN MAR MAY JUL SEP NOV JAN MAY Months (July 97 to May 99) Months (July 97 to May 99)
Figure 5. Moulting stage for both sexes of Scylla serrata from the Figure 6. Temporal variation of gonad condition, colour and size for Incomati Estuary over the study period. female mud crabs caught from the Incomati Estuary over the study period (numbers above bars = n).
Males Females 100 100
80 80
60 60
40 40
Frequency (%) Frequency 20 20
0 0 60 80 100 120 140 160 180 40 60 80 100 120 140 160 180 200 220 Carapace width (mm) Carapace width (mm)
Figure 7. Overall carapace size at first maturity for both sexes of Scylla serrata from the Incomati Estuary.
The Maputo Bay Ecossystem 5 II . Main Habitats and Ecological FunctioningCase Study 7.3. The mud crab Scylla serrata (Forskål) in Maputo Bay, Mozambique
Bibliography
Barnes, D., Dulvy, N., Priestley, S., Darwall,W., Choi- and Shelf Science 56, 325-337. sel, M.,Whittington, M., 2002. Fishery character- Piatek, M.A., 1981. Mangrove crab, Scylla serrata, istics and abundance estimates of the mangrove and its utilisation with particular reference to crab Scylla serrata in Southern Tanzania and Mozambique. A Report prepared for the Northern Mozambique. South African Journal of Research and Development of Inland Fisheries Marine Science 24, 19-25. Project. FAO, Rome, GCP/MOZ/006(SWE), No. Halare, A., 1999. Abundância de Scylla serrata no Saco 3, 53pp. da Inhaca. Licentiate Thesis, Universidade Edu- Quinn, N.J., Kojis, B.L., 1987. Reproduction biology ardo Mondlane, Maputo. of Scylla spp. (Crustacea: Portunidae) in the Labu Hill, B.J., 1994. Offshore spawning by the portunid estuary in Papua New Guinea. Bulletin of Marine crab Scylla serrata (Crustacea: Decapoda). Marine Science 42, 234-241. Biology 120, 379-384. Robertson, W.D., Kruger, A., 1994. Size at maturity, Morais, O., 1972. Estudo preliminar das capturas de mating and spawning in the portunid crab Scylla caranguejo na região de Lingamo. Relatório serrata (Forskål) in Natal, South Africa. Estuarine, Científico (1969). Missão de Estudos Biocean- Coastal and Shelf Science 39, 185-200. ológicos e de Pescas de Moçambique (MEBPM), Robertson, W.D, Piper, S.E., 1991. Population esti- Lourenço Marques. mates of crabs Scylla serrata (Forskål, 1755) Paula, J., Dornelas, M., Flores, A., 2003. Stratified set- (Decapoda: Portunidae) in two closed estuaries tlement and moulting competency of brachyuran in Natal, South Africa, from mark-recapture megalopae in Ponta Rasa mangrove swamp, methods. South African Journal of Marine Science Inhaca Island (Mozambique). Estuarine, Coastal 11, 193-202.
6 The Maputo Bay Ecossystem 6
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