Study of fatty liver disease prevalence in the raised rainbow trout (Oncorhynchus mykiss) from Shahrood city’s farms, Semnan Province

In coldwater-fish farms, 50-60% of current expenses is for feed suplly. High-lipid feed may cause an increase in the lipid storage in body. In most fish, fat storage is limited and excess fat can accumulate in liver tissue that causes lipoidosis. All salmonid fish and especially rainbow trout are susceptible to fatty liver degeneration. Fatty liver is a serious and even lethal disease in farmed fish. In this study 200 farmed rainbow trout were collected in two seasons, autumn (100 samples) and winter (100 samoles) from 20 farms and 10 samples from each farm in Shahrood city of Semnan province in 2021. After biometry the fish were classified into 300-400, 400-500- and 500-600 g weight groups and immediately transferred into the lab under chilled conditions. The livers with discoloration (yellow, milky white and light brown), enlargement, swelling, rounded edges and even fragility, hemorrhagical spots and hyperemia on the surface were selected as suspected livers. After preparing histological slides, totally, five liver samples were confirmed as positive samples that showed aggregation of fat vacuoles, cytoplasm changes and eccentric nucleus, bigger hepatocytes compare to normal ones and restricted necrosis as coagulative necrosis. No significant difference was observed between the occurrence and non-occurrence of fatty liver in autumn and winter and also in different farms.