A study was conducted to assess the possible cause for fish mortality during 14-15th February 2008 in Tuticorin Bay. Analysis showed normal values of sea surface temperature, pH, dissolved oxygen
and nutrient concentrations. The mean salinity (30.8 ppt) was lower than the normal (35.5 ppt). Though the concentration of chlorophyll was slightly higher than normal, no visible colour change was noticed
in the Bay water. The possible entry of land drainage along with freshwater influx might have caused lower salinity and higher algal concentration. Analysis of water and dead fish tissue samples were
carried out for metal load. The concentration of trace metals within safe limits ruled out the possibilities of fish mortality due to metal pollution. The estimated mean value of ammonia (1.09 mg. L-1) in water was very much higher than the toxic limit of 0.1 mg. L-1, indicating the possibility of ammonia as the major cause of fish mortality in the Bay.

Fish mortality, Tuticorin Bay, ammonia