Changes in Population Indices of a Diminishing Burbot Population in the Kootenai River, Idaho, USA and British Columbia, Canada

ABSTRACT

We quantified changes that occurred in proportional stock density (PSD), relative stock density (RSD), relative weight (W_r ), and catch per unit effort (CPUE) of a diminishing burbot (Lota lota) population in the Kootenai River, Idaho, USA and British Columbia, Canada. Sampling with hoop nets took place periodically over 46 years. After completion of the Libby Dam in Montana in 1973, the burbot population declined. The mean length of burbot increased from 459 mm total length (TL) in 1957–1958 to 615 mm TL in 2002–2004. PSD did not increase appreciably, rising only from 92 in 1957–1958 to 98 by 2002–2004; but, RSD (preferred) increased from 17 in the early samples to 86 by the last sampling interval, which suggested recruitment failures. All other RSD values increased although not incrementally, which also suggested recruitment failures. As the burbot population declined, W_r increased from 75 to 98 from the early 1980s to early 2000s, suggesting that W_r may have a length bias in burbot. We found RSD was more useful when combined with CPUE, which also decreased from 1993 through 2004. Together these indices provide good indication of burbot population change. Hoop nets are the most important sampling gear for burbot but are length selective; thus, PSD (quality) alone will not detect recruitment issues. Managers and researchers of other burbot populations should design sampling programs that consider the value in measuring RSD and CPUE when hoop nets are used.