Multiple Lines of Evidence for Determining Upper Optimal Temperature Thresholds for Bull Trout
Bull Trout have recently been the focus of intense study in the Pacific Northwest because of their listing as a threatened species. Bull Trout are increasingly found only in headwater stream reaches in watersheds that are minimally fragmented by human perturbation and still provide very cold water required by this species. Recovery of this species and other salmonids will require, among other essential habitat elements, restoration of thermal regimes on a watershed scale patterned after the natural thermal potential conditions found in historic, minimally disturbed settings. Recommendations for water temperature criteria for salmonids were recently made by an EPA Region 10 technical workgroup in its Regional Water Temperature Guidance Project. Considerable difference of technical opinion was expressed among workgroup members concerning appropriate temperature criteria that would be fully protective of Bull Trout juveniles. This debate prompted development of this paper, which provides multiple lines of evidence in deriving a recommendation. Our recommendation of 11 degrees C as a maximum weekly maximum temperature (a maximum value calculated from the rolling weekly mean of daily maximum temperatures) was based upon evaluation of evidence from growth rates under satiation feeding, distribution in the field under varying levels of interspecific competition, the effects of fluctuating temperature relative to constant temperature on growth rates, heat shock protein creation, and comparison of biological response to thermal conditions to related species. Because some of the most relevant data on growth rates relative to temperature and feeding level were available only in unpublished annual reports, and because some of this material was interpreted in different ways by workgroup members, we devoted considerable attention to explaining the basis for the results observed and contrasting these results with key literature on other salmonids. Bull Trout have a comparable optimum growth temperature at satiation to Brown Trout, a species that has been extensively studied and is known to have optimal growth rates that decline significantly with decreasing food intake. The use of multiple lines of evidence is an important tool for recommending biologically protective temperature criteria that satisfy multiple physiological or behavioral requirements. Such criteria can be applied on a stream system basis to highlight thresholds beyond which various kinds of sublethal impairment increase in intensity.
McCullough, D.A. and S. Spalding. 2002. Multiple lines of evidence for determining optimal temperature thresholds for Bull Trout. Columbia River Inter-Tribal Fish Commission Technical Report 02-4. Portland, OR. 25p.