DOES-RESPONSE EFFECTS OF RECOMBINANT BOVINE SOMATOTROPIN (POSILAC(TM)) ON GROWTH-PERFORMANCE AND BODY-COMPOSITION OF 2-YEAR-OLD RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Two hundred rainbow trout (Oncorhynchus mykiss, mean weight 301.5 g) w ere allotted to four treatments with five replicates in a randomized b lock design to determine the dose-response effects of recombinant bovi ne somatotropin (rbST; Posilac(TM)) on growth performance and carcass composition. Treatments were sham-injected controls (S), 10 mu g/g BW of rbST (L), 20 mu g/g BW of rbST (M), and 30 mu g/g BW of rbST (H). T he tanks held 135 L; water flow = 15.1 L/min; temperature = 15 degrees C. The fish were maintained in a 12-h light:dark cycle and hand-fed t wice daily. The fish received a single intraperitoneal (i.p.) injectio n on d 0 and were weighed on d 0, 14, 28, and 56. On d 56 the fish wer e killed. The whole body (WBW), eviscerated carcass (EC), viscera (VIS ), and reproductive organ weights and the increase in average daily bo dy length (ADL) were determined. Recombinant bST reduced (linear, P < .004) feed intake 17.6% from d 0 to 14 and improved ADG 44.8% from d 0 to 14 (linear, P < .001) and 8.1% from d 0 to 56 (linear, P < .022). Treated groups had improved (linear, P < .001) feed efficiencies for d 0 to 28. Treatment with rbST increased final weight (linear, P < .018 ) and length (linear, P < .001), decreased carcass dry matter (linear, P < .001) and fat(linear, P < .001), content, increased carcass ash ( linear, P < .001) and tended to increase carcass protein (linear, P < .054) content. Recombinant bST increased WBW (linear, P < .018) and EC (linear, P < .003) but decreased (linear, P < .015) testes weight. Ov ary weights, VIS and overall gonadosomatic index were unaffected (P > .05) by rbST. Recombinant bST was undetectable in serum samples taken on d 56 as determined by radioimmunoassay. Overall, the improved ADG, feed efficiency, body mass, and composition indicate that administrati on of rbST to rainbow trout may be an efficacious method of modulating growth in fish.