After being listed as a Threatened species under the Endangered Species Act in 1999, bull trout (Salvelinus confluentus) have continued to experience population declines across the southern end of their native range, the Pacific Northwest. Population declines are primarily attributed to habitat loss from degraded water quality, fish passage barriers, invasion by non-native species, destruction of in-stream and riparian habitats, and most recently, rising water temperatures due to climate change. Given that bull trout are the most sensitive to warming water temperatures compared to other native Pacific salmon, trout, and char, they will likely experience further habitat constriction as our climate continues to warm.

Until now, efforts to identify potential climate refuges have been limited to streams, which are essential spawning habitat for all bull trout regardless of their migration patterns. Researchers have identified stream reaches that may support bull trout based on climate change warming scenarios, lack of invasive species, and appropriate physical habitat characteristics, and developed a map of the area known as the “Climate Shield”, which could provide refuge for bull trout as the climate changes.

There is potential that lakes and reservoirs connected to these streams could also offer cold-water refuges for bull trout during summer. In lakes and reservoirs, cold-water habitats can persist at depth, even during the warmest periods of the year, because of thermal stratification. Through this project, Brooke and her project partners at the National Park Service seek to identify lake or reservoir habitat currently occupied by bull trout to assess their suitability as climate refugia, thereby potentially expanding the scope of the “Climate Shield”. Brooke will translate the research findings from this project into an interactive, web-based mapping tool showing potential future habitat for bull trout. A more expansive delineation of the “Climate Shield” can help decision makers prioritize management strategies given different climate change scenarios to preserve strongholds for bull trout.