Dan Isaak

US Forest Service
Rocky Mountain Research Station

Co-authors: Charlie Luce, Jason Dunham

Abstract: Bull trout have thermal requirements that are among the lowest for salmonids in North America and their distributions are strongly linked to the availability of coldwater habitats. A warming climate is driving environmental trends in air temperatures, hydrology, and conversion of terrestrial and riparian vegetation, which are generally expected to warm stream temperatures. Stream temperature response to these forcings will be spatially heterogeneous—resulting from local interactions among geomorphology, hydrology, disturbance, and biology. Models capable of accommodating this spatial complexity and accurately predicting stream temperatures will be needed to forecast distributions of thermally suitable habitats and advance conservation goals. We briefly review temperature metrics previously used to describe bull trout habitat, then contrast mechanistic and statistical approaches to modeling stream temperatures through application of several examples. Mechanistic models are based on physical processes, can capture nonlinear behaviors, and provide generalizeable results, but are data intensive and questions remain about modeling of some processes. Statistical models link empirical temperature measurements from commonly available thermograph data to variables representing surrogates of physical processes, which can often be quantified from a GIS. Statistical approaches are relatively easy to calibrate and apply across broad areas, but assume linear responses and results are less generalizeable. No temperature modeling approach offers a panacea and selection of a “best” modeling strategy will depend on the spatiotemporal scale at which predictions are needed, availability of pre-existing data, and resources available to solve the problem. Furthermore, different approaches can be complementary, and it would be useful at the outset of any effort to consider the relative merits of each in the context of desired outcomes. Accordingly, it may be useful to employ several different modeling strategies to address the complex problem of climate change, stream temperatures, and impacts on bull trout.

Video Length: 18 Minutes, 25 Seconds

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Stream temperature modeling within the context of a warming climate and bull trout recovery planning