Genetic Analyses of Juvenile Steelhead, Coastal Cutthroat Trout, and Their Hybrids Differ Substantially from Field Identifications

Abstract

Because of their similar appearance and frequent hybridization, juvenile steelhead Oncorhynchus mykiss and coastal cutthroat trout O. clarkii clarkii are difficult to distinguish visually. Nevertheless, field biologists often use visual methods to classify juvenile individuals. This study investigated hybridization between these species and determined the accuracy of field identification where hybridization occurred. Using a five-point classification system, two evaluators identified 500 fish collected from three watersheds in Humboldt County, California. Individuals were then genotyped at seven single-copy nuclear DNA genes and one mitochondrial gene, all assumed to be diagnostic for each species. Single-locus Hardy–Weinberg equilibrium, pairwise genotypic disequilibrium, and cytonuclear disequilibrium calculations revealed that subpopulations of these species were mating assortatively. Presumptive F₁ hybrid individuals were rare, whereas introgressed individuals were more common. These presumptive later-generation backcross hybrids were produced with both parental species but were more frequently produced with coastal cutthroat trout. Interspecific matings appeared to be bidirectional. Conditional classification probabilities between evaluator identifications and genotypes showed that both evaluators had moderate to substantial success identifying individuals less than 85 mm total length, whereas individuals 85 mm and larger were identified less successfully. Evaluators successfully identified coastal cutthroat trout but had moderate difficulty identifying steelhead (sometimes misidentified as hybrids) and always misidentified hybrids as coastal cutthroat trout. Although visual identifications are not without error, approximately unbiased estimates of the percentage of hybrids may be generated from a combination of visual assignments and supplementary genetic analyses.