Ideal free distribution theory as a tool to examine juvenile coho salmon (Oncorhynchus kisutch) habitat choice under different conditions of food abundance and cover

We investigated the mechanisms affecting habitat choice by juvenile coho sa lmon (Oncorhynchus kisutch) in relation to the patchy distribution of food and cover. We tested the following hypotheses: (i) the distribution of juve nile coho, both between patches in a pool and between separate pools in a c hannel, corresponds numerically to the food input rate of those habitat pat ches as predicted by the "input-matching rule" of ideal free distribution ( IFD) and (ii) the addition of instream cover, by increasing visual isolatio n among competitors, promotes input matching both within and between pools. We conducted our experimental work in artificial channels and we used two different types of cover, instream and overhead. In the absence of cover an d with either no differences or relatively small differences in food abunda nce between patches, the spatial distribution of juvenile coho responded nu merically to the input rate of food as predicted by the IFD. However, when differences in food abundance between patches were relatively large or cove r was present, fish distributions consistently undermatched food input rate in the rich patch. Coho foraged in open patches away from cover within sin gle pools but preferred pools with cover when choosing between separate poo ls. Several IFD models were used to examine the observed dispersion pattern s.