The African Catfish, Clarias gariepinus, was exposed to various sublethal concentrations (25.0, 12.50, 6.25, 3.125, 1.56 and 0.0 mgl-1) of formalin to investigate their effects on the weight gain of the fish. Decrease in weight gain, directly proportional to the toxicant concentration, was observed in fish exposed to concentrations above 3.12 mgl-1. There was significant difference in weight gain of fish exposed to the various toxicants and the fish in the control tank (P < 0.05). Though formalin is recommended for ectoparasitic infections of farmed fishes, its use in fish farms must therefore be under stipulated controlled procedures. Threshold values should be determined for the fish cultured.


Key words: Clarias gariepinus, formalin, toxicity and weight gain.


Journal of Aquatic Sciences Vol.17(1) 2002: 66-69