Fingerlings of Heterobranchus bidorsalis (mean weight 11.88 g) were exposed to various sub-lethal concentrations of cypermethrin (0.032, 0.034, 0.036, 0.038 and 0.040 ml/L), a toxicant, and the histological effects were recorded. The 96 h bioassays were also conducted to determine the lethal concentration (LC50) of cypermethrin on the test fish. The 96 h LC50 of H. bidorsalis exposed to Cypermethrin was 0.036 ml/L. The physical reactions observed in the fish were: discolorations of the skin, erratic swimming, loss of reflex, hyperactivities, surfacing, and these effects increased with increasing concentration of the toxicants and duration of exposure. The pH and dissolved oxygen of the test media showed slight decrease from lowest concentration (0.032 ml/L) to the highest concentration (0.040 ml/L) while the temperature increased slightly with increasing concentration (0.032, 0.034, 0.036, 0.038 and 0.040 ml/L). The histological examination of the gills, kidney, livers of the fish after 96 h showed pathological changes and alterations such as gills infiltration, inflammation of the livers, vacuolation and necrosis. There was excessive necrosis degeneration in higher concentrations (0.038 and 0.040 ml/L) and there was increase in mortality as the concentrations increased.
Key words: Histology, cypermethrin, xenobiotic, Heterobranchus bidorsalis, acute toxicity, LC50.
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