We evaluated the effectiveness of three‐pass depletion electrofishing for removing introduced brook trout Salvelinus fontinalis from small mountain streams that contained native populations of Colorado River cutthroat trout Oncorhynchus clarki pleuriticus. Estimates based on depletion indicated removal efficiencies of 73–100% for age‐0 brook trout and 59–100% for older brook trout. Estimates based on the number of fish caught 1 year later indicated removal efficiencies of 42–83% for age‐0 brook trout and 92–96% for older fish. The lower removal efficiencies for age‐0 brook trout reflected the difficulty in spotting small fish in streams with overhanging vegetation, undercut banks, and large woody debris. Because small brook trout were not captured as effectively as larger trout, a single electrofishing pass the year after depletion‐removal electrofishing helped further reduce brook trout numbers. Although brook trout were not eradicated from any of the study streams, a year after treatment brook trout densities (number/ 100 m2) were reduced from 11.3 to 0.6 in Nameless Creek, from 3.4 to 0.3 in Nylander Creek, and from 2.3 to 0.2 in Irene Creek. Recruitment was virtually nonexistent following 1–2 years of population control.
Habitat modification and introduced species are impacting native fishes in western North America, a situation now exacerbated by drought and anthropogenic water acquisition. One species of concern, the Bluehead Sucker Catostomus discobolus in the Bonneville basin of Utah, Idaho, and Wyoming, has experienced sharp population declines potentially augmented by hybridization with native Utah Sucker C. ardens. To evaluate this situation, we sequenced three diagnostic nuclear DNA loci and two mitochondrial genes across 81 Bluehead Suckers, 74 of which were collected in a basinwide genetic assessment and 7 from a targeted morphological evaluation. Combined, we detected 14 hybrids from the single site in the Bear River and the three sites in the Weber River that hosts the three largest remaining populations in the basin. All individuals sampled as putative hybrids were confirmed as F 1 hybrids, ascertaining efficacy of field-based morphological identifications. Hybridization may be especially problematic for the numerically reduced Bluehead Sucker because it can further depress recruitment. However, no evidence of post-F 1 backcrosses was found, suggesting in turn that hybrids are not reproducing, although the reasons for this are currently not understood. These results underscore the need to quantify the proportion of Bluehead Sucker recruitment lost to hybridization.
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