| Abstract: | Three feeding trials were conducted in an aquarium system with recirculating brackish water (6 [plus or minus] 1 ppt) to investigate the effects of dietary protein sources and diet formulations on juvenile red drum. Diets containing 35% crude protein from muscle of various marine and freshwater fishes produced weight gain (WG), feed efficiency (FE), protein efficiency ratio (PER), and body composition similar to that of the control diet containing freeze-dried muscle of red drum. However, a commercially extruded red drum diet and pelleted trout diet produced significantly (P<0.05) lower values. A similar control diet produced significantly greater WG, FE, and PER values than diets containing protein from whole croaker (Micropogon undulatus), striated beef muscle or menhaden (Brevoortia tyrannus) fishmeal; the croaker diet produced significantly greater WG than the beef and menhaden diets. Five feeding trials also assessed the effects of supplementing either 3,5,3'-triiodo-L-thyronine (T3) or thyroxine (T4) at 0, 2, 10, and 50 mg/kg to semipurified diets containing red drum muscle, on WG and proximate composition of juvenile red drum in artificial brackish (6 [plus or minus] 1 ppt) and saline (32 [plus or minus] 2 ppt) water. Regardless of salinity, dietary T3 resulted in lighter colored fish at higher doses. The highest dose also resulted in significantly reduced WG. The circulating levels of T3 appeared to be at maximal physiological levels when red drum were fed this high-quality diet. Therefore, growth was not further promoted by dietary supplementation of T3. Higher doses of T3 also resulted in significantly reduced condition factor and muscle ratio at both levels of salinity, but differences in whole-body and muscle proximate composition were not consistent. In the saline water, supplementation of T3 at the two highest doses did prevent mortality which was relatively high for fish fed the lower doses. Dietary T4 appeared to be poorly absorbed based on circulating levels and had no distinctive effects on appearance, growth, and proximate body composition... |
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