Published: 2008
Total Pages: 34
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Under the direction of the inventory and monitoring component of the Natural Resource Challenge, National Park Service biologists in Yellowstone National Park (YNP) began a three-year inventory of the Snake River watershed in 2004. Although the Snake River drainage is the third-largest in the park, less than half of the main stem and few of the tributaries had ever been sampled in a systematic manner. Not only does the Snake River contain the most diverse assemblage of native fish species in the park, but two of YNP's three types of cutthroat trout (Oncorhynchus clarki) also reside there. Although Yellowstone cutthroat trout (O. c. bouvieri) and Snake River fine-spotted cutthroat trout (O. c. subsp.) exhibit observable phenotypic differences in terms of their spotting patterns, considerable debate about their true historic distribution and taxonomic status has occurred because the two subspecies have been nearly indistinguishable when examined meristically or genetically. As a cooperative effort with U.S. Forest Service biologists of the Bridger-Teton National Forest, we sampled the watershed with the following objectives: (1) to conduct a systematic survey of the Snake River tributaries and determine the distribution of the two types of cutthroat trout there; (2) to describe population characteristics of native and non-native fish species; and (3) to obtain samples for genetic analyses in order to compare field identification with laboratory results of individual genetic identity. To accomplish these objectives, we subdivided the main stem into more than 30 sections of 2 km each. Tributary sample sections were much shorter; each section was one-tenth of that stream's total perennial length. In each survey section, the survey crew electrofished 50-100 m of the stream or until a representative sample was obtained. We attempted to catch at least three cutthroat trout longer than 150 mm from each sample reach, as individuals shorter than this length often display indistinct spotting patterns. Mountain whitefish (Prosopium williamsoni) were the most abundant species captured in the main stem, but their distribution was restricted to areas downstream from the confluence with the Heart River. Cutthroat trout and mottled sculpin (Cottus bairdi) were captured throughout the watershed. However, the seven fish identified in the field as fine-spotted cutthroat trout were restricted to the lower sections of the main stem. Most of the other native species, and the few brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) found in this survey, were collected in the same sections as the mountain whitefish. The Snake River fine-spotted cutthroat trout were the largest individuals captured during the survey; several were longer than 400 mm. Many of the other cutthroat trout were younger fish between 100 and 125 mm long. Consequently, most of the cutthroat trout caught in the main stem could not be definitively identified in the field as either the fine-spotted form or the large-spotted Yellowstone cutthroat trout. In most of the tributaries, only Yellowstone cutthroat trout and mottled sculpin were caught. Cutthroat trout, mountain whitefish, and five non-game species were captured from the Heart River. Typically, the cutthroat trout that could be identified in the tributaries were smaller and exhibited different coloration than the adult fish from the main stem.
