Mitigation for the Construction and Operation of Libby Dam

Burbot Monitoring Libby Reservoir
During the period from November 14 to April 26, 2004 we expended a total effort
of 1887 trap-days, and caught a total of 127 burbot at 10 trapping locations throughout
the reservoir (Table 1). We were unable to trap the Tobacco River, Dodge and Young
creek sites from January 3 rd to March 25 th , 2004 due to surface ice formation. Burbot
catch at all trapping locations averaged 0.064 fish per trap-day. Mean burbot catch was
highest near the mouth of Cripple Horse Creek and lowest near the mouth of Dodge
Creek (Table 2). Although catch rates differed between sites, the observed differences
were not significant when compared using an analysis of variance procedure (p = 0.316).
The mean total length of burbot captured in the hoop traps was 576.1 mm (range 350-
880 mm; Figure 13). Burbot mean length did not significantly differ between the nine
trapping locations where we captured burbot (p = 0.439). The mean condition factor (K;
Carlander 1969) for all burbot captured was 0.615. The length-weight relationship for
burbot captured in Libby Reservoir is presented in Figure 14. We recaptured 3 burbot in
the hoop traps that were previously captured and PIT tagged. One burbot was recaptured
twice near Big Creek. This fish was originally captured and PIT tagged on 11/17/03, and
subsequently recaptured on 11/20 and 12/5/03. Another burbot was originally PIT
tagged near Cripple Horse Creek on 12/8/03 and recaptured near Barron Creek on 3/1/04.
The third burbot recaptured was marked in the Tobacco Bay area on 11/25 and
recaptured near the capture release site on 4/8/04.
We used multiple linear regression to evaluate trends in burbot catch versus
trapping date and hoop trap depth. We found no evidence to suggest that catch rates
differed by trapping date (Figure 15; r 2 = 0.007; p = 0.794). We did however, find a
significant relationship between trap depths and catch rates when we grouped trap depths
into categories of 3 m intervals and averaged the proportion of those traps containing
burbot for those depth categories (Figure 16; r 2 = 0.620; p = 0.012). This relationship
suggested that catch rates increased up to approximately 11 m and then remained
relatively constant up to the 33 m, which was the maximum depth we trapped.
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