Assessing the sensitivity of Atlantic salmon (Salmo salar) to dietary endosulfan exposure using tissue biochemistry and histology

The incorporation of plant-based ingredients, and the possible carry-over of pesticides such as endosulfan, in fish feeds may present new toxicological challenges to aquacultural species. Biological responses of Atlantic salmon (Salmo salar) to a 35-day dietary endosulfan exposure at levels ranging from 4 to 710 μg kg^-1 were assessed using tissue histology and biochemistry. Liver 7-ethoxyresorufin-O-deacetylase (EROD) activity was significantly elevated in the highest exposure group (710 μg kg^-1) by day 35. Other hepatic indicators of stress impacts and responses (glutathione-S-transferase and glutathione peroxidase activities and hepatic α-tocopherol content) remained unchanged. Branchial Na⁺, K⁺-ATPase activity was significantly reduced at day 14 in the highest exposure group, but returned to control levels by day 35. Conversely, intestinal Na⁺, K⁺-ATPase activity was significantly inhibited at day 35, but again only at the highest exposure level. In contrast to the biochemical results, hepatic and intestinal histology revealed effects of exposure even at the lowest dose tested (4 μg kg^-1). In the posterior intestine, pathology was characterised by vacuolation and fusion of villi, and in the most severe cases, loss of epithelial integrity in villi tips. In the liver the primary effects were glycogen depletion and lipidosis. These changes were typical of a generalised stress response. While histology endpoints may prove to be the most sensitive indicators of dietary endosulfan exposure, the organismal relevance of these structural changes must be considered in the absence of effects in other biomarkers at dietary levels less than 710 μg kg^-1.