Movements of nonnative brook trout in relation to stream channel slope

We provide new insights on the ability of naturalized brook trout Salvelinu s fontinalis to ascend steep, headwater streams in the western USA. We test ed 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 eradi cating fish from 200-m stream sections, we assessed immigration of marked f ish into these sections. Contrary to our hypothesis, upstream movements wer e more prevalent than downstream movements during the summer, even in steep streams. Marked brook trout ascended stream channels with slopes of 13% th at extended for more than 67 m and 22% for more than 14 m; they also ascend ed a 1.2-m-high falls. Nearly vertical falls, rather than steep slopes per se, apparently inhibited upstream movements. Our hypothesis that upstream m ovements would decrease with increasing channel slope was partially support ed; fish did nor move as far upstream in steep as in gradual sites, and ups tream movements through steep channels were dominated by larger fish (>135 mm total length). Immigration by marked fish smaller than 95 mm was uncommo n in all sites. Slopes up to 13% do not ensure against upstream dispersal, although other mechanisms may inhibit brook trout invasion in steep channel s. In very steep channels, fewer dispersing fish and slower upstream moveme nt rates may increase the time required for successful invasion and reduce its likelihood of occurrence.