Effect of diet on serum thyroid hormone levels in rainbow trout ( Salmo gairdneri Richardson)

Abstract

1. 1. Rainbow trout ( Salmo gairdneri Richardson) fed a diet containing low available carbohydrate (cerelose) (0%) for 16 weeks exhibited the highest weight gain of the five experimental groups and had significantly higher plasma T4/T3 ratios than groups fed 7, 11 and 14% cerelose diets of similar calorific content; there were no differences in plasma T4/T3 ratios between fish fed the 0 and 21% cerelose diets. 2. 2. There were no significant differences in plasma T4 levels in the five groups fed diets containing different cerelose contents, but plasma T3 levels in the 0% cerelose group were significantly lower than in the 7% cerelose group. 3. 3. Plasma T4 and T3 concentrations and T4/T3 ratios in three groups of rainbow trout fed diets containing different lipid levels (10, 15 or 20% salmon oil) for 28 weeks were similar in fish maintained at 17°C, but in groups maintained at 5°C the fish fed the 10% salmon oil diet had significantly lower plasma T4 concentrations and T4/T3 ratios and significantly higher plasma T3 concentrations than fish fed the 15 or 20% salmon oil diet. 4. 4. Plasma T4 concentrations and plasma T4/T3 ratios were significantly higher and plasma T3 concentrations were significantly lower in the fish maintained at 5°C and fed the 15 or 20% salmon oil diets than in comparable groups maintained at 17°C. 5. 5. Plasma T4 concentrations and T4/T3 ratios were significantly lower in rainbow trout maintained for 20 weeks on a purified diet containing 10 mg/kg pantothenic acid when compared with fish fed purified diets containing 40 or 320 mg/kg pantothenic acid or practical diets; plasma T3 concentrations in the fish fed the purified diets were extremely variable and there were no significant differences between the three groups. 6. 6. There was no apparent correlation between weight gain and plasma thyroid hormone levels in any of the three experiments.