Daily energy intake and growth of piscivorous brown trout, Salmo trutta

1. The chief objectives were to determine the daily energy intake and growth of piscivorous brown trout (Salmo trutta), and to compare the observed values with those expected from models developed previously for brown trout feeding on freshwater invertebrates. Energy budgets for individual fish were obtained from experiments with 40 trout (initial live weight 250-318 g) bred from wild parents, and kept at five constant temperatures (5, 10, 13, 15, 18 degrees C) and 100% oxygen saturation. Each trout was fed to satiation on freshly killed sticklebacks (Gasterosteus aculeatus) over a period of 42 days. 2. Energy intake (C-IN cal day(-1)) and growth (C-G cal day(-1)) were measured directly and energy losses (C-Q cal day(-1)) were estimated by difference (C-Q = C-IN - C-G). All three variables increased with temperature. A model previously used to predict the daily energy intake (C-IN(INV)) of trout feeding to satiation on invertebrates was adapted, by changing only one parameter, to provide an excellent model (R-2 = 0.998) for predicting the mean daily energy intake (C-IN(ST)) for the piscivorous trout. Values of C-IN(ST) were 58% (range 48-67%) higher than those for C-IN(INV). A simple model was also developed to estimate mean daily energy losses for piscivorous trout (R-2 = 0.999). Both models were combined to provide excellent estimates of the daily energy gain (growth) of the piscivorous trout, and this was about three times that for trout feeding on invertebrates. The optimum temperature for maximum growth in energy terms increased from 13.9 degrees C for trout feeding on invertebrates to 17.0 degrees C (range 16.6-17.4 degrees C) for piscivorous trout. 3. The models are basically an extension of those developed for trout feeding on invertebrates. They show clearly how energy intake, growth, and the optimum temperature for growth increase markedly when trout change their diet from invertebrates to fish. The implications of this are discussed and it is shown that, in theory, these increases should continue if a more energy-rich diet was utilised by the trout.