Despite the warm, algae-filled waters of the Upper Klamath Lake in south-central Oregon, native redband trout are not just surviving – they’re thriving. This surprising behavior of a ‘coldwater’ fish is the focus of ongoing Northwest CASC research projects as scientists aim to understand how disease, invasive species, and climate change are restructuring fish habitats across the region. As environmental pressures shift with climate change, understanding how native fish use both cold and warm water habitats is essential for making effective conservation decisions. 

Unlike their invasive counterparts – brook trout and brown trout – redband trout appear to tolerate the warm nutrient-rich conditions in the lake, even though brown trout were previously thought to be more tolerant of warm water. One possible reason for this is that redband trout have co-evolved alongside a local freshwater parasite, Ceratonova shasta. In warm water, this parasite is deadly to the non-native trout while the redband trout show some resilience to it – a natural advantage that may give them exclusive access to the habitats where the other species can’t survive. Another factor may be food availability. The warm lake waters host an array of meal options for the native trout – researchers found multiple prey fish in the stomachs of nearly every redband trout they sampled, which is unusual because captured fish often have empty stomachs. 

Meanwhile, cold spring-fed streams that are typical spawning sites for the redband trout are increasingly dominated by the invasive trout who prey on young redband. By tracking fish locations and water temperatures with tools like radio tags, the researchers are building models and maps that describe and identify locations where redband trout are most likely to outcompete the invasive species. These areas would be ideal locations to focus conservation efforts. 

By mapping patterns of disease, temperature, and invasive species, decision makers can make more informed decisions about habitat restoration and barrier removal, especially in areas that were previously overlooked, like warmer water habitats.  

This research is supported by the Northwest CASC project “Incorporating Climate, Disease and Invasive Species into the Conservation of a First Food, Klamath Redband Trout.”