Authors
Dale A. McCullough, CRITFC
Shelley Spalding,
USFWS
Report Reference
#02-4
Publication Date
27 June 2002
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Multiple Lines of Evidence for Determining Optimal Temperature Thresholds
for Bull Trout
Abstract |
| 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 11ºC 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to temperature and feeding level were available only in unpublished annual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. |
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