Assessment of vital organ histopathology and plasma oxidative conditions of rainbow trout Oncorhynchus mykiss reared in earthen saltwater pond

The aim of this study was to compare gill, kidney, liver and gut histopathology, and plasma antioxidant markers of rainbow trout Oncorhynchus mykiss reared in saltwater earthen ponds in Gomishan, Iran. To this, 10000 fish were distributed in a three-ha earthen pond and 150 fish in three fiberglass tanks (2000L). Blood samples were taken after 3 months rearing with same commercial feed. The source of fish and feed was similar between the saltwater pond and fiberglass tanks. After the 3-month rearing, gill, kidney, liver and gut samples were taken from the pond fish; whereas, blood samples were taken from both the pond and tank fish. There was no significant difference in water temperature, dissolved oxygen and pH between the pond and tanks; however, water salinity and ammonia was higher in the pond compared to the tanks. Plasma superoxide dismutase and glutathione peroxidase activity of the fish in earthen ponds were significantly higher than those fish reared in fiberglass tanks; however, there was no significant in thiobarbituric acid reactive substances between the pond and tanks. The fish had various histopathological symptoms including primary and lamella hyperplasia, lamellar fusion and epithelial lifting. In the kidney section, the fish showed glomerulus shrinkage and/or disappearance, melanomacrophage aggregates and hematopoietic tissue necrosis. These fish showed necrosis and melanomacrophage aggregates in liver and goblet cell hypertrophy in gut. The results suggest that the fish in the earthen pond faced stressful conditions, which might be due to water salinity and ammonia; however, other possible factors, such as pollutants and different feeding regimen must be considered.