Acute toxicity of distillery spent wash on Nile tilapia (Oreochromis niloticus) fingerlings

Acute toxicity tests using distillery spent wash, treated by anaerobic digestion process, on Nile tilapia (Oreochromis niloticus) fingerlings were conducted under laboratory conditions for 96 h using static bioassay method. In the defmitive test, seven groups with three replications of experimental fish (body weight 5.6 plus or minus 0.45 g/fish) were exposed to seven concentrations of distillery spent wash (0, 12, 14, 16, 18, 20 and 22 ml/L). No mortality was observed during the 96 h in groups of control fish (0 ml/L) and fish exposed to 12ml/L. The 100% mortality rate was achieved only in fish group exposed to 22 m1/L of distillery spent wash. The median lethal concentration (LC50) of distillery spent wash to fish for 24, 48, 72 and 96 h were 18.547, 18.024, 17.965 and 17.965 ml/L, respectively. In addition, health of fish exposed to over 12ml/L (14, 16, 18, 20 and 22 ml/L of distillery spent wash) were affected. The behavioral changes of fish in these groups displayed rapid opercular movement and frequent gulping of air at the surface of the water. This abnormal behavior of fish increased after 24 h. Fish swimming activity decreased and later fish settled to the bottom of the aquaria, and their opercula moved slower until they eventually stopped when mortality occurred. Gill coloration was pale and the gills were clogged with sediment from distillery spent wash. Tilapia mortality was directly related to undesirable properties of distillery spent wash. Therefore, distillery spent wash should be suitably treated before disposal to the ecosystem or utilization in aquaculture.