Mitigation for the Construction and Operation of Libby Dam

Libby Reservoir Gillnet Monitoring
We documented changes in the assemblage of fish species sampled in Libby Reservoir
since impoundment. Kokanee salmon, Kamloops rainbow trout and yellow perch did not occur
in the Kootenai River prior to impoundment but are now present. Kokanee were released into
the reservoir from the Kootenay Trout Hatchery in British Columbia (Huston et al. 1984).
Yellow perch may have dispersed into the reservoir from Murphy Lake (Huston et al. 1984).
The British Columbia Ministry of Environment (BCMOE) first introduced Kamloops rainbow
trout in 1985. Eastern brook trout are not native to the Kootenai Drainage, but were present in
the river before impoundment and continue to be rarely captured in gillnets within the reservoir.
Peamouth and northern pikeminnow were rare in the Kootenai River before impoundment, but
have increased in abundance in the reservoir. Mountain whitefish, rainbow trout, westslope
cutthroat trout and redside shiner were common in the Kootenai River before impoundment,
but have decreased in abundance since impoundment.
Kokanee
Since the accidental introduction of 250,000 fry from the Kootenay Trout Hatchery in
British Columbia into Libby Reservoir in 1980, kokanee have become the second most
abundant fish captured during fall gillnetting. Fluctuations in catch have corresponded to the
strength of various year classes (Hoffman et al. 2002), and have varied by year, with no
apparent continuous trend in abundance (Figure 32). However, kokanee catch in the fall net
series follows a general trend of decreasing abundance from 1988-1995 and an increasing
trend in abundance from 1996-2003 (Figure 32). Average length of kokanee has varied
among years. Average length and weight between 1988 and 2003 was 290.3 mm and 235.2 g
respectively (Table 6), while maximum average size occurred in 1992 (350 mm, 411 g).
However, the minimum mean length was observed in 2002 (Table 6).
Catch per Net
30
25
20
15
10
5
Spring
r 2 = 0.008
p = 0.739
Fall (1988-1995)
r 2 = 0.44;p = 0.073
Y = 3773 - 1.89*X
Fall (1996-2003)
r 2 = 0.44;p = 0.072
Y = -3964 + 1.99*X
0
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
Year
Fall Floating Nets
Spring Sinking Nets
Figure 32. Average catch per net of kokanee for fall floating (1988-2003) and spring sinking
(1984-2003) gill nets in Libby Reservoir.
91