Sublethal haematological effects of zinc on the freshwater fish, Heteroclarias sp. (Osteichthyes: Clariidae)

Abstract

Laboratory study was undertaken to evaluate some haematological changes resulting from the exposure of a freshwater fish, Heteroclarias sp. to sublethal concentrations (5.0 and 10.0 mg L-1) of zinc
in water for a period of fifteen (15) days. Three groups of ten fish were subjected to serial dilutions of the stock solution of zinc of 0 (control), 5.0 and 10.0 mg L-1 in three large plastic bowls of 60 litres
capacity by the semistatic (renewal) method. At the end of the 15 days exposure period, blood samples were taken from the control and experimental fish. Blood was assayed for selected haematological
parameters (haematocrit, haemoglobin, red blood cell counts, white blood cell counts, differential white blood cell counts, erythrocyte sedimentation rate, total plasma protein and plasma glucose
concentration). The derived haematological indices of mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC) were calculated. Sublethal concentrations (5.0 and 10.0 mg L-1) of zinc caused a dose dependent decrease in haemoglobin values, coupled with a decrease in haematocrit values and red blood cell counts are obvious indication of anemia of the norm chronic type. The total white blood cell counts and the differential white blood cell counts were decreased except for the lymphocytes in which there was a slight increase. Plasma level of protein and glucose were also lower in the exposed fish when compared
to the control. The haematological indices MCHC, MCH and MCV were also lowered. In conclusion, the changes observed indicate that haematological parameters can be used as an indicator of zinc related
stress in fish on exposed to elevated zinc levels.