Evaluation of Depletion-Removal Electrofishing of Brook Trout in Small Rocky Mountain Streams

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.