Eutrophication is a serious problem occurring in ponds or estuaries where fish culture is practised. A possible solution to this problem is to raise oysters in an integrated culture with fish to improve water clarity and to help remove excess nutrients. In order to determine the effects that cultured oysters have on the environment, an experimental study was performed in the Moothakunnam region of the Vembanad lake in the Ernakulam District during February-November, 2012. The study was performed in four cages (1× 1× 1 m) referred to as treatments T1, T2, T3 and T4. The four treatments used fish ( Etroplus suratensis ) and oysters ( Crassostrea madrasensis ) in the ratio of 1:0.3, 1:0.5, 1:0.7 and 1:0 respectively. Several parameters were measured and compared between the four treatments, and these included temperature, pH, salinity, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, total suspended solids, ammonia, nitrite, nitrate and orthophosphate. Results showed that the parameters, DO, BOD, COD, TSS, ammonia and orthophosphate varied with different treatments and months. However, the contents of nitrite and nitrate showed variation only with respect to months. The eutrophication index (E) values calculated for the four treatments were 9.45 ± 4.41, 5.25 ± 3.23, 11.32 ± 6.17 and 11.52 ± 5.45 respectively. The lower E value obtained was for T2 which supports the conclusion that oysters can control eutrophication in an integrated aquaculture system. The optimal co-cultivation proportion of fish to oysters obtained from the present study was 1:0.5.

Water quality, oysters, integrated culture, nutrients.