A NEW FUNCTIONAL-MODEL FOR ESTIMATING THE MAXIMUM AMOUNT OF INVERTEBRATE FOOD CONSUMED PER DAY BY BROWN TROUT, SALMO-TRUTTA

1. A model developed over 20 years ago has been used to estimate daily food intake in brown trout living in streams and lakes over a wide ge ographical range. The chief disadvantages of this early model are that it is not continuous and requires twelve parameters, not all of which can be interpreted biologically. A new model, using a larger data set , was therefore developed to overcome these problems and estimate the mean daily energy intake. 2. The two data sets used to develop the ori ginal model were also used to develop the general form of the new one, but a third data set was used to specify the model more precisely and to estimate the parameters. This third data set originated from exper iments in which 185 trout (live weight range 1-350 g) were kept indivi dually at 19 constant temperatures (range 3.8-21.7 degrees C) usually for 5-6 weeks. They were fed freshly killed shrimps (Gammarus pulex) a nd their food consumption was recorded throughout each experiment. 3. Five, six and eight parameter versions of the new model were all excel lent fits to the data (P < 0.001, R-2 > 0.99), with the eight paramete r version being slightly the best. All parameters can be interpreted i n biological terms; three define threshold temperatures, three define the curvilinear slopes in the model over different temperature ranges, one is a weight exponent and one is the maximum daily energy intake o f a 1g trout. The simpler six parameter model was adequate at temperat ures above 7 degrees C. 4, An additional experiment with twenty-eight trout feeding on six different invertebrate foods provided estimates o f energy intake that were very similar to those predicted from the mod el. However, when daily intake was converted to dry weight, agreement with values from the model (also as dry weight) was poor. Possible rea sons for this are discussed, as are other studies using the earlier mo del, and it is shown that different conclusions can be reached dependi ng upon whether comparisons are based on units of energy, dry weight o r wet weight.