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

We tested the elevation refuge hypothesis that colder 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 8-20 degrees C in the laboratory. In allopatry, age-0 bull trout and brook trout grew at similar rates at temperatures of 8.0-14.3 degrees C, 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 8 degrees C and 16 degrees C. The modeled growth of age-0 bull trout and brook trout based on tributary temperature data from a high-elevation site (mean summer temperature, 10 degrees C) and a low-elevation site (14 degrees C) 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 min (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.