A new energetics model for brown trout, Salmo trutta

1. The chief objective of the present study was to develop a functional mod el for the daily change in the total energy content of a brown trout, Salmo trutta, (equivalent to growth when positive) in relation to the difference between energy intake (energy content of food) and energy losses (metaboli sm + losses in faeces and excretory products). Energy budgets for individua l fish were obtained in earlier experiments with 210 hatchery trout (live w eight = 11-270 g) kept at fairly constant temperatures (mean values ranging from 3.6 to 20.4 degrees C), but without strict control of temperature or oxygen, and in later experiments, with 252 trout (1-300 g) bred from wild p arents and kept at five constant temperatures (5, 10, 13, 15 and 18 degrees C) and 100% oxygen saturation. Each trout was fed a fixed ration of shrimp s, Gammarus pulex, the ration level varying between zero and maximum. 2. Energy intake (C-IN, cal day(-1)) was measured directly and expressed as a proportion (p) of the maximum energy intake (C, cal day(-1)), the latter being estimated from a model developed earlier. In a new model, energy los ses (C-Q, cal day(-1)) were expressed as a function of temperature, fish we ight and ration level. This model was continuous over the 3.6-20.4 degrees C range, had twelve fitted parameters and was an excellent fit to the data for the 462 trout (P < 0.001, R-2 = 0.9970). In an extended model, the weig ht exponent for energy losses was not assumed equal to that for energy inta ke, the difference between the two exponents being very small, but signific ant, with a slight improvement in the fit of the model (R-2 increased to 0. 9972). 3. The limits of model use were discussed. An example of its utility was to elucidate the complex relationships between both positive (growth) and neg ative daily changes in the total energy content of the trout, and temperatu re, fish size and variable energy intake. The model has raised several ques tions for future work, including the effect of increasing energy intake by a change of diet from invertebrates to fish or fish pellets, and a comparis on of growth models based on weight or energy changes.