Distribution of Bull Trout in the Waterton River Watershed, Alberta, 2012 – 2013
Jason Blackburn, Brad Hurkett and Tyler Johns
In recent decades, bull trout (Salvelinus confluentus) in the Waterton River watershed have become restricted to the coldest headwater tributaries on which they rely to satisfy narrow thermal requirements for successful spawning and rearing. Current threats to their continued persistence include hybridization with brook trout (Salvelinus fontinalis) and habitat fragmentation, which have impacted genetic integrity and interrupted gene flow across the watershed. Compounding these threats is the geography of the watershed, which rests at the fringe between suitable and unsuitable thermal habitat, and the sensitivity of fringe populations to environmental change. A current description of bull trout distribution is essential to identify and prioritize remediation efforts; however, data collection efforts to date have been uncoordinated.
We completed a comprehensive two-year inventory of the Waterton River watershed to determine current distribution of bull trout populations relative to thermal habitat quality, existing migration barriers, and the presence of non-native fish species. We used an intensive systematic sampling pattern across the watershed to detect bull trout and obtain a detailed description of the existing sport-fish community. We electrofished approximately 63 km of stream in the watershed, including 71 reaches on tributaries, the river reach between Upper and Middle Waterton Lakes (Dardanelles), the main-stem Waterton River, and perimeter of Maskinonge Lake, using backpack, tote-barge, raft and jet-boat electrofishing gear. We installed data loggers to monitor in-stream temperature at 29 stations across the watershed to characterize thermal habitat quality relative to juvenile bull trout tolerances (classified as high, medium or low), detect temperature gradients across the watershed, and delineate populations separated by thermal barriers.
Of 71 tributary sample reaches, 11 were upstream of waterfall barriers and yielded no fish. In the remaining 60 reaches, we captured 283 bull trout, of which 94% (n = 265) were captured in three short reaches where stream temperatures were coldest (high quality). Catch-per-unit-effort of bull trout was highest in Spionkop Creek (139 fish/km), upper Yarrow Creek (58 fish/km) and Blakiston Creek (32 fish/km). Catch-per-unit-effort decreased exponentially with increasing temperature, and bull trout were not captured in streams averaging >15°C in the summer. In streams where bull trout were detected, probability of capture exceeded 50% for juveniles (≤150 mm fork length; FL) and for all bull trout where temperatures averaged <8.6°C and <10.25°C, respectively. We identified six high-quality thermal habitat areas: North Drywood, South Drywood, Spionkop, Yarrow, upper Galwey and Blakiston creeks. Bull trout were not captured in two of the high-quality habitats of Galwey Brook and North Drywood Creek, which were fragmented by seasonal and permanent migration barriers. Summer stream temperature conditions in lower reaches of the watershed were seasonally unsuitable for bull trout and appear to limit their distribution. Waterton Reservoir provides a potential thermal refuge at depths >10 m, whereas mid-summer surface temperatures generally exceeded those suitable for bull trout survival.
The combined length frequency distribution of all bull trout captured in the watershed exhibited a relatively broad range of size classes (44 – 668 mm FL); however, juvenile and adult sizes represented in the overall frequency distribution were physically separated from each other by permanent and temporary barriers, such as dams and subsurface stream reaches. Based on size-class distributions, the tributary most likely to support fluvial bull trout was Spionkop Creek, where 44% of fish >300 mm FL, 71% of fish >400 mm FL, and 100% of fish >500 mm FL were captured.
The number of non-native sport species captured (n = 1,659) was roughly double that of native sport species (n = 814). Brook trout was the most abundant sport species in the study area, comprised 36% (n = 901) of the total catch, and exceeded the total catch of all native sport species combined. Brook trout were captured in every sub-watershed and in 78% of fish-yielding sample sites. Rainbow trout (Oncorhynchus mykiss) was the second most abundant and widely distributed sport species, comprising 18% of the total catch (n = 447). Rainbow trout were captured in 47% of tributary sample sites and were the primary sport species present in the Drywood/Yarrow sub-watershed, captured in 82% of sample sites. Brown trout (Salmo trutta) represented 12% of the total catch (n = 296) and were dominant in the Waterton River and in the Cottonwood Creek sub-watershed. Mountain whitefish (Prosopium williamsoni) was the most abundant native sport species, comprising 16% of the catch (n = 399), as well as the most abundant species in the Waterton River. Bull trout accounted for 12% of the catch, and the remaining 6% was comprised of cutthroat trout (Oncorhynchus clarkii lewisi) (n = 44), cutthroat trout x rainbow trout hybrids (cutbow trout) (n = 35), burbot (Lota lota) (n = 34), northern pike (Esox lucius) (n = 14), bull trout x brook trout hybrids (n = 11), and lake trout (Salvelinus namaycush) (n = 1).
We observed contrasting fish community compositions above and below one seasonal, one semi-permanent, and two permanent fish passage barriers on upper Yarrow creek, Dungarvan Creek, and the Gulf and Shell dams, respectively. Natural barriers appeared to isolate native species from non-native species, whereas human-made barriers appeared to limit the upstream distributions of native bull trout and mountain whitefish while benefitting non-native species like brook trout and rainbow trout above the barriers.
Overall, the Waterton River watershed is dominated by non-native fish species. Brook trout appear to be a significant threat to remaining bull trout populations as we captured bull trout x brook trout hybrids in nearly every tributary where bull trout were captured. Our findings will help fisheries managers decide how and where to best conserve Alberta’s provincial fish while maintaining viable recreational fisheries for Alberta anglers.