Mud crabs (Scylla spp.) are considered luxury seafood items and high market value; therefore mud crab farming is well established throughout Southeast Asia. In Vietnam, mud crab S. paramamosain is one of the most promising candidates, and is often seen as an alternative to shrimp or poly-culture in the improved extensive shrimp farming in the Mekong delta. A nursery period as an intermediate step in between hatchery and grow-out, to grow postlarvae to a size appropriate for transport and release into large extensive to intensive production systems, is considered necessary. However, the prevalence of cannibalistic behavior in crabs thus using shelter in the culture system to minimize conspeci?c cannibalism and improve survival of crab.
The present study aims to find out the suitable seaweed species as shelter to improve survival and production of mud crab S. paramamosain crablets. The experiment was conducted consisting of seven treatments, and randomly designed in triplicate tanks. The control treatment had no shelter (without seaweed in the culture tank) and other six treatments, red seaweed (Gracilaria tenuistipitata) or green seaweed (Enteromorpha intestinalis) were placed to the rearing tanks at three density levels of 0.5, 1 and 2 kg/m2. Instar 2 crablets with a mean weight of 0.019 g were reared at density of 300 ind./m2 in the 250-L tanks with salinity of 20 ppt, and provided continuous aeration. The crablets were fed frozen Artemia biomass to satiation for four weeks.
Results showed that survival of crabs in all treatments decreased with the rearing period. At the end of experiment, average survival of crab in the control group was lowest (17.4%) while survival of crabs were significantly improved and varied in the ranges of 53.3-70.7% and 27.8-35.9% in the Gracilaria and Enteromorpha groups, respectively. Notably, higher survival resulted in lower growth rates in the seaweed treatments but enhanced biomass and production of crab juveniles. Moreover, the concentrations of TAN and NO2-, in the Gracilaria treatments were much lower than in the control and the Enteromorpha treatments that improved water quality in the rearing tanks. The present finding proved that Gracilaria could be considered a suitable shelter for reliable production of mud crab juveniles in nursery phase.