Factors Influencing Distributions of Salmonids in the Little Bighorn River Drainage, Wyoming

Abstract

We assessed the influences of geomorphology and introductions of nonnative salmonid species on the distributions of salmonids in headwater streams of the Bighorn Mountains in Wyoming. We sampled geomorphic features and fish at 71 sites among streams in four watersheds near the headwaters of the Little Bighorn River in 1999. Brook trout (Salvelinusfontinalis), rainbow trout (Oncorhynchus mykiss), cutthroat trout (0. clarki), and cutthroat trout x rainbow trout hybrids were found in the study area. Distributions were influenced by stream size, channel slope, natural barriers to upstream movement by salmonids, spatial patterns of nonnative salmonid introductions, and recent stocking of nonnative salmon id species. Salmonids were found at sites with wetted widths ≥ 1.1 m and channel slopes ≤ 16 percent, and the probability of salmonid presence increased as wetted width increased and channel slope decreased. Natural barriers to upstream movement appeared to have excluded nonnative salmonids from colonizing some small headwater streams. Two allopatric populations of genetically pure Yellowstone cutthroat trout were isolated from nonnative salmonids by natural barriers. Past introductions of brook trout and rainbow trout have led to naturalized populations in large portions of the study area. Recent stocking of cutthroat trout and rainbow trout accounted for occurrence of these species in some headwater streams where evidence of natural reproduction was not found.