Levels of Introgression in Westslope Cutthroat Trout Populations Nine Years after Changes to Rainbow Trout Stocking Programs in Southeastern British Columbia

Abstract

Introgressive hybridization (introgression) between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout O. mykiss in the upper Kootenay River, British Columbia, appears to be recent and is partly attributed to rainbow trout stocking in Koocanusa Reservoir starting in 1988. In 1998, rainbow trout stocking was stopped or replaced with the stocking of triploids throughout the watershed. The goal of this research was to determine the effect of the change in stocking practices on the level of introgression between westslope cutthroat trout and rainbow trout. We monitored 14 sites from 1999 to 2007 using four co-dominant, diagnostic nuclear markers. Sample sites were categorized as (1) open if no fish migration barriers existed between the site and the reservoir or (2) closed if migration barriers were present between the site and the reservoir. We classified fish as pure westslope cutthroat trout if no rainbow trout alleles were detected; fish were classified as hybrids if they had at least one rainbow trout allele present. We also classified fish into parental and hybrid types based on their multilocus genotypes. Open sites had more hybrids (15.8%) than closed sites (4.8%; χ² = 23.38, df = 1, P < 0.0001). Westslope cutthroat trout backcrosses were the most common hybrid genotype at both open and closed sites. At open sites near the reservoir, introgression levels stayed relatively high (20–30%) and pure rainbow trout and rainbow trout backcross individuals were common. Introgression significantly increased at two of three sites surveyed at intermediate distances upstream from the reservoir (i.e., 27–87 km). The new stocking program does not appear to have reduced introgression or prevented the spread of hybrid individuals in the upper Kootenay River, and more active management strategies will be required to prevent further introgression and loss of unique westslope cutthroat trout populations.