Mitigation for the Construction and Operation of Libby Dam

Discussion
Long-term monitoring of bull trout redd numbers can be an important tool to
assess bull trout population trends (Rieman & McIntyre 1993). Bull trout redd counts in
the tributaries that we monitor below Libby Dam have not increased in proportion to the
increases we have observed in redd counts in bull trout spawning tributaries located
upstream of Libby Dam over the past 9 years. We are however, confident that we have
identified the important core spawning tributaries below Libby Dam within the Montana
portion of the basin. Although drought conditions in 2001 through 2003 likely
exacerbated the effects of debris jams in some of these streams by forming impassible
and substantial barriers in some of the tributaries may have reduced redd counts, the bull
trout redd count information does not correlate well with our adult bull trout population
estimate conducted in the spring of 2004 below Libby Dam. We observed a total of 126
bull trout redds in Quartz, Pipe, O’Brien, and Bear Creeks in the fall of 2003. Baxter and
Westover (2000) estimated an average of 1.55 fish per redd (range = 1.2-2.1 fish per
redd) for spawning bull trout in the Wigwam River in 1996-1999. If we apply these
ratios to bull trout redd counts in the 4 spawning tributaries below Libby Dam 2003
(Table 1; 126 total redds), there may have been 195 (range 150 – 265) spawning bull
trout in the Kootenai River below Libby Dam in 2003. We estimated a total of 920 bull
trout (95% confidence interval 698 – 1142). We believe most of these fish were likely
adult fish. Baxter and Baxter (2002) operated a fish trap on Skookumchuck Creek in
2001 and estimated the mean size of spawning adults to be 640 mm (range 400-920 mm),
which was similar to the size of fish we found below Libby Dam during our spring 2004
population estimate (mean total length 649 mm; range 343 – 861 mm). Although we
acknowledge that the 2003 bull trout redd counts within the tributaries below Libby Dam
may have underestimated of the total Kootenai River bull trout population because they
did not take into account alternate year spawning individuals, the disparity between our
adult population estimate and the redd count data is nevertheless large. One possible
explanation for these large differences may bull trout entrainment through Libby Dam
from within Libby Reservoir. The bull trout redd counts, juvenile estimates and adult
bull trout estimates collected by Montana FWP provide critical information required to
assess the status and trends of bull trout within the Kootenai River Basin. This
information will be essential to determine whether recovery criteria (USFWS 2002) are
met within this basin. Therefore, collection of these data will remain a high priority for
long-term monitoring conducted by Montana FWP. This project will also investigate the
use of other methodologies, such as scale and otolith micro-chemistry (Wells et al. 2000;
Wells et al. 2003; Kennedy et al. 2000) to determine the origin of adult bull trout below
Libby Dam.
Catch rates of burbot in our baited hoopnets in the Kootenai River directly below
Libby Dam have precipitously and significantly decreased in recent years. Our burbot
catch rates using baited hoop traps in the stilling basin below Libby Dam were the lowest
since we began monitoring this site in 1994/1995, and averaged only 0.0139 burbot per trap
day, or one burbot every 72 trap-days during the 2003/2004 trapping season. In
comparison, our catch rates for burbot using similar gear and techniques in Libby Reservoir
during the same period were approximately 5 times higher. However, since this was the
first year of trapping burbot in Libby Reservoir, using techniques similar to those we use in
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