Impacts of dam construction in the upper Columbia Basin, British Columbia, on bull trout production, fisheries, and conservation status

Author: J. Hagen

Bull trout have probably fared better than most other species, sport or non-sport, in the flooded valleys of the upper Columbia Basin, due to the great efficiency with which they can exploit a cold, lacustrine environment, even one which is of extraordinarily low primary productivity, once a prey fish base is established. Negative footprint impacts on bull trout production associated with BC Hydro dams are nonetheless likely to be significant. By far the most significant loss of potential bull trout spawning and rearing habitat was the 142 km estimated to have been inundated due to the construction of the Mica Dam. When integrated over the entire life cycle, however, these losses appear to be mitigated in large part by good growth and survival in Kinbasket Reservoir, and bull trout in the dam unit under post-impoundment conditions are large and abundant. Fluvial populations rearing in accessible tributaries to Revelstoke Reservoir now have adopted an adfluvial life history or have been replaced by/amalgamated with adfluvial populations, which probably experience greater growth and survival than in the pre-impoundment Columbia River. Although significant tributary habitat losses (29 km) also occurred, production of adult bull trout from wholly within the dam unit has increased because of recruitment from adfluvial populations with natal streams above the Revelstoke Dam but that would have recruited to the Arrow Lakes prior to dam construction. This recruitment, however, has been permanently cut off from the Arrow Lakes Reservoir. Although the breaching of barriers on the Illecillewaet and Halfway rivers has compensated for much of this lost recruitment, the estimated shortfall remaining is 19,300 age-1+ and older juvenile bull trout, equivalent to 770 spawners annually. Bull trout rearing habitats (8.9 km) were also inundated by Duncan Reservoir. Because connectivity remains with Kootenay Lake, however, 30 km of access above a breached barrier on the Kaslo River has mitigated these tributary losses. Pend d’Oreille system bull trout, which are now isolated within the Salmo River, have been most severely affected by dam construction, with the Seven Mile Reservoir now being too warm for bull trout production and not allowing passage into productive habitats in the Columbia River mainstem.

Probably the gravest effect of the dam construction and flooding, from the perspective of those who are entrusted with the conservation of the species, has been the compromised relationship between diverse natural environments, adaptations within bull trout populations to these environments, and genetic diversity. Genetic diversity within upper Columbia Basin appear already to have been lost as a result of amalgamation of diverse adult and subadult rearing habitats into single reservoirs, and, to some degree, compensation efforts to date in the ALR basin. The deteriorated conservation status of two southern populations in the upper Columbia Basin, the isolated, small populations in the Blueberry Creek and Salmo River watersheds, are directly the result of dam construction, and the requirement for mitigation measures in these watersheds may be urgent even if restoration of populations to preimpoundment levels is no longer possible.

Bull trout have probably fared better than most other species, sport or non-sport, in the flooded valleys of the upper Columbia Basin, due to the great efficiency with which they can exploit a cold, lacustrine environment, even one which is of extraordinarily low primary productivity, once a prey fish base is established. Negative footprint impacts on bull trout production associated with BC Hydro dams are nonetheless likely to be significant. By far the most significant loss of potential bull trout spawning and rearing habitat was the 142 km estimated to have been inundated due to the construction of the Mica Dam. When integrated over the entire life cycle, however, these losses appear to be mitigated in large part by good growth and survival in Kinbasket Reservoir, and bull trout in the dam unit under post-impoundment conditions are large and abundant. Fluvial populations rearing in accessible tributaries to Revelstoke Reservoir now have adopted an adfluvial life history or have been replaced by/amalgamated with adfluvial populations, which probably experience greater growth and survival than in the pre-impoundment Columbia River. Although significant tributary habitat losses (29 km) also occurred, production of adult bull trout from wholly within the dam unit has increased because of recruitment from adfluvial populations with natal streams above the Revelstoke Dam but that would have recruited to the Arrow Lakes prior to dam construction. This recruitment, however, has been permanently cut off from the Arrow Lakes Reservoir. Although the breaching of barriers on the Illecillewaet and Halfway rivers has compensated for much of this lost recruitment, the estimated shortfall remaining is 19,300 age-1+ and older juvenile bull trout, equivalent to 770 spawners annually. Bull trout rearing habitats (8.9 km) were also inundated by Duncan Reservoir. Because connectivity remains with Kootenay Lake, however, 30 km of access above a breached barrier on the Kaslo River has mitigated these tributary losses. Pend d’Oreille system bull trout, which are now isolated within the Salmo River, have been most severely affected by dam construction, with the Seven Mile Reservoir now being too warm for bull trout production and not allowing passage into productive habitats in the Columbia River mainstem.

Probably the gravest effect of the dam construction and flooding, from the perspective of those who are entrusted with the conservation of the species, has been the compromised relationship between diverse natural environments, adaptations within bull trout populations to these environments, and genetic diversity. Genetic diversity within upper Columbia Basin appear already to have been lost as a result of amalgamation of diverse adult and subadult rearing habitats into single reservoirs, and, to some degree, compensation efforts to date in the ALR basin. The deteriorated conservation status of two southern populations in the upper Columbia Basin, the isolated, small populations in the Blueberry Creek and Salmo River watersheds, are directly the result of dam construction, and the requirement for mitigation measures in these watersheds may be urgent even if restoration of populations to preimpoundment levels is no longer possible.