The scientists found that during the 2015 drought year in southeastern Oregon, only a fraction (about 15 percent) of locations sampled throughout the fish’s known range had water of suitably cool temperature.

The Lahontan cutthroat, the largest subspecies of cutthroat trout, is native to northern Nevada, eastern California, and southern Oregon. Historically, it was an important food source for the Paiute peoples of the Great Basin. When John Fremont explored the region in 1844, he remarked on the presence of these large fish, “salmon trout,” as he called them, in his journal of the expedition.

Thanks to a series of concerted conservation actions, the Lahontan cutthroat has made a remarkable recovery from near extinction in recent years, much to the delight of local anglers.

However, these fish are sensitive to environmental conditions, including water temperature. Lahontan cutthroat trout depend upon clear, cold-water streams to reproduce. Scientists from the USGS Forest and Rangelands Ecosystem Science Center, Bureau of Land Management, University of Georgia, and Oregon State University examined how the twin phenomena of drought and wildfire – both of which are expected to become more frequent and intense over the next century – might affect the habitat available to this species.

The team collected historical and current stream temperature data from locations throughout the Willow and Whitehorse creeks watershed of southeastern Oregon. This watershed supports one of the largest existing populations of Lahontan cutthroat trout. It also offers a natural laboratory in which to observe the effects of drought and wildfire. The region experienced severe drought between the years of 2012-2015, peaking in 2015. In August 2012, the Holloway Fire burned over 85 percent of the study area.

The study’s findings indicate that drought and wildfire can have noticeable and prolonged impacts on stream temperature. In locations affected by the Holloway Fire, the biggest change in water temperature was observed immediately following the fire, when temperatures increased up to 4 degrees Celsius. Warmer than normal water temperatures persisted for up to two years, perhaps sustained by continuing drought.

In both burned and unburned areas, the scientists found that stream temperatures observed during the peak drought year of 2015 varied considerably from historical baselines. At low elevations, many sites had water temperatures that were warmer than those predicted by a model constructed with past data. At high elevations, some sites had cooler than expected temperatures.

Collectively, the effects of drought and fire resulted in considerably diminished habitat for the Lahontan cutthroat trout. Of 59 sampled locations within the trout’s known range, 40 had surface water present and only nine contained water within the thermal limit of the species. The scientists note that water availability is a concern not only for the Lahontan cutthroat trout and other aquatic species but also for animals ranging from the rare greater sage-grouse to game species such as the mule deer.  

The full paper, “Spatial and temporal variability in the effects of wildfire and drought on thermal habitat for a desert trout,” is available in early release view from the Journal of Arid Environments. This study was partially funded by the USGS National Climate Change and Wildlife Science Center and the Department of the Interior Northwest Climate Science Center.