Bull trout distributions related to temperature regimes in four central Idaho streams
bull trout Salvelinus confluentus distributions and water temperature regimes were studied in four streams in the Weiser River basin, Idaho, in 1992 and 1993. bull trout occurred at elevations ranging from 1,472 m to 2,182 m and at densities up to 9.5 fish per 100 m2. Bull trout were sympatric with rainbow trout Oncorhynchus mykiss in all streams and with brook trout S. fontinalis and brook trout x bull trout hybrids in two streams.
Summer temperatures were continuously recorded in 1992 and 1993. The 1993 temperature monitoring sites were at the upstream and downstream distribution limits of juvenile bull troub, the upstream limits of either the brook trout or the rainbow trout distribution, and six additional locations. Temperatures recorded from June through September 1993 varied from 0.8 to 18.2oC. In 1992, juvenile and adult bull trout were found in 20.5oC water where no other species were present. Some bull trout occupied a cool water refugium, which may have been critical to their survival in one stream reach.
Cumulative temperature units were similiar among the downstream distribution limits of bull trout in three of the four streams. Temperature units may be more indicative than maximum temperatures of the downstream limits of the bull trout distribution. apparently, summer water temperature is an important, but no singular, factore determining bull trout distributions in the Weiser River Basin.