Mitigation for the Construction and Operation of Libby Dam

Grave Creek
Juvenile salmonid monitoring within the Grave Creek Demonstration Project had two
primary objectives, to determine fish population trends through time and to evaluate the fish
community response to the restoration activities completed during the fall of 2001 (Grave
Creek Demonstration Project). Cutthroat and Rainbow trout were the two combined most
abundant fish species present at this site in all years except 2003, when juvenile bull trout
were the most abundant species present (Table A2). We compared mean fish abundance (by
species) for pre (2000-2001) and post (2002 and 2003) restoration projects using t-tests
(Figure 23). However, the variability in pre and post project fish abundance estimates is high
(Figure 23 and 24), and sampling methodology differed between years. These factors
reduced our ability to distinguish statistical differences in abundance before and after project
completion. Brook trout and bull trout abundance increased after at this site after project
completion, although the differences were not significant (Figure 23). Rainbow trout
abundance at this site did significantly increase from 9.0 to 24.5 fish per 1,000 feet after
project completion (p = 0.099). Mean westslope cutthroat trout abundance decreased
slightly, although not significantly (p = 0.38; Figure 23). We used linear regression to assess
whether there was a temporal trend in abundance for the four fish species at this site (Figure
24). Although the r 2 values for the regression analyses for rainbow, brook and bull trout all
exceeded 0.50, none of the trends differed significantly from a zero slope (p > 0.1; Figure
24). There was no apparent trend in westslope cutthroat trout abundance over the period
2000-2003 (Figure 24).
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