Effect of seaweed on water treatment for mud crab fattening in the recirculating condo system

Management of water quality is the most important limiting factor for the mud crab (Scylla paramamosain) fattening process in recirculating condo system (RCS). The effluent especially ammonia circulating in the system affects the survival and growth of the crabs. The purpose of this study was to assess the efficiency of different types of seaweeds in reducing ammonia concentration in the RCS. Two experiments were conducted; (1) a preliminary study in the experiment tank without crabs and (2) in the RCS system with crabs fed regularly with fresh fish. Four species of seaweed were used in the experiment 1 for 5 days, namely, Gracilaria fisheri, Chaetomorpha linum, Ulva rigida, and Caulerpa lentillifera. Result showed different filtering rates of ammonia between these five seaweed species (P<0.01) and indicated that G. fisheri and C. linum were the most efficient seaweeds in removing ammonia in water. In the experiment 2, these two seaweed species were used in the RCS system together with control. It was again confirmed that seaweeds can effectively remove ammonia from the RCS system (P<0.05) and G. fisheri was the most efficient compared to C. linum. Apart from that other water parameters indicated that using seaweeds in the RCS system can improve water quality by resulting in higher dissolved oxygen value and lower pH value. Therefore, it is suggested that using seaweeds, especially G. fisheri is essential to remove ammonia in water and help the healthy status of the system leading to healthy crab production and reducing toxic to nature. Additionally, it is observed that apart from reducing ammonia, C. linum also helps in trapping sediment in the RCS, thus a combination of using both seaweed species is recommended.