Abstract
Specimens of Oreochromis niloticus were exposed to different concentrations of nickel chloride. Lc-50 at 96 hours was computed at 27.2 mg/l. Remarkable changes in the behaviour of fish exposed to lethal (26.0 to 28.5 mg/l) and sublethal (1.5, 3.0 and 5.0 mg/l) concentrations were observed. Significant (P < 0.01) changes in respiratory behaviour (cough and yawns) were registered in fish exposed to sublethal concentrations. Aggressive behaviour (nudge and nip) and stress-related discomfort movements (fin flickering, jerking movement and swimming) were also increased in exposed fish. A positive dose-response relationship was demonstrated for these changes in behaviour. Significant (P < 0.01) changes in haematological parameters such as a decrease in total leucocyte count and an increase in erythrocyte count, haemoglobin concentration and haematocrit were also recorded. Differential leucocyte counts showed a significant (P < 0.01) fall in the number of thrombocytes and lymphocytes and an increase in granulocytes (neutrophils and eosinophils) and monocytes. MCV, MCH and MCHC values were also affected but the changes were insignificant (P > 0.01).