The study was aimed to determine the toxicity of cypermethrin, a synthetic pyrethroid pesticide on the freshwater catfish Heteropneustes fossilis. The experiment was conducted in two parts for a period of 96 h, under static conditions. In the first part, the LC 50 value of cypermethrin for 24, 48, 72 and 96 h was estimated. Behavioral changes in fish such as increased frequency of surfacing and gulping of atmospheric air, decrease in opercular movement (OCM), alteration in swimming pattern, violent shaking and jerking of head and gradual loss of balance due to cypermethrin exposure were regularly monitored for 96 h. In the second part of the study, the fish were subjected to short term (96 h) exposure of 3.42 µg/l (75% of 96 h LC 50 ) cypermethrin. Thereafter, fish were released in the cypermethrin-free water in order to study the recovery patterns in serum electrolyte levels. The data obtained from the acute toxicity tests were evaluated using EPA Probit analysis, Version 1.5, software based on Finney's Probit Analysis method. The LC 50 values for cypermethrin 24, 48, 72 and 96 h were 5.43, 5.12, 4.82 and 4.56 µg/l, respectively. The Ca +2 levels decreased in the exposed fishes up to 96 h, whereas, Mg +2 and Pi recorded an increase. During recovery period the serum electrolytes recorded a pattern towards normalcy when compared with 96 h exposed fishes.

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