Outside Influences

Because pygmy whitefish in the upper Cedar River Watershed 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 Cedar River Watershed. 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 on the status of global climate change can be found on the US Global Change Research Program website.

Pygmy whitefish require cold water temperatures (<10°C), typically in deep lake environments, during most of the year.  The effect of climate warming on this species is largely unknown, but changes in snowpack and spring runoff could influence the timing and/or success of alevin emergence from watershed streams.  Changes in the thermal regime of the reservoir related to climate change could also negatively impact all age classes of pygmy whitefish by altering their food supply or the range of habitat they are able to occupy in the lake environment.  These changes are of great importance to the bull trout population of Chester Morse Lake as pygmy whitefish are one of their most important food resources.

Natural disturbances
Pygmy whitefish spawning habitat can be affected by natural disturbances such as flooding, landslides, and windstorms.  Flooding can result in scour to gravels where pygmy whitefish eggs and alevins are deposited; 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 typically both negative effects, but LWD input would be beneficial by creating pools for cover during the pygmy whitefish spawning run as long as the amount of LWD entering the system fell within a natural range of variation.  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.