Predicting stream temperatures for native fish habitat management in White River National Forest, Colorado

2014 Summer.Includes bibliographical references.Stream temperature is a critical habitat parameter for many cold-water fish, particularly the salmonids family that includes trout. Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus), a native fish in the Colorado River Basin, currently exists in fragmented, isolated populations as a result of degraded thermal habitat, competition with nonnative trout and other reasons. Managers of the White River National Forest in northwestern Colorado want to reintroduce this native trout to additional streams within its historic range to help protect the subspecies from extinction. To identify additional streams within the Forest that have the appropriate thermal regime for Colorado River cutthroat trout, this research has created two multiple regression models to predict summer stream temperature metrics related to lethal and sublethal thermal tolerances for the subspecies. The 7-day mean of daily maximum stream temperature for the warmest 7 days can be equated with the critical thermal maximum, which is the extreme high temperature beyond which the fish cannot survive. The mean temperature of the warmest month can be equated with the upper limit of the optimum temperature range for the species, beyond which the fish experience sublethal temperature effects. The models can be used to identify streams cool enough throughout the year to support Colorado River cutthroat trout populations. The strongest predictor variables of these metrics were the drainage area, the discharge and the residual pool volume. Most previous studies found that air temperature was the strongest predictor variable in stream temperature models, but for the mountain headwater streams in this study, variables related to stream flow volume and stream morphology had better explanatory power. The models, created from and tested against field data, were able to explain 66% and 51% of the variability in monthly mean and 7-day mean stream temperatures, respectively, and had prediction errors of less than 2°C. Results from the models suggest that many of the streams in White River National Forest still have cool enough summer thermal regimes to support Colorado River cutthroat trout populations. Management of cold-water fish that reconnects fragmented populations by reintroducing species to thermally appropriate habitat is a step toward reducing the vulnerability of the species to extirpation by future climate changes or other disturbances