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.