Acute toxicities of commercial grade malathion (EC50) on Esomus danricus were determined by static-with-renewal tests and found to be 0.026, 0.022, 0.02 and 0.017 mgl-1 respectively for 24, 48, 72 and 96 hours. During acute toxicity, the normal colour was found to fade, along with copious mucous secretions, irregular, erratic and jerky movements. Three sub lethal doses (0.0017, 0.00017 and 0.000017 mgl-1) were taken for determining changes in growth, somatic indices and oxygen consumption at weekly interval for 28 days. A decline in body weight accompanied by a dose and exposure dependent increase in hepato-somatic index and decline in brain and kidney-somatic indices were observed. Decline in rate of oxygen consumption in 0.0017, 0.00017 and 0.000017 mgl-1 malathion exposed fish for 7 days were 33, 28 and 10%; for 14 days were 49, 43 and 17%; for 21 days were 55, 50 and 31% and for 28 days were 69, 57 and 41% respectively.
Acute toxicity experiments for Fenvalerate technical grade and 20% EC formulation were conducted using static renewal bioassay and continuous flow through systems for 24h, 48h and 96 h on the three major carps, Labeo rohita, Catla catla and Cirrhinus mrigala. Although toxicity studies were conducted on both fry and fingerling stages, further experiments were carried out with fingerling stages only. After determining the LC50 concentrations for the three fish individually, one-tenth of the 24 h LC50 was taken as sublethal concentration for studies on oxygen consumption. The toxicant exposed fish showed anomalous behaviour like surfacing phenomenon, irregular, erratic and darting swimming movements, hyperexcitability, loss of equilibrium and hitting to the walls of the test tank before finally sinking to the bottom just before death. Oxygen consumption studies for a period of 12 hours, at intervals of 2 hours, in both sublethal and lethal concentrations indicated that lethal concentrations had profound effect than sublethal concentrations and 20% EC was found to be more deleterious than technical grade of fenvalerate. During experimentation, severe respiratory distress, rapid opercular movements leading to the higher amount of toxicant uptake, increased mucus secretion, higher ventilation volume, decrease in the oxygen uptake efficiency, laboured breathing and gulping of air at the surface were observed in all the three carps studied.
