Use of a Riverscape Census to Evaluate the Influences of Space, Time and Environment on the Summer Habitat Selection and Distribution of Three Salmonid Species

Associations between stream habitat and salmonids have been well studied at the habitat unit and stream reach scales.  In contrast, the influences of stream habitat on the abundance and distribution of salmonids at the scale of the entire watershed or 'riverscape' are less clear.  Based on the smaller-scale research, juvenile salmonid and trout abundance is assumed to vary in relation to stream habitats that offer differing opportunities for growth and survival.  Accordingly, salmonid abundance should peak and dip across the riverscape in relation to stream habitat, and temporally, the distribution may expand and contract annually depending on climate and environment.  The extent to which the abundance and distribution of salmonids varies and is influenced by space, time and stream habitat has important implications for population viability, and in turn, stream restoration and salmonid recovery efforts.  To improve knowledge on salmonids at the riverscape scale we censused stream habitat features and coastal cutthroat trout, juvenile coho salmon and juvenile steelhead across 85%  and 75% of the Calawah River basin during late summer in 2002 (a drought summer) and 2003 (a summer with average precipitation), respectively.  We delineated the riverscape into mainstem river sections and tributaries. In the field we measured slope, depth, surface area, substrate, and instream cover. Fish were enumerated via snorkeling in all habitat units that met minimum depth requirements.  Preliminary analyses revealed a high level of spatial variation in the abundance of salmonids: in some streams, a few habitat units were disproportionately used while in others there was a more consistent level of abundance.  The patterns also varied dramatically between species and among life stages of juvenile steelhead, and the distribution of salmonids was reduced during the drought summer compared to the wetter summer.  Using natural breaks in the fish abundance data and electivity indices we effectively distinguished the characteristics of the habitat units and stream reaches that were disproportionately used and determined that the relative influences of particular habitat characteristics varied between summers, presumably in relation to stream flow.