Distribution and habitat use of bull trout following the removal of nonnative brook trout

Graduation date: 2006Invasion by nonnative brook trout (Salvelinus fontinalis) often results in\ud replacement of bull trout (Salvelinus confluentus) in western North America, but the\ud causal mechanisms are not well understood. Removal of brook trout from 1992 to 2000\ud from Sun Creek in southern Oregon, provided an opportunity to investigate the changes\ud in distribution and abundance of bull trout. This study investigated bull trout\ud distribution over from 1994 to 2003, during and following the extirpation of brook\ud trout in 2000. In 2001 over, 8 km of contiguous stream habitat was examined to\ud investigate bull trout habitat utilization. Bull trout abundance increased almost 300%\ud after the removal of brook trout; however, bull trout distribution did not shift either\ud upstream or downstream into habitat previously occupied by brook trout. This finding\ud suggested that the occupied habitat was either critical for bull trout persistence or that\ud factors restricted dispersal. Temperature did not appear to limit bull trout distribution\ud remaining below 17°C with averages less than 10°C throughout Sun Creek. The\ud downstream distribution of bull trout coincided with a point source increase of turbidity.\ud In regions of the stream where turbidity was low bull trout abundance was positively\ud associated with deep pools, higher average temperatures, and correlated with high densities of springs. The results of this study suggest that brook trout did not displace\ud bull trout, underscoring the importance that point source disturbances (i.e. turbidity) can\ud have on trout distributions. These findings reiterate the value of sampling contiguous\ud streams as a means to identifying factors influencing trout distribution throughout a\ud stream system