Experimental evidence for the effect of depth and structure on the distribution, growth, and survival of stream fishes

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Abstract
Water depth and physical structure are important components of habitat complexity for stream fishes. Experiments in a semi-natural stream, containing four depth and structure treatments, quantified the effect of these two habitat components on the distribution, growth, and survival of five fish species common to coastal streams of Washington State. When fishes were permitted to move freely among the various habitat types, most species and age-classes were underrepresented in shallow pools lacking structure. In some cases water depth or physical structure alone appeared to explain these distributions. However, assemblage- and species-level responses were strongly influenced by the combined effects of depth and structure. A subsequent 30-d experiment revealed that mortality (likely due to bird predation) of water-column species using the simplest habitat type was as much as 50% greater than in the other treatments. However, there were no differences in survival of the benthic species, coastrange sculpin (Cottus aleuticus), among the different treatments, nor were there significant differences in growth among treatments for any species. These results indicate that predation risk may help to explain the importance of both deep water and physical structure to fishes in coastal streams in Washington. The results of our habitat-selection experiment also support the growing view that a community-level approach may be more effective than the single-species approach in evaluating the effects of human activities on stream fishes.

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