Outside Influences

Because bull trout in the upper CRMW have been isolated by the natural fish passage barrier at Cedar Falls since glacial times, many outside influences that could negatively affect their population, such as non-native species or introduced diseases, are reduced. Nonetheless, there are other outside influences, of both natural and human origin, that can affect bull trout populations in the CRMW. The most important outside influences are likely to be climate change and natural disturbance events.

Climate Change
In the Pacific Northwest, global climate change models indicate that winter and summer temperatures in the region are likely to be warmer, with possibly higher amounts of winter precipitation. More information: US Global Change Research Program.

Researchers have documented that bull trout prefer, and may require, cold water (8-10 °C) during most of their life cycle, although some exceptions are known. Many activities and shifts in behavior (e.g., foraging, spawning) are associated with these cold temperatures in many stream/lake systems. Consequently, a warming climate could negatively affect a species with temperature preferences and/or requirements in this range.

In addition to changes in stream water temperatures with climate change, the frequency of rain-on-snow events and more intense winter flooding may increase. Because high flow events can cause scouring of the stream bed, they can result in high levels of mortality to early life history stages (eggs and alevins). With higher frequency of stream bed scouring due to climate change, there may be less recruitment to the adult bull trout population because of higher egg and alevin mortality.

Increased temperatures in lakes may also result in critical adverse changes in prey availability or distribution, potentially causing significant shifts in bull trout foraging behavior and/or population status. These effects of a changing climate are outside of SPU’s control, although restoration of habitat in the CRMW under the HCP should help ameliorate some impacts to bull trout due to climate change.

Natural disturbances
Bull trout spawning and rearing habitat can be affected by natural disturbances such as flooding, landslides, and windstorms. Flooding can result in scour to bull trout redds; landslides can deliver large amounts of coarse and fine sediment and large woody debris (LWD) to streams; and windstorms can contribute LWD, as well. Scour and sediment input are both typically adverse effects, at least on a short-term basis, and LWD input is generally considered beneficial to bull trout habitat. Although natural disturbance frequency or impacts can be increased by human influence, these events are difficult to predict and are largely out of our control.