Grizzly bears (Ursus arctos) and black bears (U. americanus) prey on spawning cutthroat trout (Oncorhynchus clarki, formerly known as Salmo clarki) in tributary streams of Yellowstone Lake. These tributary streams were surveyed from 1985 to 1987 to determine the presence and level of trout spawning activity and bear use. Indices were developed to enumerate spawner density and levels of bear use. Of 124 known tributaries of Yellowstone Lake, 48% had a spawning run. Of these spawning streams, 93% had associated bear activity, and 61% had associated evidence of bear fishing. Bears were apparently using more spawning streams and fish compared to 10 years earlier. Bear use of cutthroat trout spawning streams appeared to be largely a positive function of volumetric spawner density. We hypothesize that abundance and quality of stream-side vegetation relative to other foraging options influenced bear use. Intra- and interspecific avoidance among bears was suggested by patterns of spawning stream use. Less bear use of spawning streams than expected occurred within 1 km of park developments.
