Climate change reducing stream habitat for fish

Bull trout (Salvelinus confluentus)A warming climate is already having an impact on bull trout and potentially other fish in mountain streams, according to a study conducted in Idaho. The impacts may not become severe for another decade, however, and the scientists suggest several management actions to improve these species' resilience to climate change.

Aquatic species in mountain streams are especially vulnerable to climate change because most are ectotherms whose movements are confined to linear habitats where conditions such as water temperature are easily disturbed by human impacts. In particular, warmer air temperatures and increased wildfires—both associated with climate change—have been predicted to make mountain streams less suitable for some fishes.  

Daniel Isaak and several collaborators from the U.S. Forest Service and CSIRO discovered that these impacts of climate change are already happening in Idaho. Using a large database of stream temperatures throughout the upper Boise River basin from 1993 to 2006, they analyzed trends in stream temperature and identified the environmental factors responsible.

During the study period, average water temperatures in the streams increased by 0.38°C, or 0.27°C per decade, and maximum temperatures increased by 0.48°C, or 0.34°C per decade. Thirty- to fifty-year trends in air temperature and stream flow played the biggest role in the stream temperature increase, while wildfires accounted for 9 percent.

Based on the temperature models, the scientists determined that bull trout lost up to 20 percent of the cold, headwater streams previously suitable for spawning and early juvenile development. If this trend continues, bull trout will lose half of their habitat in the next fifty years.

In contrast, habitat for rainbow trout was scarcely affected from 1993 to 2006, aside from a small shift toward higher elevations. This difference makes sense because rainbow trout prefer slightly warmer temperatures than those favored by bull trout.  

“Our results suggest a warming climate has begun to affect thermal conditions in streams and that impacts to biota will be both species and context specific,” according to the scientists. “Our models suggest climate change will have important implications for species like bull trout, although biologically significant changes may take a decade or more to occur. This time lag may provide a window of opportunity for management actions to conserve or recover some resilience in vulnerable populations.”

To buffer against further warming due to climate change, Isaak and his colleagues suggest that management actions include planting riparian vegetation, restoring streamflow, and reducing the stream impacts of grazing, road building, and timber harvest. Fire suppression may be appropriate in some cases to protect rare fishes and critical stream habitats. In addition, managers should remove barriers to fish passage at road crossings, water diversions, and dams so that fish can move to areas with suitable water temperatures.

--Reviewed by Peter Taylor

Isaak, D., Luce, C., Rieman, B., Nagel, D., Peterson, E., Horan, D., Parkes, S., & Chandler, G. (2009). EFFECTS OF CLIMATE CHANGE AND RECENT WILDFIRES ON STREAM TEMPERATURE AND THERMAL HABITAT FOR TWO SALMONIDS IN A MOUNTAIN RIVER NETWORK Ecological Applications DOI: 10.1890/09-0822