Linking microhabitat availability and local density of rainbow trout in low‐gradient Japanese streams

Abstract

Quantification of reach‐based microhabitat availability for rainbow trout (Oncorhynchus mykiss), considering their microhabitat requirements in two low‐gradient streams, northern Japan, was attempted to test for habitat space limitation of local trout density. Underwater observations revealed that fish selected microhabitats of moderate current velocity, relatively greater depth and shorter distance to overhead cover in both streams, although habitat features used and available differed slightly between the streams. Habitat space for fish potentially available in the channel environment was evaluated using principal component analysis (PCA) of both available and used microhabitat. A close relationship was evident in both streams between reach‐based microhabitat availability and fish density, which was assessed by a three‐pass removal method. Direct estimates of fish microhabitat availability using PCA can contribute to accurate predictions of local fish density and provide insight into the mechanisms responsible for fish–habitat relationships in streams.