Commensalism in the High Desert: Juvenile Steelhead Abundance, Movements and Microhabitat Use around Beaver Dams

Steelhead (Oncorhynchus mykiss) of the Pacific Northwest coevolved with the once ubiquitous American beaver (Castor canadensis).  Extirpation of beaver and their associated dams are thought to have had negative impacts on stream function in desert watersheds where high water temperatures, low water levels. And simplified habitat can limit steelhead productivity.  Bridge Creek, Oregon is an Intensively Monitored Watershed (IMW) where construction of beaver dam analogs were used to encourage beavers to build and maintain more stable dams to restore ecosystem function.  Following restoration in 2009, dams and structures greatly altered habitat complexity by increasing pools and side channels.  In addition, we observed increases in juvenile steelhead abundance and survival compared to an external control watershed.  To evaluate the mechanisms by which beaver dams influenced this steelhead population, we employed PIT tag arrays to observe microhabitat use and movements of juvenile steelhead in beaver dam complexes.  Results from a week-long observation period revealed that in a single day juvenile steelhead exploit a variety of habitat and velocity zones associated with beaver dams.  We surmise that habitat complexity around beaver dams increased the opportunities for individual fish to access a variety of habitat types not present when beaver dams are absent.