Step 3 - Risks and Threats to Native Trout Populations and Associated Stream-Riparian Habitats

Extinction risks for salmonids are influenced by complex and interacting factors that are often difficult, if not impossible, to identify and measure. Despite this difficulty, understanding the nature of the extinction process and the characteristics of local fish populations and habitats can lead to management prescriptions that minimize risks. Relative extinction risks include deterministic, stochastic, and genetic factors at several spatial and temporal scales (Rieman and others 1993). The risk portion of step 3(A-C) (figure at right) was generated for cutthroat trout by agency biologists through application of an extinction risks matrix (Rieman and others 1993) that matched population characteristics, such as size, isolation, and survival rates, with a variety of environmental and demographic parameters. The outcome was an estimate of relative risk of extinction (low to extreme) for trout in a given reach, subwatershed, or across a subbasin. For the Central Bear River subbasin, step 3(A) maps extinction risks in subwatersheds based on trout population size and isolation, respectively.

The persistence of trout populations in a subwatershed can be threatened by land use practices such as grazing, fire suppression, mining operations, recreational activities, road construction, dams and water diversions, and introduction of invasive species. Threats to fish populations or their habitat may be direct, as in the alteration of riparian and channel structure by overgrazing, or indirect, such as when high road density in the uplands impacts water quality and timing of flows. For the Central Bear River subbasin, biologists spatially displayed possible threats to cutthroat populations by mapping road densities (step 3(B)), and water quality integrity (step 3(C)). Aquatic specialists then evaluated the threats and identified those that Forest Service management or cooperative partnership actions could influence.

In the national forest plan revision process, risks and threats to native fish species on the subwatershed level would be included in the CER.