Effects of Diet, Ambient Temperature, and Holding Conditions on Plasma Thyroxine Levels in Rainbow Trout (Salmo gairdneri)

Rainbow trout (Salmo gairdneri) acclimated to 19 °C and fed a pelleted diet of either 6% soybean oil or 6% tallow fat had similar plasma thyroxine levels (0.62 ± 0.10 and 0.69 ± 0.08 μg/100 ml, respectively), although fish acclimated to 7 °C and fed the soybean diet gave significantly (P < 0.05) higher plasma thyroxine concentration (1.32 ± 0.13 μg/100 ml) than the group fed the tallow fat diet (0.95 ± 0.12 μg/100 ml). The plasma thyroxine concentration in the fish fed the soybean oil diet was significantly (P < 0.01) higher at 7 °C than at 19 °C. There was no significant difference between the two groups of fish given the tallow fat diet. Epithelial cell heights were largest in groups with the highest plasma thyroxine levels. Epithelial cell height in the two groups at 19 °C were similar, but in both groups of fish at 7 °C, the cell height was significantly greater than in either of the groups at 19 °C (P < 0.01 for comparisons of the 19 °C tallow fat diet with both groups of 7 °C acclimated fish and com...