Movements of nonnative brook trout in relation to stream channel slope

We provide new insights on the ability of naturalized brook trout (Salvelinus fontinalis) to ascend steep, headwater streams in the western USA. We tested hypotheses that upstream movements by brook trout are limited or absent in reaches of steep streams and are more prevalent and longer in gradually sloping streams. We compared brook trout movements in headwater streams in Idaho at sites with varied channel slopes (averages of ,1-12%). After eradicating fish from 200-m stream sections, we assessed immigration of marked fish into these sections. Contrary to our hypothesis, upstream movements were more prevalent than downstream movements during the summer, even in steep streams. Marked brook trout ascended stream channels with slopes of 13% that extended for more than 67 m and 22% for more than 14 m; they also ascended a 1.2-m-high falls. Nearly vertical falls, rather than steep slopes per se, apparently inhibited upstream movements. Our hypothesis that upstream movements would decrease with increasing channel slope was partially supported; fish did not move as far upstream in steep as in gradual sites, and upstream movements through steep channels were dominated by larger fish (.135 mm total length). Immigration by marked fish smaller than 95 mm was uncommon in all sites. Slopes up to 13% do not ensure against upstream dispersal, although other mechanisms may inhibit brook trout invasion in steep channels. In very steep channels, fewer dispersing fish and slower upstream movement rates may increase the time required for successful invasion and reduce its likelihood of occurrence