1. The chief objectives were to determine the daily optimum energy intake (COPT cal day−1) for growth and the gross efficiency (KG%) for converting energy intake into growth for brown trout, Salmo trutta. Energy budgets for individual fish were obtained from experiments with 292 trout (initial live weight 1–318 g) bred from wild parents, and kept at five constant temperatures (5, 10, 13, 15, 18 °C) and 100% oxygen saturation. Most trout (252) were fed over a period of 42 days on a fixed ration of shrimps, Gammarus pulex, the ration levels varying between zero and maximum, but 40 of the larger trout were fed to satiation on freshly-killed sticklebacks (Gasterosteus aculeatus).
2. Energetics models developed in earlier studies on the same data were summarized briefly and were used to predict the relationship between the change in the total energy content of a trout (CG cal day−1) and its energy intake (CIN cal day−1), and hence to estimate COPT. The models were also used to predict the relationship between KG and CIN. In both comparisons, there was good agreement between observed values from the experiments and expected mean values predicted from the models. For trout feeding on invertebrates, COPT lay closer to the maximum, rather than the maintenance, energy intake. When the diet changed from invertebrates to fish, there was a marked increase in CIN, CG and KG.
3. For trout feeding on invertebrates, KG exceeded 30% within 7–11 °C, with a maximum KG of 31.8% at 8.9 °C. For piscivorous trout, KG exceeded 30% within 4–16 °C and 40% within 6.5–12 °C, with a maximum KG of 41.8% at 9.3 °C. These differences were discussed in relation to the results of previous workers, and the models used in the present study provided a method of exploring the limitations of the ‘K-line’ hypothesis for the relationship between KG and CIN.