Assessing the Impacts of Climate Change on Bull Trout in Oregon
Bull trout are an endangered species in the Pacific Northwest and their populations may be particularly susceptible to the impacts of a warming climate. Bull trout are thought to avoid warmer water temperatures but little is known about their thermal habitat requirements in streams and lakes. To better assess the impacts climate change may pose to bull trout, USGS scientists tagged and tracked 42 trout in the Lostine River, Oregon to monitor their thermal tolerances. Results indicated that the Lostine River bull trout population did not preferentially avoid warmer waters. These results could be best explained by the fact that Lostine River is located at a more southerly latitude, thus the bull trout population in this river has acclimated to warmer than expected temperatures for centuries. Similar studies to the Lostine River are being conducted in other bull trout habitat in Oregon, Washington, Idaho, Nevada, and Montana.
Bull trout, Salvelinus confluentus, populations in Washington, Oregon, Idaho, Montana and Nevada were listed as threatened under the U.S. Endangered Species Act in 1999. Their populations exist in 21% of their historic range. Bull trout require relatively pristine streams and lakes, specifically habitat that has cold, clean, complex, and connected waters. Because of these requirements, bull trout populations are viewed as an indicator of water quality. There are roughly 70 bull trout populations in the Columbia Basin and nine populations in the Klamath Basin.
Climate change may threaten bull trout populations because with a warming climate, cool-enough spawning and rearing grounds are expected to shrink. Bull trout are known to prefer cold water and cooler temperatures; thermal preferences have been identified as a limiting factor for this species.
Bull trout can migrate over hundreds of kilometers, through lakes and rivers, throughout their life history. Most bull trout populations spend a portion of their life in lakes and return to smaller streams to spawn. Their large spatial range has made it difficult for managers and scientists to assess the thermal regimes naturally experienced by bull trout. One way to better assess the thermal habitats used by bull trout is by attaching miniature thermographs to individual fish. The goal of this project was to better describe the temperatures experienced by migrating bull trout and to look for evidence of cold-water refuge use by adult fishes.
Data was collected along the Lostine River in southern Oregon in 2001-02 and 2004-05. In 2001, 15 bull trout were tagged with thermographs; in 2004, 27 bull trout were tagged. The thermographs were designed to record data for roughly one year. The fish were tracked generally once per week from May to November, when the fish were migrating, and once per month from December to April, when the fish were overwintering. After a year, the fish were recaptured and the data was downloaded.
Outcomes and Conclusions
In the Howell et al. (2010) study, the bull trout did not appear to alter their behavior based upon thermal cues. In general, increasing temperatures caused fish to migrate upstream but responses varied by individual. The fish in this study did not show any evidence of using cold-water thermal refuges when in an area with generally warmer waters. The researchers argue that the discrepancy between the Lostine River bull trout population response and other known cases may be due to the fact that the Lostine River is located at a more southerly latitude. Thus, the bull trout population in this river may be pre-acclimated to warmer than normal temperatures in contrast to populations that live in temperatures further to the north. This study highlights the heterogeneity of individual populations and the need to study each system on a case-by-case basis; rarely are there absolutes in nature. In 2010, the U.S. Fish and Wildlife Service listed 2,836 stream miles and 30,256 acres of lakes or reservoirs in Oregon as critical habitat for bull trout. The Service opted to prioritize cold-water spring habitats for conservation because they might be among the most resistant habitats to climate change, while lower elevation and warmer habitats were given less protection.
Similar studies to the Lostine River are being conducted in other bull trout habitat in Oregon, Washington, Idaho, Nevada, and Montana through a Northwest Climate Science Center project, Range-wide climate vulnerability assessment for threatened bull trout. Investigators are mapping bull trout habitat and measuring threats through the bull trout's range in the region.
Feifel, K.M. (2013). Assessing the Impacts of Climate Change on Bull Trout in Oregon. [Case study on a project of the USGS Forest and Rangeland Ecosystem Science Center]. Product of EcoAdapt's State of Adaptation Program. Retrieved from CAKE: http://cakex.org/case-studies/assessing-impacts-climate-change-bull-tro… (Last updated December 2013)