Asian Journal of Research in Zoology

Heavy metal contamination in freshwater ecosystems remains an environmental concern because metals such as lead persist in aquatic habitats and may affect exposed organisms. The present study evaluated the effect of lead nitrate on blood glucose content in the freshwater air-breathing fish Heteropneustes fossilis under laboratory conditions. Healthy fish were acclimatised before exposure and maintained either in control water or in water containing lead nitrate at a sublethal concentration of 5 mg/L for 28 days. Blood samples were collected on days 0, 7, 14, 21 and 28, and glucose concentration was estimated by the glucose oxidase-peroxidase method. The control group showed only minor variation in blood glucose values, ranging from 82 to 85 mg/dL. In contrast, lead nitrate-exposed fish showed a gradual decline from 82 mg/dL on day 0 to 49 mg/dL on day 28, with a more evident reduction during prolonged exposure. Behavioural changes, including reduced swimming activity, irregular movement, decreased feeding response, mucus secretion and lethargy, were also observed in treated fish. These findings indicate that lead nitrate exposure may disturb carbohydrate metabolism and energy balance in H. fossilis. The observed decline in blood glucose may be associated with increased energy demand, depletion of glycogen reserves, altered liver metabolism, hormonal imbalance and disruption of enzymatic processes involved in glucose regulation. Blood glucose content may be used as a biochemical indicator of lead-induced stress in freshwater fish. Further studies with stronger replication, statistical validation and supporting physiological or histopathological observations are needed to clarify mechanisms of lead toxicity.