Haematological and Parasitological Biomarkers of Sub-Lethal Manganese Stress in African Catfish Clarias gariepinus: Implications for Fish Health and Freshwater Monitoring

Abstract

The African catfish, Clarias gariepinus, is a major freshwater aquaculture species in Africa due to its fast growth, environmental tolerance, and high market value. Manganese (Mn) enters aquatic systems through agricultural runoff, industrial discharge, and geogenic sources and, at elevated levels, can induce oxidative stress, tissue damage, and immunosuppression in fish. This study assessed haematological and parasitological responses of C. gariepinus to sub-lethal Mn exposure. Forty-eight sub-adult C. gariepinus (mean length 200 ± 0.9 mm; weight 500 ± 1.5 g) were obtained from a fish farm in Ondo State, Nigeria. Following 14 days of acclimation in dechlorinated water (pH 7.7 ± 0.2; 27 ± 1°C), fish were exposed for 144 h to Mn concentrations of 6.48 mg/L (T1) and 8.64 mg/L (T2) in 60-L glass aquaria, with a Mn-free control. Blood was collected at 24-h intervals for haematological analysis, while gills and skin were examined for parasites. Mn exposure significantly increased packed cell volume, haemoglobin, RBC and WBC counts, indicating compensatory erythropoiesis and immune activation, but significantly decreased erythrocyte sedimentation rate and lymphocyte count, suggesting immunosuppression. MCV, MCH, MCHC and neutrophils were unaffected, and no significant differences occurred between T1 and T2. Trichodina sp. prevalence and mean intensity increased with Mn concentration, from 0% in the control to 70% at the highest dose. Short-term Mn exposure alters haematological parameters and increases susceptibility to parasitic infection in C. gariepinus. These biomarkers may facilitate early detection of Mn contamination and fish health decline in tropical aquaculture systems.