Effect of a commercial trout diet on liver ultrastructure of fed and fasted yearling coho salmon, Oncorhynchus kisutch Walbaum

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Abstract
Yearling coho salmon were fed either a commercial trout (TC) diet or a diet consisting of homogenized adult Pacific Ocean coho salmon (PS), supplemented with vitamins and minerals. The hepatosomatic index of the TC‐fed fish killed 12 h after the last feed or after 4 or 8 weeks of fasting were significantly larger (P 0.05) than comparable PS‐fed groups.‘Light’ and‘dark’ hepatocyte cell types were evident in all groups although the dark cells were less numerous in the fasted fish. Hepatocytes of fasted fish contained markedly more endoplasmic reticulum than fed fish. Hepatocytic glycogen content of the TC‐fed fish was higher than in PS‐fed fish. Whereas in the TC‐fed group hepatic glycogen was still evident in fish fasted for 8 weeks, the hepatocytes of the fasted PS‐fed group contained little or no glycogen. The suitability of commercial fish diets for salmonids is discussed.