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ORIGINS AND TAXONOMIC THEORY While it is not the intent of this HCA to provide detailed documentation of phylogeny and prehistoric zoogeography of Yellowstone cutthroat trout, it is important to review theory represented in the most recent work on native western trout (Behnke 1992). For an indepth discussion of the genesis of native western trout, including Yellowstone cutthroat trout, the reader should refer directly to Behnke's monograph. Phylogenetic and zoogeography theory supports the concept that, while evolutionary sequences leading to present cutthroat trout species and subspecies originated in the mid- Pleistocene era (more than a million years ago), most historical and prehistorical distributions were determined by events occurring in the last glacial and post-glacial periods (about 70,000 years ago to the present, Behnke 1992). For the purpose of this assessment, reference to the prehistoric status addresses conditions existing before the presence of white expiorers (prior to 1800 AD) within the Yellowstone cutthroat distributional area (Figure 1.). Reference to the historic status addresses conditions occurring during and subsequent to the exploration and settlement of the western Rocky Mountains by early white explorers (after 1800). Yellowstone cutthroat trout became isolated in the headwaters of the Snake River following creation of Shoshone Falls (between 30,000 and 60,000 years ago) and in two distinct areas of the lower Columbia River basin (Waha Lake, Idaho and Crab Creek, Washington)(Behnke 1979 and 1992). Behnke (1992) also presents the most logical method of movement between the Snake River and Yellowstone River drainages following the last glacial period (about 8,000 years ago). This headwater transfer allowed for movement and expansion of Yellowstone cutthroat trout into suitable habitats east of the Continental divide within the Yellowstone River drainage (Behnke 1979 and 1992). During the period of time associated with historic exploration and settlement (1800 to 1900) of the West, cutthroat trout could be found in essentially the entire Snake River drainage above Shoshone Falls to the headwaters located within the Teton Wilderness area of Bridger-Teton National Forest and Yellowstone National Park (Rollins 1935; Jordon 1891; Evermann 1891; Gilbert and Evermann 1894). Even though the earliest explorations were linked to the fur trade, many diaries associated with these expeditions mentioned fish and specifically trout (Trotter and Bisson 1988). Robert Stuart, an early explorer of the upper Snake River in 1812, maintained a diary which mentioned eating trout taken from Marsh Creek, South Fork of the Snake and the Hoback River (Rollins, 1935). During the late 1800's, several scientific expeditions directed by the U.S. Fish Commission, focused attention on fishery conditions existing within the upper Snake and Yellowstone River basins. Both the Jordan (1889) expedition and the Evermann expedition (1891) visited portions of the upper Snake River drainage. The Gilbert and Evermann expedition, of 1892 and 1893, not only returned to sites within the upper portion of the Snake River drainage, they visited the Snake River between Shoshone Falls and Idaho Falls, including three tributary streams in that vicinity. Although not mentioned by early naturalists, the Snake River and its tributaries associated with Jackson Lake along with the mainstem Snake River and most tributaries downstream to the present Palisades Reservoir supported cutthroat having a unique spotting pattern characterized by a profusion of very small spots covering most of the body. They are referred to as "finespotted Snake River Cutthroat trout" and have the potential to represent a separate subspecies (Behnke 1979). Continued genetic comparison (Loudenslager and Kitchin 1979; Leary et a!. 1987; Allendorf and Leary 1988) of the two cutthroat forms has not provided definitive proof that would lead to total acceptance that the "finespotted Snake River cutthroat" is indeed a separate subspecies. The fact that most early naturalists (David Starr Jordan being the exception) did not acknowledge the presence of the "finespotted" cutthroat form on their visits to the Snake River above Jackson Lake and within the Two Ocean Pass area is somewhat surprising given the tendency of the time to classify organisms based on outward appearance. The spotting pattern that is so noticeable at present was either inadvertently overlooked or deemed not important. Jordan (1891) acknowledges a difference in spot size and pattern between fish in Heart Lake (upper Snake 12