Population-Level and Behavioral Responses As Tools for Short-Term Effectiveness Monitoring of Instream Habitat Restoration

In-stream habitat restoration is a commonly employed technique locally, regionally, and globally for the recovery of impacted stream fish populations; however relatively few of these restoration projects is followed by monitoring to determine the project’s effectiveness.  In the Entiat River (Washington, USA), in-stream habitat restoration structures were installed to enhance juvenile rearing habitat, particularly for chinook salmon (Oncorhynchus tschawytscha) and steelhead (O. mykiss).  To complement large-scale, census-based monitoring in several treated and untreated reaches in this basin, we examined fish density, growth, and movement for both species in small-scale assays using replicated structures in a single treated and similarly sized pools in a single untreated reach over two study seasons.  Fish density declined during the study seasons in both reaches and each species responded differently to reaches with habitat structures. In both years, chinook salmon were substantially more abundant in the treated reach than in the untreated reach, but steelhead were frequently more abundant in the untreated reach.  Changes in density during the study season occurred and indicated that observed density is to some extent independent of habitat type.  Change in fish density over time made estimates of density-dependent growth problematic, but we did find that movement patterns were both density- and habitat-dependent.  Fish tended to move less frequently in the treated reach relative to the untreated reach, which is suggestive of higher habitat affinity despite increased density in pools with structural enhancement.  Steelhead showed higher affinity for treated reach pools despite the fact that their density was often higher in the control reach.  A comparison of density-dependent growth in each reach type was only possible for steelhead, given that numbers of recaptured chinook were not sufficient in the control reach for analysis.  Steelhead growth indicated that restoration potentially increased carrying capacity in treated reaches.  Finally, some of the benefits of instream habitat restoration were mediated through basic environmental variables.  Treatments created deeper pools with slower flowing water that favored juvenile chinook, potentially explaining our observation of consistently higher density there relative to the control reach.  Our results suggest that microhabitat scale, within-reach studies of restoration treatments might be necessary to detect impacts to fish populations, although further analysis and more data are required to establish this conclusively.