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Title: Temperature and competition between bull trout and brook trout: A test of the elevation refuge hypothesis.

Author
item MCMAHON, THOMAS - MT STATE UNIV,BOZEMAN
item ZALE, ALEXANDER - USGS/BRD
item Barrows, Frederic
item SELONG, JASON - MT STATE UNIV, BOZEMAN
item DANEHY, ROBERT - WEYERHAUSER COMPANY

Submitted to: American Fisheries Society Transaction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2007
Publication Date: 8/2/2007
Citation: McMahon, T.E., Zale A.V., Barrows F.T., Selong J.H., Danehy R.J. 2007. Temperature and competition between bull trout and brook trout: A test of the elevation refuge hypothesis. Transactions of the Ameriican Fisheries Society 136:1313-1326.

Interpretive Summary: Bull trout is a species native to the Pacific Northwest and the remaining populations are in jeopardy. Introduction of brook trout, a species native in eastern US, has been identified as a major factor contributing to the decline of bull trout populations. Bull trout are now found only in the cold headwater streams of major drainages. Laboratory trials were conducted to evaluate the effect of water temperature on growth of brook trout and bull trout, when they were reared separately or in mixed populations. The results indicated that brook trout will out-compete bull trout at warm temperatures, and that other factors besides water temperature are relevant to protecting remaining bull trout populations from displacement by brook trout.

Technical Abstract: We tested the elevation refuge hypothesis that colder water temperatures impart a competitive advantage to bull trout Salvelinus confluentus and thus account for increased biotic resistance to invasion and displacement by brook trout S. fontinalis in headwater streams. Growth, survival, and behavior were compared in allopatry and sympatry at temperatures of 8-20oC in the laboratory. In allopatry, age-0 bulltrout and brook trout grew at similar rates at temperatures of 8.0-14.3C, but brook trout grew significantly faster at higher temperatures. In sympatry, bull trout grew significantly less than brook trout at all test temperatures, with growth differences increasing linearly with increased temperature. Age-1 brook trout had significantly higher feeding and aggression rates than did similar sized bull trout at 8oC and 16oC. The modeled growth of age-0 bull trout and brook trout based upon tributary temperature data from a high elevation site (mean summer temperature 10oC) and a low elevation site (14oC) was similar for both species in allopatry. However, brook trout achieved much greater size than bull trout in sympatry, particularly at the warm site, where the predicted size of brook trout was 21.7 mm (23%) greater in length and 4.9 g (60%) greater in weight. Brook trout therefore had a marked size and growth advantage over bull trout at warm temperatures, but bull trout do not appear to gain a similar advantage over brook trout at low temperatures. Thus, factors in addition to water temperature are relevant to protecting remaining bull trout populations from displacement by brook trout in headwater streams.