Impact of Fin Erosion on Biomarker Responses in Salmo trutta: Implications for the Reliability of Biological Effects Monitoring in Aquatic Environments

Fin erosion is a common condition in aquaculture-reared salmonids used in river restocking programs and may influence multiple biomarkers applied in aquatic biomonitoring. The health status of Salmo trutta from a river with good ecological status was evaluated by analysing biometric traits, glucose concentration, haematological indices, erythrocyte morphometry, and erythrocytic nuclear abnormalities in relation to fin condition. Fish with eroded fins were larger and exhibited pelvic and dorsal fin erosion, suggesting a hatchery origin, but showed lower haemoglobin concentration, packed cell volume, and glucose levels, together with altered erythrocyte morphology and increased frequencies of cytotoxic nuclear abnormalities. In contrast, leukocyte profiles and overall erythropoietic activity did not differ between fin condition groups. These findings indicate that fin erosion can alter baseline biomarker responses independently of environmental contamination, highlighting the importance of considering fin integrity when interpreting biomonitoring data and evaluating the suitability of fish for restocking programs.