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

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 X 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 X rainbow trout hybrids, thus causing difficulty in the use of morphology alone to classify trout.