A Model Using Phenotypic Characteristics to Detect Introgressive Hybridization in Wild Westslope Cutthroat Trout and Rainbow Trout

Abstract
Introgressive hybridization is a substantial threat to native populations of cutthroat trout Oncorhynchus clarki ssp. To assess the status of native cutthroat trout and protect existing pure (nonhybridized) populations, fisheries managers need to identify introgressive hybridization in wild populations. Genetic techniques are the most reliable methods for detecting introgression but are typically expensive and time-consuming. Phenotypic characteristics are generally easy to measure in the field and have been investigated for their value in identifying hybrids in several genera of fish. We developed a practical quantitative tool for detecting introgressive hybridization in westslope cutthroat trout O. c. lewisi by fitting a classification tree model to the phenotypic characteristics of known pure and hybrid individuals. We then tested it as a means of making site-level assessments of the level of introgression. The genotypes of individuals were determined using noncoding sequences of nuclear DNA. ...