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

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Abstract
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 comparison of the two groups on the soybean oil diet; P < 0.05 for comparisons of the 19 °C, soybean oil diet with the group held at 7 °C and given the tallow fat diet). Fish deprived of food for 40 days had a significantly (P < 0.05) lower plasma thyroxine concentration when compared with fish that had been fed daily. There was no significant diurnal variation in plasma thyroxine over the light phase of a 12 h light: 12 h dark photoperiod, nor did the limitation of light entering the aquaria significantly affect plasma thyroxine values. Key words: endocrine, thyroid, Salmo gairdneri, diet, environmental temperature

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