Distinguishing Yellowstone Cutthroat Trout, Rainbow Trout, and Hybrids by Use of Field‐Based Phenotypic Characteristics

Abstract

Native and nonnative salmonids within the same genus sometimes hybridize, and the hybrids are often difficult to visually distinguish from parental species. We compared phenotypic delineations (based on several visual characteristics) and genotypic screening (using seven nuclear DNA loci) for 323 fish collected from an Idaho stream where Yellowstone Cutthroat Trout O. clarkii bouvieri were introgressed with Rainbow Trout O. mykiss to evaluate our ability to visually distinguish Yellowstone Cutthroat Trout from Rainbow Trout and hybrids. Assuming that the genotypes were 100% accurate, correct phenotypic classification was highest for Yellowstone Cutthroat Trout (the genotype confirmed the phenotype 94% of the time), followed by hybrids (79%) and Rainbow Trout (71%). All errors were between pure and hybrid fish. All of the measured phenotypic characteristics were useful for differentiating Yellowstone Cutthroat Trout from hybrids, but the most informative characteristics were the lack of a white leading tip on the anal and dorsal fins, the presence of fewer than five spots on the top of the head, and the presence of a bright red‐orange throat slash. Fish size did not influence correct phenotypic identification, although fry were excluded from our study. A logistic regression model predicted that biologists were more than 50% likely to visually detect Rainbow Trout traits in a hybrid when its level of hybridization was greater than 22% (95% confidence interval = 4–30%). For an additional 189 fish in three other Idaho streams, phenotypic differentiation of Yellowstone Cutthroat Trout from Rainbow Trout and hybrids was 91% accurate. These results can be used to screen for introgression in Yellowstone Cutthroat Trout populations or to selectively remove Rainbow Trout and hybrids from Yellowstone Cutthroat Trout populations. However, in most cases, genetic confirmation of purity or introgression would still be valuable for identifying low levels of hybridization in individuals or in the population at large. Received September 19, 2016; accepted December 29, 2016 Published online March 15, 2017

Leveraging public harvest to reduce invasive hybridization in Yellowstone National Park: field identification and harvest of cutthroat × rainbow trout hybrids

“…Field identification of non-native species can complicate maintenance control efforts, especially when invading species hybridize with native taxa further convoluting identification (Meyer et al 2017a). If hybrids between introduced and native taxa are fertile, non-native genes can spread across the native species through introgressive hybridization (Allendorf et al 2001).…”

“…1 The Lamar River watershed in Yellowstone National Park, depicting the general frequency of non-native and hybrid trout, reaches designated for estimating angler catch (named with black font, separated by dotted lines), and sampling sites where fish were collected for genotype-phenotype comparisons. Sample locations correspond with Table 1 RT had faint cutthroat slashes, as did nearly 90% of all CTX (Meyer et al 2017a). Therefore, if trout in the Lamar River had similar phenotype to genotype relationships, then regulations enacted in 2013 were protecting a majority of the fish the managers wished to remove.…”

Understanding Life History Diversity of a Wild Steelhead Population and Managing for Resiliency

“…Additionally, scales were systematically sampled from PIT‐tagged juvenile steelhead to determine the age composition of the emigrating cohort (Bettoli and Miranda 2001). Fish that expressed characteristics of resident maturation (e.g., high frequency of spots, milt excretion) or hybridization with Cutthroat Trout O. clarkii (e.g., partial or faded throat slash, distinct pattern of spots) were excluded from analyses (Kennedy et al 2009; Meyer et al 2017).…”

Proactive Rainbow Trout Suppression Reduces Threat of Hybridization in the Upper Snake River Basin

“…Generally, visual identification of Yellowstone Cutthroat Trout from hybrids is accurate (Meyer et al. ), particularly for individuals with Rainbow Trout admixture that exceeds 20% (Kovach et al. ).…”