Abundance and Distribution of Bull Trout in the Muskeg River Watershed, 2014
Author(s)
Mike Rodtka and Chad Judd
Summary
The Muskeg River bull trout population is currently considered at High Risk of extirpation. The population is one of only a handful of bull trout populations in Alberta that has been assessed periodically since the 1960s. Our objective was to provide fisheries managers with an update on the abundance of bull trout in the Muskeg River and on their distribution throughout the watershed, information which can be used when determining species status in the Muskeg River watershed.
To assess abundance of adult (i.e., ≥250 mm fork length [FL]) bull trout in the Muskeg River, we sampled two reaches of the river established for population monitoring using angling and float electrofishing gear in 2013. Following unexpectedly low catches at both reaches, we returned to the watershed in 2014. We angled 4 and 5 km sections of the upper and lower reaches, respectively. In addition, we electrofished 4.5 km of the lower reach from July 3 to 6. We used Program MARK to estimate bull trout abundance, capture probability and associated 95% confidence limits (CL) using our 2014 catch from the first 2 km of the upper reach. Our bull trout catch in the lower reach was too low for a formal estimate of abundance. To describe bull trout distribution in the watershed, we sampled 25 sites distributed randomly throughout the upper Muskeg River and its larger tributaries upstream of Muskeg Falls using backpack electrofishing gear between July 7 and 29, 2014. We used occupancy estimation and modelling to quantitatively describe sport fish distribution, with an emphasis on the juvenile life stage of bull trout (i.e., ≤150 mm FL) because these fish may be more indicative of a local population than adults.
We captured 73 fish, including 56 individual bull trout, in the two reaches of the Muskeg River in 2014. The remainder of our catch was comprised of rainbow trout (n = 10) and brook trout (n = 7). Bull trout size ranged from 262 to 514 mm FL, with a mean (± standard deviation) of 381 ± 59 mm. All bull trout captured were ≥250 mm FL and were classified as mature. Despite capturing very few fish in the same reach in 2013 under comparable conditions, we estimated bull trout abundance in the first 2 km of the upper reach to be 50 (95% CL = 41 – 76) or 25 fish/km with an associated capture probability (p) of 0.47 (95% CL = 0.30 – 0.64). We captured 479 fish throughout the 7.8 km of stream sampled while backpack electrofishing, including 89 bull trout (42 of which were juveniles). The remainder of our catch included brook trout (n = 231) and rainbow trout (n = 159). Bull trout was the most widely distributed species and the only fish captured in the headwaters of the Muskeg River. Modelling identified moderate potential for false absences to bias estimates of bull trout occupancy. The naive estimate (i.e., assuming detection probability = 1) of juvenile bull trout occupancy was 0.24, whereas our model-averaged estimate (± standard error) was 0.34 (0.16) although our small sample size did not permit strong inference. Assuming an approximately average stream conductivity (i.e., 200 μS/cm) and fish availability of 0.85, our top-ranked model predicted cumulative conditional detection probability of bull trout exceeded 95% within the first 200 m of effort. Our study provides fisheries managers with an update on the abundance and distribution of bull trout in the Muskeg River watershed, which can be used when determining species status.