Mitigation for the Construction and Operation of Libby Dam

Mountain Whitefish
Mountain whitefish are one of three native species that have declined in abundance
since impoundment of the Kootenai River (Huston et al. 1984, Figure 28). A natural logarithm
transformation provided the best fit to the sinking gillnet catch data (Figure 33; r 2 = 0.598, p <
0.05). The trend in catch data for mountain whitefish during the first 13 years (1975-1988;
mean catch = 3.5 fish per net) after reservoir impoundment decreased annually, until it reached
a significantly (p = 0.0003) lower equilibrium with mountain whitefish catch rates since 1989
averaging 0.81 fish per net (r 2 = 0.14; p = 0.20). Catch rates since 1988 remained low; with
mountain whitefish comprising an average of 1.1% of the spring catch during 1988 through
2003. We attribute the initial (1975-1988) mountain whitefish decline in Koocanusa Reservoir
to the loss of spawning habitat and rearing habitat that resulted from a conversion of lotic to
lentic habitat through reservoir construction.
Rainbow and Westslope Cutthroat Trout
Rainbow trout and westslope cutthroat trout catch have both significantly declined since
the impoundment of Koocanusa Reservoir (Figure 33). Similarly to mountain whitefish gillnet
catch data, rainbow and westslope cutthroat trout gillnet catch data was best with linear
regression after performing a natural logarithm transformation (Figure 33). Although both
species exhibit similar declining trends in catch since 1975, rainbow trout catch per net since
1975 has declined more precipitously than cutthroat trout catch per net. Rainbow trout have
exhibited two general trends since impoundment. The first trend was the initial decline in
abundance from 1975 to 1988, which is characterized by significant decline (Figure 33),
followed by a period of relative stability from 1989 to 2003, where the average catch per net
during this period (mean fish per net = 0.344) was not significantly different than a stable
population (zero slope; Figure 33). Gill net catch of cutthroat trout in Koocanusa Reservoir
exhibit a similar pattern, with the exception that that cutthroat trout catch rates exhibit 3 general
trends through the same period. The first is a significant and precipitous decline during the
early years of impoundment from 1975 to 1986 (Figure 33), where mean catch rates averaged
1.37 fish per net. The second general trend reduced abundance (0.38 fish per net), but at a
level of stability from 1987 to 1993 (r 2 = 0.337; p = 0.172). The third general trend occurs
from 1994 to 2003,and is characterized by a significantly lower level of abundance (0.136 fish
per net; p = 0.001), and a somewhat stable level (r 2 = 0.023; p = 0.674). We believe that the
period of general equilibrium during the period 1987-1993 may have been artificially elevated
by the presence of hatchery cutthroat trout that were extensively stocked in the reservoir during
this period (Table 7). Hatchery cutthroat trout were last stocked in the reservoir in 1994.
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