Quantitative analysis of histopathological lesions in rainbow trout (Oncorhynchus mykiss) from northern Iran farms

This study was conducted to evaluate the health of rainbow trout on three farms in Mazandaran Province using a quantitative analysis of the histopathological alteration method. The fish were sampled within three classes: <100, 300-700, and >1500 g. Ten fish were sampled per size class from each farm. The fish gill, liver, kidney, and intestine pieces were collected, fixed, sectioned, mounted on slides, and stained for histopathological examinations. A quantitative approach was followed to examine the tissues alternation and fish. The results showed that the organ index of the gill in 300-700 and >1500 g fish was significantly higher than <100 g ones. The organ index of the liver in >1500 g fish was significantly higher than in the other two groups. There was no significant difference in the organ index of the kidney and intestine. Organ index within each fish size was as follow: gill> liver> kidney> intestine. Moreover, the total index was calculated based on the organ index in a fish and, accordingly, <100 g fish had the lowest total index than 300-700 and >1500 g fish. In conclusion, the fish gill exhibited more pathological changes due to direct contact with the ambient environment. Moreover, the fish intestines were generally healthy, suggesting suitable diet quality, although vacuolization and hypertrophy of the hepatocytes show that the dietary fat and feeding rate should be controlled.