Optimum energy intake and gross efficiency of energy conversion for brown trout, Salmo trutta, feeding on invertebrates or fish

1. The chief objectives were to determine the daily optimum energy intake ( C-OPT cal day(-1)) for growth and the gross efficiency (K-G%) for convertin g energy intake into growth for brown trout, Salmo trutta. Energy budgets f or individual fish were obtained from experiments with 292 trout (initial l ive weight 1-318 g) bred from wild parents, and kept at five constant tempe ratures (5, 10, 13, 15, 18 degrees C) and 100% oxygen saturation. Most trou t (252) were fed over a period of 42 days on a fixed ration of shrimps, Gam marus pulex, the ration levels varying between zero and maximum, but 40 of the larger trout were fed to satiation on freshly-killed sticklebacks (Gast erosteus aculeatus). 2. Energetics models developed in earlier studies on the same data were sum marized briefly and were used to predict the relationship between the chang e in the total energy content of a trout (C-IN cal day(-1)) and its energy intake (C-IN cai day(-1)), and hence to estimate C-OPT. The models were als o used to predict the relationship between K-G and C-IN. In both comparison s, there was good agreement between observed values from the experiments an d expected mean values predicted from the models. For trout feeding on inve rtebrates, C-OPT lay closer to the maximum, rather than the maintenance, en ergy intake. When the diet changed from invertebrates to fish, there was a marked increase in C-IN, C-G and K-G. 3. For trout feeding on invertebrates, K-G exceeded 30% within 7-11 degrees C, with a maximum K-G of 31.8% at 8.9 degrees C. For piscivorous trout, K- G exceeded 30% within 4-16 degrees C and 40% within 6.5-12 degrees C, with a maximum K-G of 41.8% at 9.3 degrees C. These differences were discussed i n relation to the results of previous workers, and the models used in the p resent study provided a method of exploring the limitations of the 'K-line' hypothesis for the relationship between K-G and C-IN.