THE EFFECTS OF DIETARY-FAT CONTENT ON PERFORMANCE AND BODY-COMPOSITION OF FARMED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Three studies were conducted, one in autumn with decreasing water temp eratures (11 degrees to 5 degrees C), one in spring with rising temper atures (7 degrees to 12 degrees C) and one in summer in relatively sta ble temperatures (mean 13.2 degrees C). Three diets (T1, T2, T3) conta ining 470/100, 450/150, 430/200 g kg(-1) of protein and fat, respectiv ely, were fed to provide equivalent energy intakes. Actual mean feedin g levels were: Trial 1, 13, 12, 11 g kg(-1) body weight; Trials 2 and 3, 16, 15, 14 g kg(-1) body weight, respectively, for T1, T2, T3. In T rial 2 a fourth treatment (T4) was diet T3 given at 16 g kg(-1) and in Trial 3 a fourth treatment was diet T4 formulated to contain 410/230 g kg(-1) of protein and fat and fed at 13 g kg(-1) body weight. Initia l weights of trout were 170 g in Trial 1, 145 g in Trial 2 and 215 g i n Trial 3. Initial pond loads and stocking densities were 184, 40, 30 kg and 12.4, 18.4, 13.8 kg m(-3). All three trials were of 6 weeks dur ation. It is concluded that in the spring and summer trials protein in take was adequate on the diet with the highest energy:protein ratio bu t that protein may have been limiting at the lower feeding levels whic h occurred with falling temperatures in the autumn trial. Observed imp rovements in feed conversion ratios with the higher energy diets were in line with expected values in Trial 2 and were better than expected in Trial 3. The effects of high fat diets on fillet and offal composit ion were also examined. It was found that fillet and offal fat levels on a fresh and dry matter basis were significantly increased as increa sing amounts of fat were fed. (C) 1997 Elsevier Science B.V.