Westslope Cutthroat Trout Recovery-Stocking Potential of Select Alberta Mountain Lakes, 2022
Final Report
Author(s)
Jason Blackburn, B.Sc.
Summary
Native populations of westslope cutthroat trout (Oncorhynchus lewisi; WSCT) occupy only 5% of their historical range in Alberta and the species is currently listed as Threatened under Canada’s Species at Risk Act. Protection of headwaters from introduced species is imperative for the persistence of native WSCT in Alberta; however, mountain lakes in the headwaters have historically supported a variety of recreational fisheries that include non-native species. Dispersal examples from invasive species in the headwaters could serve as a model for the introduction and reintroduction efforts necessary to ensure long-term persistence of WSCT in Alberta, using mountain lakes with impassible waterfall barriers along outlet streams as secure habitats for WSCT gamete reserves. We investigated 21 mountain lakes in the Bow River and Oldman River watersheds, representing native, naturalized, and stocked fisheries, and lakes with unknown fish assemblages, and known to occur within or adjacent to stream networks identified as having genetically pure WSCT populations. We assessed spawning suitability based on spawning substrate availability, evidence of redd building, overhead coarse woody debris, and fish spawning behaviour observed in the outlet tributaries, the lake near the outlet, and the littoral lake perimeter. We used environmental DNA (eDNA) to determine fish species currently present in 11 lakes.
The lake with the best likelihood to support reproducing WSCT was Rawson Lake, followed by Chester Lake, Rainy Ridge Lake, Lys Lake, and South Scarpe Lake. Picklejar Lake 4 already has a confirmed reproducing WSCT population, and Picklejar Lake 2 is suspected to support shoal-spawning WSCT. Targeted (species-specific) eDNA presence/absence results at four known positive lakes confirmed the presence of species, with very high detection rates, validating our methods. However, we also observed uncertainty in presence/absence results where trace eDNA detections occurred in six of ten lakes, highlighting limitations of eDNA results where transport of external genetic material is possible. Our results support the extirpation of brook trout in Grizzly Lake based on zero eDNA detections and a fishless overnight gillnet set, suggesting this lake could serve as vacant habitat to explore for recovery-stocking considerations. Of the lakes with high spawning potential, Rainy Ridge Lake, Rawson Lake, and Picklejar Lake 4 were among those with the highest growth potential based on summer growing degree days (SGDD). We documented mountain lake tributaries and lake water columns that approach and exceed upper incipient lethal temperature for WSCT even at mountain lake elevations. Conversely, we documented inflowing tributaries that do not exceed a 5°C SGDD base temperature during the summer months. This assessment will help fisheries managers determine the role of mountain lakes for recovery-stocking of WSCT in Alberta.