Effect of stocking density on the survival rate and grow-out of blue swimming crab (Portunus pelagicus) in brackish water ponds

Crabs only contribute a small percentage to Indonesia's total exports of aquaticproducts. Due to the growing popularity of the Blue swimming crab (Portunuspelagicus ), fishery catches of this species have increased since the early 1950s.This increased fishing pressure has led to a decline in the P. pelagicus population in Southeast Asian countries such as Thailand, Philippines, Vietnam and Indonesia. As a result, fishermen hardly catch large crab, and market also juveniles. Juvenile crabs are usually sold at lower prices; therefore, fattening small crab to market size may have a future. Furthermore, fattening to marketable size for an acceptable cost and effort might also reduce the fishing pressure. This study aimed to determine the optimal stocking density of Blue swimming crab for rearing in cages in shallow brackish water ponds. The research was conducted at Tambakbulusan village, Demak, Central Java, Indonesia. In a pond with 0.5 meter water depth we suspended 20 cages of 2.5 x 2 meter, for a Randomized Block Design of 4 stocking densities (2, 3, 4, and 5 individual's m-2) in five replications. Sea weed was added as shelter. The initial average size of these crabs was 5+1.6 cm in carapace length and 20+1.5 g in body weight. The crabs were fed trash-fish twice a day at a total dosage of 5 % of body weight; the quantity was adjusted bi-weekly after measuring and weighing all individuals. The effect of stocking density on the growth was not significantly different (P>0.05) but the survival rate was (P<0.05). Lowest densities (2 individual's m-2) resulted in highest survival rate (50 %). Research is needed oncauses and solutions for mortality as survival seems to be the most limiting factor for a feasible grow-out farming of Blue swimming crab.