Fish out of water: Quantifying the distribution limits for native and nonnative trout in the northern Great Basin

Background/Question/Methods

Geographical range is the basic unit of biogeography and understanding the area over which a species occurs is a key first step in identifying threats to a species and to establishing conservation priorities. In this study we first determined the elevation envelope within which native redband rainbow trout, native cutthroat trout, nonnative brook trout, and nonnative brown trout occur in the northern Great Basin.  Our study area included closed basins encompassing the Oregon Lakes region and eastern Lahontan basin.  Using quantile regression analysis we determined the latitude by elevation relationship to describe the range in which 90% of trout were present. Then, to explore distribution changes potentially linked to climate change, we moved the elevation envelope upstream by 100m and quantified the degree of range contraction.

Results/Conclusions

Our preliminary results indicate a high degree of overlap in occupied elevations among the four trout species, suggesting common responses to spatial variability in climate. At the northern end of their native range, native trout and nonnative brook trout occupy the majority of their potential elevation range (as determined by quantile regressions against maximum stream elevation and terminal basin elevation).  In this portion of the study area, preliminary results suggest that topographic constraints were more important than climate in constraining trout distributions.  In contrast, at southern latitudes, trout occupy a smaller portion of the total elevation range in a pattern that suggests climate is the primary factor that constrains distributions, and that topography is less important.  Although climate conditions are more suitable for trout in the north, due to the shallow topography in this region, upstream shifts in the envelope (e.g., in response to climate change) have much more severe impacts than similar losses in the south.  The preliminary analyses show that in the most extreme case, nonnative brook trout, a 100m shift upstream in the elevation envelope leads to a loss of about 1 km in the south but as much as 7 km of river in the north.  These geographical contrasts in constraints imposed by both elevation and topography have critical implications for the response of both native and nonnative trout to climate impacts in the region.