Abstract
Clinical, gross and histopathological investigations were carried out into large‐scale mortalities on eastern Mediterranean bluefin tuna, Thunnus thynnus (L.), farms. Fish showed only nervous signs and darkened colour. At post‐mortem the liver was bronze coloured and the pyloric area waxy in consistency. There was no evidence of any other gross pathology. Histopathology showed severe hepatic necrosis and lipidosis. Peri‐pancreatic lipoid tissue was heavily infiltrated with an inflammatory round cell infiltrate. Fish on all three farms had been fed on a North African pilchard diet rather than traditional local or Baltic species. Once the diet was modified, losses ceased. A diagnosis of pan‐steatitis as seen in other farmed fish species, as well as in terrestrial animals, on particular fish‐based diets was made, although the actual factor within the diet which induced the inflammatory effect is not known.