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