Bull Trout Population Assessment in the Upper Oldman River Drainage, 2009


Brad Hurkett


Historically, Bull Trout (Salvelinus confluentus) were the only native ‘char’ species to occupy all East Slope drainages in Alberta (Post and Johnston 2002). Since the early 1900’s, the abundance and distribution of this species has declined and has been restricted to the headwater drainages throughout the upper Peace, upper Athabasca, upper North Saskatchewan, and upper South Saskatchewan River basins. The decline is attributed to the effects associated with anthropogenic impacts, including angling pressure, habitat fragmentation and degradation, migratory barriers and introduction of non-native fish stocks (Post and Johnston 2002). These disturbances have led to a significant decline in Bull Trout populations and have resulted in threatened designations and extirpation throughout the Bull Trout native range (Rieman et al. 1997, Nelson et al. 2002). The General Status of Wild Species (2000) has ranked Alberta’s Bull Trout as a ‘sensitive’ species. In addition, Bull Trout are listed as a “threatened” species in the United States (lower 48 states) under the United States Endangered Species Act (1999).

Bull Trout inhabit cold-water stream, river and lake habitats throughout Alberta’s East Slopes which generally have lower productivity and carrying capacity. Reduced growth rates, late maturity and alternate-year spawning of Bull Trout are related to a low production coldwater habitat (Nelson and Paetz 1992). The specific habitat preferences and low reproduction and growth rates of Bull Trout are reflective to the species sensitivity to environmental, biological or anthropogenic effects (Nelson and Paetz 1992, Post and Johnston 2002). Bull Trout prefer diverse habitats with stable flows, low proportions of fine sediments, available cover, suitable water temperatures, and open migratory corridors (Haas 2001, McCart 1997, Watson and Hillman 1997). Bull Trout habitat diversity and connectivity allow for the expression of all types of life history strategies and are responsible for the persistence of Bull Trout (Rieman and Clayton 1997).

Bull Trout exhibit three main life history strategies: resident, fluvial, and adfluvial. Stream-resident Bull Trout permanently reside in small headwater tributaries. Migration is minimal for stream-resident fish, and spawning and overwintering occur within the same stream (Nelson and Paetz 1992). Stream-resident Bull Trout populations are generally isolated from other Bull Trout populations, typically segregated by fish barriers (McPhail and Baxter 1996). Stream-resident Bull Trout mature earlier and are typically smaller than migratory fish (< 300 mm) because they reside in low production waterbodies (Bellerud et al. 1997 and Earle and McKenzie 2001).

Fluvial and adfluvial Bull Trout are migratory fish that migrate from larger waterbodies and spawn in smaller headwater tributary streams. The difference between the two migratory life history forms is that fluvial Bull Trout inhabit larger mainstem streams and rivers throughout most of their life, whereas adfluvial Bull Trout reside in lakes and reservoirs throughout most of their life (McPhail and Baxter 1996). Migratory Bull Trout grow larger than stream-resident Bull Trout (> 400 mm) given that they reside in waterbodies that are much more productive (Allan 1980, Brewin 1994, Clayton 1999, Hvenegaard and Thera 2001, Rhude and Rhem 1995). Bull Trout migration can be quite extensive, as fish have been known to migrate distances of up to 400 km (McPhail and Baxter 1996). Upstream migration occurs early in the summer when water temperatures rise and stream flows subside and typically vary depending on the migration distance (Monnot et al 2008, Mushens 2003, Popowich and Paul 2006, Allan 1980, Bellerud et al. 1997, Burrows et al. 2001, Clayton 1998).

Alternate-year spawning is common in migratory Bull Trout populations, where individuals migrate and spawn in spawning tributaries once every two years (Nelson and Paetz 1992). As adult spawning populations become denser, alternate-year spawning occurs less and non-repetitive spawning (> 2-year spawning cycle) becomes more common, especially in male fish (Johnston and Post 2009). Successive-year spawning is also present in migratory populations and comprises 20% of the spawning population. Successive-year spawning is also density dependent and occurs less frequently as the spawning density increases (Johnston and Post 2009).

Bull Trout contain complex habitat requirements and exist in habitats where life history strategies overlap (Post and Johnston 2002). It is common for stream-resident and migratory Bull Trout to inhabit the same waterbody. Historically, where resident and migratory forms coexist, the migratory form is dominant (Fredenberg et al. 2005).

Whitesel et al. (2004) indicates that as an apex predator species, the migratory life cycle of Bull Trout is a highly successful strategy.

The upper Oldman River (UOM) Bull Trout population historically exhibited two of the three life history strategies, stream-resident and fluvial populations. And since the construction of the Oldman River Dam, an adfluvial Bull Trout population has become established in the Oldman Reservoir (Warnock in press).

Bull Trout distribution within the Oldman watershed has declined to 34% of the watershed’s historic range, largely since the 1950s (Fitch 1997). Since this time, human activity has increased in the UOM drainage, as well as other East Slope drainages. Logging, gas exploration and extraction, off-highway vehicle use, random access camping and angling are just a few activities that have been cumulatively impacting Bull Trout and Bull Trout habitat in the UOM drainage. Few assessments have been completed on the UOM Bull Trout population. Therefore, a drainage-wide Bull Trout population assessment is required to update the current status of the species within the UOM drainage.

The Alberta Conservation Association (ACA) is currently conducting a four-year Bull Trout population assessment in the UOM drainage. The study focuses on intercepting and marking migratory post spawn Bull Trout in key spawning tributaries in an effort to estimate the adult migratory Bull Trout population in the UOM drainage. Redd surveys have also been conducted to identify critical Bull Trout spawning habitats throughout the UOM drainage and also to determine the abundance and distribution of spawning activity within these identified spawning habitats. Currently, three consecutive years of fish trapping has been completed in Hidden Creek, since 2007. Similar efforts were completed for two consecutive years in the Livingstone River, Racehorse Creek and Dutch Creek. Redds surveys have been completed throughout the UOM drainage in 2008 and 2009.

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