The role of seaweed (Gracilaria sp) and green mussel (Perna viridis) in reducing organic waste in shrimp culture water

Organic waste, originating from inlet water, feed or excrements, reduces the water quality in brackish water pond used for aquaculture of e.g. shrimp. High concentrations of organic waste are a threat to ecosystem stability since decomposition reduces dissolved oxygen content, and can result in toxic compounds such as ammonia and nitrite. Various aquaculture technologies can reduce the concentration of organic waste or prevent the consequences of its decomposition. Two options .are the use of seaweed and green mussel since these organisms use organic and an-organic particles for their nutrition. This research aims to analyse the impact of seaweed and green mussle densities in reducing organic waste in shrimp culture water. Tt:iis research was done in Tambakbulusan, Demak regency, Central Java, lndone'sia. Densities of seaweed (Gracilaria verucosa) and green mussel (Perva viridis), were compared in a randomized block design of 9 treatments and 4 replications. Densities for seaweed treatments were 50, 100, 150 and 200 gram m-2, while treatments for green mussel were 60, 90, 120 and 150 gram m-2. Both seaweed and mussels were hanged on rope in fiber tanks of 1 x1 x1 m with 800 L of brackish water and substrate to create a brackish mesocosm. The densities of seaweed and green mussle influenced organic waste reduction. The optimum seaweed density was 100 g m-2 and reduced ammonia with 34% and nitrite with 24%. The higher densities of seaweed resulted in concentration of nitrate lower than advised for shrimp culture (0.36 mg.m-2). The highest densities of green mussel increased the content of organic matter and nitrite compared to the control. The green mussel density of 90 g.m-2 gave the highest absorption of organic matter (38%) and nitrate (49%).