Purifying a Yellowstone Cutthroat Trout Stream by Removing Rainbow Trout and Hybrids via Electrofishing

Abstract

AbstractThe South Fork of the Snake River in Idaho supports one of the few remaining fluvial populations of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri, but long‐term persistence of Cutthroat Trout in the drainage is threatened by introgression with introduced Rainbow Trout O. mykiss. We completed eight backpack electrofishing removals from 2010 to 2015 to remove Rainbow Trout and Rainbow Trout × Cutthroat Trout hybrids from a 9.3‐km isolated reach of Palisades Creek (a tributary of the South Fork) in an attempt to improve the purity of the population. For two removals, fish from a subsample of Oncorhynchus were genetically screened at seven diagnostic nuclear DNA loci. A total of 14,092 fish were captured across all removals, of which 3,446 were putative Rainbow Trout or hybrids, which were removed from the stream. The proportion of the total catch that Yellowstone Cutthroat Trout comprised (across all size‐classes combined) increased slowly over time, from 67% in 2010 to 86% for the second removal in 2015. By the end of the study, an estimated 90% of the alleles at the loci we screened were Cutthroat Trout compared with 80% at the beginning. Capture efficiency across all years and size‐classes averaged 38%, ranged from 23% to 52%, and was much higher for larger fish (i.e., ≥250 mm TL) than for smaller fish. Considering the capture efficiencies achieved, initial hybridization levels observed, and number of removals conducted, ending phenotypic purity should have been 94% rather than the 86% we observed. This discrepancy was at least partly due to low capture efficiency for fish < 150 mm and extremely high flows throughout 2011 that prevented electrofishing removals that year, but also suggests that Rainbow Trout and hybrids somehow might have been recolonizing the stream or had a competitive or survival advantage over Cutthroat Trout.