Streamflow regulation and fish community structure

Many regulated streams are characterized by high variable and unpredictable flowregimes. Since changes in streamflow directly modify physical habitat, streams with such highlyvariable flows provide highly unstable aquatic habitats. We evaluated the effect of artificialstreamflow fluctuations on stream fish communities by comparing fish densities, in species andhabitat groups, between two rivers differing in daily flow regime: on with with a natural flow, andone with highly regulated flows.We developed a simple model describing the relationship between available streams habitat andits use by 15 species or size classes of fish in the natural river. Species and size classes thatused a specific set of microhabitat conditions were identified by comparing habitat characteristicsfor samples with and without each type of fish; for fish that used a particular type of microhabitat,we grouped species and size classes according to similarity in microhabitat use. Next, wecategorized stream habitat samples in both the natural and regulated rivers into groups on thebasis of fish habitat use criteria. Fish densities for each fish and habitat group were tenindividually compared between the two rivers.An abundant (\u3e90% of all fish) and diverse (nine species) group of small-fish species and sizeclasses were restricted to microhabitat that was characterized as shallow in depth, slow in currentvelocity, and concentrated along stream margins. This group of fish was reduced in abundance inthe regulated river and absent at the study site with the greatest flow fluctuation. Another fishgroup included species and size classes that used either a broad range of habitat or amicrohabitat that was deep, fast, or both, and was concentrated in midstream areas. The densityof fish in this group was higher in the regulated river and peaked at the sites with the greatestfluctuations in flow. Highly variable and unpredictable flow regimes appear to be a high-frequency disturbance that effects fish differently depending on the way they use stream habitat and acts toreduce community complexity. http://www.esajournals.org/doi/abs/10.2307/194043