Effects of coarse woody debris accumulation, channel structure and land use on fish populations in forested lowland streams

Abstract

A detailed study of forested streams in the New Forest in southern England has shown that the accumulation of coarse woody debris (CWD) in channels affects the within-reach structure of the stream channel and the abundance, diversity and community structure of the fish fauna. Samples in a priori selected habitat units, namely 43 riffles, 80 pools and 39 CWD accumulations showed significantly higher densities of fish in the riffles than in the other habitat units and significantly higher biomass in the CWD accumulations. Effects on each of the six species present differed. There was a significant correlation between maximum size of salmonids and abundance of CWD in a habitat unit.

Habitat diversity and fish diversity were lowest in riffles and dense CWD accumulations and highest in habitats with moderate amounts of CWD present. Communities of habitat units could not be separated clearly based on the a priori selection, but both the physical structure and fish communities showed a gradient of change from an erosional to a depositional condition with riffles and the deepest CWD habitats as the opposing extremities of the gradient. On the reach scale habitat diversity was related to the abundance of CWD but overall fish diversity and abundance were not. Salmonid density was negatively correlated with CWD abundance on the reach scale. The fish community of the forested streams was highly structured and characteristic of a deterministic community relatively undisturbed by human influences. A simple, partly hypothetical model is proposed to predict the effects of varying abundance of CWD on the fish communities of the forested streams. Future stream habitat management is discussed and potential applied and fundamental topics for research outlined.

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