The Biology and fishery potential of the crab portunus pelagicus (Linnaeus, 1766) along the coast of Dar es Salaam.

The blue swimming crab, Portunus pelagicus Linnaeus, is of great potential for fishery expansion in Tanzania. This study investigated some aspects of the biology and fishery potential of P. pelagicus. The study also established the species composition and diversity of portunid crabs along the coast of Dar es Salaam. Samples were obtained from artisanal fishermen operating beach seine nets and basket traps. Studies on reproductive biology revealed that immature crabs dominated between December and June. There were two peaks of percentage mature crabs, one was between February and April while the second between July and November. Females attained sexual maturity slightly earlier (8.5 cm) than males (8.7 cm). The breeding cycle was continuous throughout the year with two peaks, a minor between January and April and a major one was between July and October. Studies on fecundity revealed that the number of eggs laid ranged between 112,300 and 1,061,000 for crabs of 6.73 cm and 12.22 cm carapace width, respectively. The incubation period was nine days at a temperature of 27° C. Investigations on the food and feeding habits of P. pelagicus showed that the food items taken comprised mainly sessile and slow moving invertebrates. The major food items included molluscs (51.3%), crustaceans (24.1%), and fish bones (18%). There was no significant difference in the percentage occurrence of the food items in relation to sex, crab size, season and habitats. However, non ovigerous females fed more frequently than ovigerous females. Studies on moulting and growth revealed that, P. pelagicus moults continuously throughout the year and in all size classes with some distinct peaks. Moult increments were higher in males (23.99% for immature and 9.77% for mature individuals) than in females (21.88% for immature and 7.94% for mature individuals). The modal progression analysis for males and females recorded more or less the same growth rate, 0.86 and 0.80 cm/month, respectively. It was concluded that P. pelagicus is a fast growing species. Production studies of P. pelagicus showed that there were seasonal variations in catch rates with a peak between December and May. The harvestable biomass for Dar es Salaam coast, was estimated as 34.5 tonnes/year while the contribution of recruitment to the biomass was about 20.2 tonnes/year. Data on species composition of portunid crabs showed that there were 11 species, with P. pelagicus being the most abundant in all study sites, followed by P. sanguinolentus, and the least represented was Charybdis natator. P. pelagicus and P. sanguinolentus were abundant in shallow sub-littoral waters while C. natator was abundant in deep waters. For P. pelagicus small individuals prefer shallow waters while large ones prefer deep waters. Development of P. pelagicus fishery is mainly hampered by the exploitation of juvenile crabs. The study concludes with recommendations for further studies and management of the crab fishery.