Morphological Patterns of Hybridization between Yellowstone Cutthroat Trout and Introduced Rainbow Trout in the South Fork of the Snake River Watershed, Idaho and Wyoming

Abstract

Hybridization between native and introduced species is considered a serious threat for many native fish populations. In western North America, native cutthroat trout Oncorhynchus clarkii subspecies are declining dramatically across their ranges; competition and hybridization with nonnative rainbow trout O. mykiss are recognized as key factors in their decline. Presently, identification of cutthroat trout × rainbow trout hybrids is commonly made using molecular genetic markers because morphological differences are thought to be inaccurate. We sampled trout from the South Fork of the Snake River and 10 tributary streams to test for morphological differences that could predict genetic identity of individual Yellowstone cutthroat trout O. clarkii bouvieri, rainbow trout, and their hybrids. We found distinctive body shape differences among the three groups: Yellowstone cutthroat trout and hybrids had larger heads than rainbow trout. Yellowstone cutthroat trout had shallower bodies and caudal peduncles than rainbow trout, whereas hybrids exhibited intermediate body depth and caudal peduncle depth. Using logistic regression with a suite of morphological measurements, we found an overall rate of 92% for correctly classifying trout as Yellowstone cutthroat trout, hybrids, or rainbow trout. Over 99% of Yellowstone cutthroat trout and 80% of rainbow trout were correctly categorized; however, the correct classification rate for hybridized individuals was considerably lower. This study demonstrates that consistent morphological differences exist between Yellowstone cutthroat trout and rainbow trout in the wild, and the degree of introgression accounts for a significant portion of morphological variation in Yellowstone cutthroat trout × rainbow trout hybrids, thus causing difficulty in the use of morphology alone to classify trout.

Hybridization between Yellowstone Cutthroat Trout and Rainbow Trout in the Upper Snake River Basin, Wyoming

“…It appears that hybridization is relatively localized to the lower Gros Ventre River, in spite of the fact that there is strong genetic connectivity throughout the basin. The removal of hybrid individuals through the use of relatively noninvasive methods (i.e., nonchemical treatment, such as electroshocking or angling) appears feasible given the high levels of success we found in visually identifying individuals with greater than 20% rainbow trout alleles as hybrids (R. P. Kovach, unpublished data), a success rate highly concordant with that of other studies (Henderson et al 2000;Weigel et al 2002;Robinson 2007;Muhlfeld et al 2009b;Seiler et al 2009). In general, this is markedly different than other cutthroat trout populations where rainbow trout genetic introgression can quickly lead to genomic extinction and the loss of genetically pure natural populations (e.g., Allendorf et al 2001;Hitt et al 2003;Boyer et al 2008).…”

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

“…First, for all three taxa (i.e., Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids), age-0 fish were small (i.e., <60 mm TL) at the time of sampling and were extremely rare in the catch. Second, some of the phenotypic traits that distinguish Cutthroat Trout from Rainbow Trout are not fully expressed in age-0 fish (Miller 1950;Martinez 1984;Seiler et al 2009).…”

“…Although this was partly due to the fact that we excluded fry from our analyses (since they were generally too small to capture), nevertheless 84 of the 511 fish examined across all four streams were smaller than 150 mm TL, and error rates were essentially equivalent between small fish (18%) and large fish (15%). Spotting patterns and other phenotypic characteristics are highly variable or not fully expressed in juvenile trout (Seiler et al 2009), and small Cutthroat Trout are notoriously difficult to distinguish from Rainbow Trout (Miller 1950;Martinez 1984;Campton and Utter 1985), so we expected a marked difference in error rates between smaller and larger fish. However, previous studies have also demonstrated successful differentiation between small fish of these two species based on phenotypic traits (e.g., Weigel et al 2002;Seiler et al 2009).…”

“…Spotting patterns and other phenotypic characteristics are highly variable or not fully expressed in juvenile trout (Seiler et al 2009), and small Cutthroat Trout are notoriously difficult to distinguish from Rainbow Trout (Miller 1950;Martinez 1984;Campton and Utter 1985), so we expected a marked difference in error rates between smaller and larger fish. However, previous studies have also demonstrated successful differentiation between small fish of these two species based on phenotypic traits (e.g., Weigel et al 2002;Seiler et al 2009). For fry, we suspect that white leading tips on the anal, dorsal, or pelvic fins may be the most fully expressed Creek, Idaho, based on phenotypic characteristics (AIC = Akaike's information criterion; ΔAIC = difference in AIC value between the specified model and the best-performing model; w i = Akaike weight;R 2 = adjusted R 2 for discrete models).…”

“…For other subspecies, such as the Yellowstone Cutthroat Trout O. clarkii bouvieri, phenotypic differentiation has been more successful. Seiler et al (2009) recently summarized 29 morphological measurements, which they used to distinguish between groups of Yellowstone Cutthroat Trout, Rainbow Trout, and hybrids with a high degree of accuracy. However, their results were based on digitized photographs taken in the field but processed in the laboratory, and processing involved numerous meristic measurements, such as eye diameter and the length of each fin.…”