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Chapter 3 Bonneville Cutthroat Trout Oncorhynchus clarki utah Don Duff, Aquatic Ecologist Forest Service/Trout Unlimited USDA Forest Service 8236 Federal Building 125 South State Street Salt Lake City, Utah 84138 INTRODUCTION The Bonneville cutthroat trout (Bel) is the only trout native to the Great Basin within Utah. This subspecies is endemic to the Bonneville Basin, the largest endorheic basin in the Great Basin of the western North America, comprising about 132,650 km2.lake Bonneville once extended over 51,840 km2, with a maximum depth of over 300m. About 30,000 years ago the Bear River lost its connection with the Snake River through volcanic activity and changed its course into the Bonneville Basin. Ancestral cutthroat trout could have been present before this divergence but would have been exposed to Yellowstone cutthroat trout from the upper Snake River basin as the Bear River flowed into the Bonneville Basin. Historically, subspecies are geographical races, isolated and differentiated from all other subspecies of the species. Probably two separate ancestral invasions of the Bonneville basin occurred. Both invasions would have probably come from similar ancestor Yellowstone cut- throat trout of the Snake River and some mixing-blending should have occurred (Dr. Robert Behnke, personal communication 1996). Behnke states that although the Bear River utah do share more close-relatedness to the Yellowstone cutthroat, because they came into the basin later than the first invasion, they share enough similarities to the other utah for all to be regarded as one subspecies. This mixing and the apparent genetic differentiation is evident today between the Bonneville cutthroat trout from the Bear River basin and those from the main Bonneville Basin (Behnke 1992, Hickman 1978, Shiozawa et al. 1993). The subspecies was historically abundant throughout all suitable waters of the vast Bonneville basin area encompassing portions of Utah, Nevada, Idaho, and Wyoming until about 8,000 years ago during the final desiccation of ancient Pleistocene lake Bonneville (Broecker and Kaufman 1965, Hunt 1953). At this time the cutthroat trout were forced into streams throughout the basin, forming isolated, disjunct populations (Figure 1). For example, the Snake Valley strain of the Bonneville cutthroat has been geographically isolated from the eastern Bonneville Basin for about 8,000 years. However, reproductive isolation may have occurred long before that time as glacial lake Bonneville was noted for many large lake level fluctuations during its history. Many populations throughout the basin became isolated from one another since that time by this natural habitat fragmentation. The lake's Snake Valley arm was rather ephemeral in geographical time, and only during the lakes maximum elevation (ca. 18,000 BP) was it connected, and then isolated from the rest of the basin as the lake level dropped (Behnke 1976). Zoogeographical evidence for fish distribution in the Great Basin points to their occurrence habitats in which they have been able to reach through surface-water migrations. Thus, their dispersal is very closely linked with climate and hydrogeographic history (Hubbs and Miller 1948). 35