Relationships among nutrition, salinity, and the exogenous growth promoter, 17alpha-methyltestosterone, and growth in the euryhaline tilapia, Oreochromis mossambicus

Effects of long-term 17{dollar}alpha{dollar}-methyltestosterone (MT) treatment (10 mg MT/kg feed) and ration size on the growth of the euryhaline tilapia, Oreochromis mossambicus, in fresh water (FW) and seawater (SW) were examined. On limited ration, control fish in SW grew to twice the size of their FW counterparts. A reduction in routine metabolism in O. mossambicus in SW may account for the increase in growth of the SW animals over their FW counterparts.; Treatment with MT increased growth in both FW and SW tilapia. Doubling the feed ration produced a significant increase in growth in SW, but not in FW fish. Varied protein content in the diet appeared to affect the ability of MT to stimulate growth. The use of MT makes it worthwhile to use a more expensive, high-protein feed to achieve greater growth in SW.; Distribution and metabolism of 3,5,3{dollar}spprime{dollar}-triiodothyronine (T{dollar}sb3{dollar}) were studied in FW O. mossambicus to better understand the clearance of the endogenous growth promoter and to develop a protocol for MT clearance. Time serial analysis that was done after injection of {dollar}sp{lcub}125{rcub}{dollar}I-3,5,5{dollar}spprime{dollar}-T{dollar}sb3{dollar} (T{dollar}sb3{dollar}*) into the tilapia ventral aorta indicated the presence of 2% of total body T{dollar}sb3{dollar}* in plasma, 5% in rapidly exchanging tissues and 93% in slowly exchanging tissues. Plasma-clearance rates and minimum production rates were quite different from those of rat and human, and 5 to 50 times smaller than values reported for rainbow trout. Enzymatic-chromatographic analyses indicated substantial amounts of labeled glucuronide, but not sulfate, conjugates of iodothyronines appearing in plasma. T{dollar}sb3{dollar}* uptake studies indicated equal uptake of T{dollar}sb3{dollar}* into kidney, liver and intestine ({dollar}sim{dollar}5% of the dose per min) in fed fish, but half as much in the kidney and no significant uptake in liver or intestine in fish fasted for 24 hr.; Radioimmunoassay detected no MT in tilapia plasma 8 hr after cessation of MT feeding, and none in bile or muscle by two days after cessation of MT feeding.