Quantifying the direct and indirect effects of flow-related disturbance on stream fish assemblages

Flood-related disturbances are predicted to be seriously altered by climate change effects, and this will have strong implications for stream communities. Predicting how and why community structure responds to changes in disturbance regimes will require measures of disturbance that are closely linked to community variability. A range of disturbance measures have been tested for their ability to explain patterns in stream periphyton and invertebrate assemblages, but assessments of fish have largely focussed on flow measures as predictors. Consequently, the mechanisms driving fish assemblage responses to disturbance are poorly known. Stream disturbance was quantified using seven measures, three based on disturbance of the streambed (assessed using painted tracer particles), three associated with variation in stream discharge and one from a subjective measure of stream channel stability. Twenty streams were sampled on five occasions to evaluate which disturbance measure explained the most variation in fish assemblages and also to quantify the influence of disturbance on spatial and temporal assemblage variability. To determine whether disturbance affected fish directly or via its effect on benthic food supply (i.e. stream invertebrates), a one-off survey of 52 streams was conducted. Path analysis on data from the 20 streams indicated that fish biomass responded most strongly to flow-mediated bed movement, not flow variability. Consequently, bed-movement measures were better predictors of spatial and temporal variability in fish biomass and assemblage structure than purely hydrological measures. Furthermore, variations in fish biomass and assemblage structure were higher in more disturbed streams. A second path analysis conducted on data from the 52 stream survey compared the effects of physical habitat and food-related components of bed disturbance on fish biomass. It indicated that both had a similar influence on disturbance-related reductions in fish biomass. These results indicate that variation in fish assemblages can best be understood by measuring or predicting bed movement, because bed disturbance is likely to encompass more effectively major factors that influence stream fish, such as habitat availability and food supply. A shift in disturbance regime is likely to change the biomass of fish a habitat can support and fish assemblage structure.