Ideal Free Distributions of Stream Fish: A Model and Test with Minnows, Rhinicthys Atratulus

Ideal Free theory has furthered our understanding of the processes determining the distribution of mobile foragers in a spatially heterogeneous (patchy) habitat. The Input Matching rule derived from Ideal Free theory has been used to predict forager distributions, but does not account for unshared environmentally induced costs that individuals may incur. Drift—feeding stream fish typically contend with such costs in the form of (1) an energetic costs of maintaining position while foraging in moving water, and (2) a decrease in the proportion of drift food items they can capture as current speed increases. These costs often differ between patches, and the cost of position maintenance is unaffected by changes in the number of fish foraging in a patch. Here, we developed an Ideal Free Distribution (IFD) model (IFD With Costs) to describe forager distributions in a habitat composed of patches in which foragers compete for food and incur an energetic cost when foraging that is unaffected by the number of competitors in the patch. We tested the ability of two versions of the IFD With Costs as well as two alternative IFD models to predict the distribution of minnows (Rinichtys atratulus) foraging in a laboratory flow—tank with two side—by—side patches. In all trials the patches differed in their supply of drift food, and in some trials the patches had different water velocities. When the patches' water velocities differed, the fish distributions (1) changed as the total supply of food in the two patches increased while the ratio of food supply in the patches remained constant, (2) did not match relative food availabilities in the patches, and (3) favored the slow water velocity patch. One version of the IFD With Costs predicted each of these results while none of the alternative models could account for these findings. The results support the hypothesis that fish quantitatively integrate energetic gains and costs in a manner similar to that described by the IFD With Costs when making patch selections.