Local and geographic variability in the distribution of stream-living Lahontan cutthroat trout

We investigated local and geographic variability in the up- and downstream distribution limits of threatened Lahontan cutthroat trout (Oncorhynchus cl arki henshawi) in stream habitats of the eastern Lahontan basin in northern Nevada and southeastern Oregon. At a geographic scale, elevations of upstr eam distribution limits were significantly correlated with latitude and lon gitude, suggesting a potential influence of climatic gradients. Elevations of upstream distribution limits also were positively correlated with maximu m basin elevation, which suggested topographic, rather than climatic constr aints may be important. Upstream distribution limits were not significantly affected by local variation in stream size or presumptive dispersal barrie rs. Stream gradient was related to upstream distribution limits, but this w as again confounded by maximum basin elevation. Stream gradients used by La hontan cutthroat trout at upstream limits were considerably steeper than th ose observed for other subspecies of cutthroat trout in other areas. Geogra phic variation in elevations of downstream distribution limits was also app arent, paralleling variability in summer air temperatures and presumed rest riction of Lahontan cutthroat trout from lower elevation habitats by unsuit ably warm temperatures in summer. Previous regional models assumed downstre am distribution limits for salmonids correspond to isotherms of 22-24 degre es C (mean July air temperature), but we found the distribution of Lahontan cutthroat trout to be considerably more restricted, most closely correspon ding to a mean July air temperature of 18 degrees C. Occurrence of nonnativ e brook trout Salvelinus fontinalis was associated with a significant upstr eam shift of the downstream distribution of Lahontan cutthroat trout, sugge sting a highly variable, but negative interaction between these species.