Size structure, female sexual maturity and growth of the mangrove crab, Scylla serrata, in the North-West of Madagascar, were studied from data obtained from a commercial catch sampling programme. Gonad Stage III was utilised for the determination of maturity. Although mature females of Scylla serrata were found year-round, the reproductive activity of the population showed a clear seasonality. Most females matured at a size between 12 and 14 cm carapace width (CW). High proportions of mature females were found during the dry season, and low proportions during the rainy season. Towards the end of the dry season, most mature females undertake a spawning migration to the open sea. The migration was most pronounced for the larger size classes of the females. It is uncertain which environmental factor is responsible for the onset of maturity in the smaller females. Maturation of the small females occurred in the rainy season only, at the time of a reduction in salinity, but other environmental factors, such as food availability, could also have an influence on the onset of maturation. Large seasonal variations in maturation by size class were observed. Two size classes (viz. 13-14 and 14-15 cm CW) contributed most to the female reproductive population and to its reproductive potential. The antagonism between ovarian and somatic growth plays a major part in the growth of Scylla serrata. Therefore, a "size structured" analysis of the maturity data is of utmost importance in the understanding of the growth process in this crab species. For the males, growth parameters could be identified by modal progression analysis of size frequency data. The estimates of K were close to 0.8, but the Linf's (around 17.6 cm CW) were rather low as compared to the values reported in the literature. For the females, no sensible growth parameters could be derived from the size frequency data. This could be attributed (a) to the antagonism between gonad development and somatic growth, and (b) to the spawning migration, which affects the apparent size structure of the female population. Females may mature (and accomplish their terminal moult) at different sizes, and therefore, the maximum size of female Scylla serrata is a highly variable parameter. The growth rate found in this study also indicates that, in the North-West of Madagascar, there is about one year between recruitment and the onset of the spawning migration. Female crabs thus spend about one year on the shallow grounds in the mangroves, before moving to the open sea. There is some evidence that large males may also take part in these migrations. These findings lead to the hypothesis that the movements of Scylla serrata follow an annual rhythmicity, in accordance with the seasonal fluctuations in the environmental conditions. |
