DIETARY-EFFECTS ON THYROID-HORMONES IN THE RED DRUM, SCIAENOPS-OCELLATUS

Juvenile red drum (Sciaenops ocellatus) were cultured at 25-degrees-C on a variety of diets and blood sampled over eight weeks to examine th e relationship between growth and plasma thyroid hormone levels. Maxim um growth rates were achieved on formulated experimental diets and a s imulated natural shrimp diet. Associated with these maximal rates was a significant increase in triidothyronine (T3), but no consistent chan ge in thyroxine (T4). Reduced rations of diets resulted in low growth rates associated with significantly lowered levels of T3 but not T4. T o determine whether weight gain could be increased by application of e xogeneous hormone, diets were supplemented with T3 or T4 at 2, 10, and 50 mg hormone/kg diet. Significantly elevated T3 was induced by suppl ementation with 10 and 50 mg T3/kg diet, although there were no indica tions of an anabolic effect of T3 incorporation, and 50 mg T3/kg diet was in fact associated with decreased weight gain. Incorporation of T4 into diets had no effect on growth or T3, and had effects on T4 which were small and inconsistent, indicating that T4 may not be effectivel y absorbed from the gut. No difference was found in response to hormon e feeding between low (6 ppt) or high (35 ppt) water salinity. T3 leve ls thus appear to closely parallel growth in fish on unsupplemented di ets, whereas T4 remains relatively insensitive to dietary manipulation . Supplementation with T3 is not an effective means of stimulating gro wth in red drum fed optimum diets. Whereas thyroid hormones may functi on to regulate intermediary metabolism in red drum, elevated endogenou s thyroid hormone levels appear adequate to supply tissue needs during juvenile growth in culture.