Growth performance and carcass composition of rainbow trout, Oncorhynchus mykiss (Walbaum), fed the beta-agonist ractopamine

Ractopamine is a synthetic catecholamine analogue which is known to improve feed efficiency, reduce carcass fat and increase lean muscle growth in terrestrial livestock (e.g. sheep, pigs, cattle and chickens), and therefore, may be useful in aquaculture production if similar effects can be elicited in fish. In the present study, rainbow trout, Oncorhynchus mykiss (Walbaum), (mean initial weight of 196 g) were fed the beta-agonist ractopamine at 0.5, 10, 20 or 40 parts per million (p.p.m.) of the diet for 12 weeks in a complete, randomized experimental design. Growth and feed intake were monitored throughout the experiment. Carcasses were analysed for protein, fat, energy and ash prior to the initiation of the trial, and subsequently, every 4 weeks. The growth rate, expressed as thermal-unit growth coefficient (TGC) was higher (P less than or equal to 0.05) in the 10 p.p.m. groups during weeks 1-4 and when averaged over all treatment weeks. Feed efficiency was significantly higher (P less than or equal to 0.05) in fish fed 10 and 5 p.p.m. ractopamine compared with controls during weeks 1-4 and 4-8, respectively, and was significantly higher (P less than or equal to 0.05) in all treated groups when averaged over the 12-week treatment period. Compared with controls, carcass protein was elevated in fish fed 5 p.p.m. ractopamine for 4 weeks, and in those offered 10 and 20 p.p.m. for 8 weeks. Carcass fat was significantly lower (P less than or equal to 0.05) in fish fed 10 p.p.m. ractopamine for 8 weeks, although the 40 p.p.m. group exhibited elevated fat levels in comparison with control fish. The data suggest that rapidly growing rainbow trout demonstrate age-related growth and carcass compositional trends similar to those of mammals, in that feed efficiency and carcass protein decrease while carcass fat increases with age. Oral administration of ractopamine to rainbow trout only modestly altered growth performance, and the trends were not as dramatic nor as consistent as those observed in homeothermic vertebrates fed ractopamine. Out of the concentrations tested, feed incorporation of ractopamine at 10 p.p.m. elicited the most pronounced effect on growth performance.