Abundance of Bull Trout in the Clearwater River Alberta 2012-2013
Chad Judd and Mike Rodtka
The Clearwater River watershed bull trout population is at considerable risk of extirpation. Of the four identified subpopulations, the Tay and Prairie Creek subpopulations are essentially extirpated, while the Elk Creek subpopulation was assessed in 2010, leaving only the Clearwater River mainstem population unassessed. Our objectives were to estimate bull trout size structure and adult abundance at three established index reaches of the river and to compare river habitat at index reaches to river habitat throughout the study area. We used electrofishing and angling to capture bull trout at four reaches in the Clearwater River. We used field measurements as well as broad‐scale, remotely sensed measurements to identify habitat qualities not already captured in the index reaches. Due to low catches, we could not use the electrofishing data to derive abundance estimates. In April 2013, we used angling gear to estimate bull trout abundance at the Elk and Corkscrew reaches. Estimated bull trout abundance was 9 (95% CL = 9 – 22) or 1.5 fish/km and 19 (95% CL = 14 – 41) or 3.2 fish/km at the Elk and Corkscrew reaches, respectively. Remotely sensed measurements were comparable to field measurements. Rooted widths at index reaches ranged from 56 to 155 m and 51 to 181 m for field measurements and remote sensing measurements, respectively. A fourth reach (Prairie), near the mouth of the river, contained markedly different habitat qualites from the index reaches and was added to the study. The Prairie reach was confined to a single channel, low gradient and mostly flatwater habitat. We conclude that bull trout abundance in the Clearwater River has remained unchanged for the past 35 years and the core area may still be at high risk of extirpation.