Local and Geographic Variability in the Distribution of Stream-Living Lahontan Cutthroat Trout

—We investigated local and geographic variability in the upand downstream distribution limits of threatened Lahontan cutthroat trout (Oncorhynchus clarki henshawi ) in stream habitats of the eastern Lahontan basin in northern Nevada and southeastern Oregon. At a geographic scale, elevations of upstream distribution limits were significantly correlated with latitude and longitude, suggesting a potential influence of climatic gradients. Elevations of upstream distribution limits also were positively correlated with maximum basin elevation, which suggested topographic, rather than climatic constraints may be important. Upstream distribution limits were not significantly affected by local variation in stream size or presumptive dispersal barriers. Stream gradient was related to upstream distribution limits, but this was again confounded by maximum basin elevation. Stream gradients used by Lahontan cutthroat trout at upstream limits were considerably steeper than those observed for other subspecies of cutthroat trout in other areas. Geographic variation in elevations of downstream distribution limits was also apparent, paralleling variability in summer air temperatures and presumed restriction of Lahontan cutthroat trout from lower elevation habitats by unsuitably warm temperatures in summer. Previous regional models assumed downstream distribution limits for salmonids correspond to isotherms of 22–248C (mean July air temperature), but we found the distribution of Lahontan cutthroat trout to be considerably more restricted, most closely corresponding to a mean July air temperature of 188C. Occurrence of nonnative brook trout Salvelinus fontinalis was associated with a significant upstream shift of the downstream distribution of Lahontan cutthroat trout, suggesting a highly variable, but negative interaction between these species. Lahontan cutthroat trout Oncorhynchus clarki henshawi is a threatened subspecies (Office of the Federal Register 40:29864) endemic to the Lahontan basin of northeastern California, southeastern Oregon and northern Nevada (Coffin and Cowan 1995; Figure 1). It is estimated that Lahontan cutthroat trout occupy less than 3% of the * Corresponding author: [email protected] 1 Order of last four authors is alphabetical. 2 Deceased. Received March 23, 1998; accepted November 30, 1998 species’ historical habitat in the Walker, Carson and Truckee River basins of northeastern California and western Nevada, and about 14–15% of historical habitat in the eastern Lahontan basin, which includes the Quinn and Humboldt river and Coyote Lake basins (Coffin and Cowan 1995). Like other subspecies of cutthroat trout (e.g., Gresswell 1988; Young 1995; Duff 1996; Hall et al. 1997), the presently contracted distribution of Lahontan cutthroat trout has been attributed primarily to interactions with hatchery and nonnative salmonids, and habitat degradation (Coffin and