Distribution and status of redband trout in the interior Columbia river basin and portions of the Klamath river and great basins
We summarized existing knowledge (circa 1996) of the potential historical range and the current distribution and status of non-anadromous interior redband trout Oncorhynchus mykiss ssp. in the U.S. portion of the interior Columbia River Basin and portions of the Klamath River and Great Basins (ICRB). We estimated that the potential historical range included 5,458 subwatersheds and represented about 45% of the species? North American range. Two forms of interior redband trout were considered, those sympatric with steelhead Oncorhynchus mykiss ssp. and allopatric forms that evolved outside the range of steelhead. Data were compiled from existing surveys and expert opinions of over 150 biologists during the scientific assessment for the Interior Columbia River Basin Ecosystem Management Project (ICBEMP). We also predicted fish presence and status in unsampled areas, using statistical models to quantitatively explore relationships among redband trout status and distribution, the biophysical environment, and land management. Redband trout had the highest likelihood of being present or supporting strong populations in mid-size or smaller streams, of higher gradients, in highly erosive landscapes with steep slopes, with more solar radiation, and mean annual air temperatures less than 8?9ºC. Variables reflecting the degree of human disturbance within watersheds (road density, land ownership, and management emphasis) were also important. Redband trout remain the most widely distributed native salmonid in the ICBEMP assessment area and the second most widely distributed native fish, occupying 47% of the subwatersheds and 64% of their potential range. Sympatric redband trout are the most widely distributed of the two forms, present in an estimated 69% of their potential range. Despite their broad distribution, important declines in distribution and status are apparent from our analysis, although finer scale extirpations of redband trout populations were more difficult to quantify. Both forms of redband trout have narrower distributions and fewer strong populations than historical populations; neither form supported strong populations in more than 17% of their potential ranges. Habitat degradation, habitat fragmentation, and nonnative species introductions are primary factors that have influenced status and distribution. Because of the likelihood of introgressive hybridization with introduced salmonids, actual status of some strong populations may be worse than suggested. Although much of the potential range has been altered, core areas remain for conserving and rebuilding more functional aquatic systems in order to retain the ecological diversity represented by redband trout. Protection of core areas critical to stock persistence and restoration of a broader matrix of productive habitats will be necessary to ensure the full expression of phenotypic and genotypic diversity in interior redband trout. We recognize the limitations of this database and acknowledge that estimates based on expert opinion and modeling involve inherent uncertainties. A more refined synthesis of redband trout distribution and status will require documentation, consistency, and rigor in sampling and data management that does not currently exist.