INFLUENCE OF DIETARY-FAT LEVEL AND INCREASED ADIPOSITY ON GROWTH AND FAT DEPOSITION IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM)

Rainbow trout, Oncorhynchus mykiss (Walbaum), (initial weight approxim ate to 90 g) were fed diets differing in fat content (27.5% versus 12. 6%) to examine the effects of dietary fat level on growth and fat depo sition. During phase 1, which was run for 11 weeks, the fish were fed either a high- (27.5%) or low-fat (12.6%) diet. The fish were held at 14.5 degrees C under a 24 h:0 h light:dark photoperiod and were fed fo r 4 h each day during this phase. Subsequently, in phase 2, both diets were presented simultaneously, and the influence of adiposity on grow th and fat deposition was examined in a growth trial which lasted for 8 months, There were no significant differences in body weights at the end of phase 1 (360.7 +/- 7.7 g versus 348.2 +/- 18.7 g for the fish on the high- and low-fat diets, respectively), but body composition wa s influenced by the fat content of the diet. The trout fed the high-fa t diet had higher viscerosomatic indices (VSI: 11.7 +/- 2.0% versus 9. 1 +/- 0.7%) and higher visceral fat contents (per cent fat: 38.0 +/- 6 .7% versus 23.3 +/- 3.7%) than trout fed the low-fat diet, The percent age fat contents of the carcass (per cent fat: 12.4 +/- 1.0% versus 9. 3 +/- 0.7%) and whole body (per cent fat: 15.4 +/- 1.6% versus 10.5 +/ - 0.7%) were higher amongst the trout fed the high-fat diet, and these fish had a higher energy gain than those fish fed the low-fat diet. D ifferences in adiposity between the trout fed the high- and low-fat di ets did not result in any differences in weight gain when the fish wer e provided with both diets simultaneously. During phase 2, the fish th at had previously been fed the low-fat diet deposited more fat than th ose fed the high-fat diet during phase 1. The latter maintained whole body fat concentrations at approximate to 15% (phase 1: 15.4 +/- 1.6%; phase 2: 15.2 +/- 1.2%), whereas the fish that had been fed the low-f at diet increased their body fat concentration during phase 2 (phase 1 : 10.5 +/- 0.7%; phase 2: 14.7 +/- 1.3%). Thus, there were no signific ant differences between groups in either body weight or proximate comp osition by the end of the experiment.