Paraquat, a chlorinated contact herbicide is mainly used to control weeds in agriculture. The use of this herbicide may lead to its excess being washed away into aquatic ecosystems during surface runoff thereby affecting non-targeted organisms including fish. This study examined the alterations in haematology and histopathology of fish exposed to sub-lethal concentrations of Paraquat. 100 juveniles of Clarias gariepinus (mean weight, 17.1€6.0g; mean total length, 14.0+/-1.4cm) were exposed in a 48 hour renewal bioassay to sub-lethal concentrations (0.00, 1.125, 2.25, 4.00 and 9.00mg/L) of paraquat for 14 days with 10 fishes per concentration in duplicates. Five fish samples from each concentration were used for the haematological study and the haematological parameters examined were; Packed Cell Volume (PCV), Haemoglobin (Hb), Red Blood Cell (RBC), Mean Corpuscular Volume (MCV), Mean Corpuscular Haemoglobin (MCH), Mean Corpuscular Haemoglobin Concentration (MCHC) and White Blood Cells (WBC). There were significant decreases (p<0.05) in PCV (from 35.70€2.97 % in the control to 16.00€0.58 % at 9.00 mg/L), Hb (from 11.90€1.10 g/dl in the control to 4.50€0.09 g/dl at 9.00 mg/L), RBC (from 3.36€0.06 10 6 /•L in the control to 1.70€0.03 10 6 / •L at 9.00 mg/L), MCV (from 105.90€9.30 fl in the control to 96.14+/-1.80 fl at 9.00 mg/L) and MCHC (from 33.40+/-0.40 g/dl in the control to 28.40€0.81 g/dl at 9.00 mg/L) values of the exposed fishes which was concentration dependent but, there was no significant difference (p>0.5) in the MCH values. The White Blood Cells showed significant (p<0.5) increase at 4.00mg/L (24217+/-2377 10 3 /•L) from that of the control (15350€2828 10 3 /•L). Platelets and Lymphocytes increased significantly (P<0.05) different from that of the control while Heterocytes decreased (from 29.30€3.50 % in the control to 24.00€4.10 % at 9.00mg/L) significantly (p<0.05) with increasing concentrations. The major histopathological alterations observed were: multilayered epidermis with polygonal cells at 4.00 mg/L, thickening of epidermal cells at 9.00 mg/L, congestion of dermal blood vessels and gill capillaries at 1.125 mg/L, sloughing of the secondary gill lamellae at 2.25 mg/L and necrosis of the gill epithelium at 9.00 mg/L. This study therefore showed that chronic exposure to sub-lethal concentrations of Paraquat can affect fish adversely. Most importantly, farmers should be made to consider these when using this herbicide in controlling weeds on farmlands.

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Accepted: 2 November, 2017.