Assessment of Hepatic Enzyme Biomarkers in Northern Pike (Esox lucius) from Lotic and Lentic Freshwater Habitats: Implications for Monitoring Metal Pollution and Ecological Stress in Aquatic Ecosystems

Rapid urbanization and increased anthropogenic activities have led to the release of an increasing number of pollutants, including metals, into freshwater ecosystems, posing a significant threat to aquatic life. In this study, the bioaccumulation of metals and hepatic enzyme activities in northern pike (Esox lucius) from two contrasting freshwater ecosystems in Serbia, the lotic Tisza River and the lentic Bela Crkva Reservoirs, were investigated. A total of 22 specimens (11 per site) were sampled in autumn 2024. The liver tissue was analyzed for the concentrations of 11 metals (As, Cd, Cr, Co, Cu, Pb, Li, Fe, Zn, Mn, Ni) and the activities of the liver enzymes (ALT, AST, AP, GGT). The results showed ecosystem-specific patterns of metal accumulation, with the northern pike in the rivers showing significantly higher Cu and Fe levels, while the fish in the reservoirs showed increased Zn concentrations. Enzymatic biomarkers showed different responses between ecosystems, with river fish showing increased ALT and AST activities, indicating hepatocellular stress, while reservoir fish showed increased GGT, indicating enhanced detoxification processes. The body condition factor was negatively correlated with liver Mn and Zn concentrations, emphasizing its utility as an integrative bioindicator of metal-induced stress. No significant sex-specific differences in metal or enzyme levels were found. These results suggest the suitability of northern pike as a sentinel species for environmental monitoring in freshwaters and highlight the different physiological adaptations to local metal stress in lentic and lotic habitats.