| Citation |
Wood, C.C., D. Sneep, S. McAdam, J. Korman and T. Hatfield (2007) Recovery potential assessment for white sturgeon populations listed under the Species at Risk Act. Fisheries and Oceans Canada, B.C. Ministry of Environment, University of British Columbia and Solander Ecological Research. Canadian Science Advisory Secretariat Research Document 2007/003. Fisheries and Oceans Canada. |
| Abstract/Description or Keywords |
We assessed recovery potential for each of four populations of white sturgeon now listed as Endangered under the Species at Risk Act by considering current status, potential direct sources of human-induced mortality, and various strategies to mitigate harm and promote recovery. We used a simulation model to evaluate scenarios that span the range of plausible human activities that cause mortality or change the quantity or quality of important habitat. Best estimates of the abundance of mature fish in each population in 2006 are 185 in the Upper Fraser River, 305 in the Nechako River, 455 in the Kootenay River and 1000 in the Canadian portion of the Columbia River. Habitat is believed to limit current abundance in all populations. The Nechako, Kootenay, and Columbia populations are declining following decades of recruitment failure related to extensive habitat changes, primarily associated with dams and river regulation. Potential critical habitats (but not residences) have been identifed for all populations and include key areas for spawning, larval and juvenile rearing, adult feeding and staging prior to spawning migration. Threats to habitat include river regulation; instream activities such as dredging for gravel or sand; linear development; alterations or development of riparian, foreshore, or floodplain areas; upstream use of land and water; and effluent discharge from both point and non-point sources. Specific sources of harm or mortality to individual white sturgeon include targeted or incidental capture in recreational fisheries, bycatch in salmon gillnet fisheries, passage through dams, and sampling for research and hatchery broodstock. Best estimates for total annual mortality directly induced by humans range from 0.01% in the Upper Fraser to 0.07% in the Columbia population for small sturgeon (ages 2 to 10); and from 0.02% in the Upper Fraser to 0.3% in the Nechako population for large sturgeon (ages >10). The recovery goal specified in the draft national recovery strategy for white sturgeon is to ensure the long-term viability of naturally-reproducing populations throughout the species’ natural range, and restore opportunities for beneficial use, if and when feasible. Specified quantitative recovery objectives that could be assessed in simulation scenarios include (1) to ensure no net loss of reproductive potential, (2) to achieve within 50 years (a) 1000 mature individuals, (d) ongoing natural recruitment, and (e) population growth when below the abundance target. For the Upper Fraser population, simulation model projections suggest that all recovery objectives except 2a can be achieved if total human-induced mortality does not exceed twice the estimated status quo level. Simulation results based on our assumptions about historic abundance lead us to question the necessity of achieving 1000 mature fish (recovery objective 2a) and continued population growth (recovery objective 2e). An alternative approach is to recognize that the naturally small size of the Upper Fraser population makes it inherently vulnerable to extinction, and to seek to maintain its current viability by preventing further deleterious impacts. Concerns about the potential loss of genetic diversity over the longer term that motivate recovery objective 2a might be addressed by intervention to manage gene flow with other populations. For the Nechako, Kootenay, and Columbia populations, simulation model projections indicate that unless human intervention can restore natural recruitment, extinction in the wild is inevitable, even in the absence of further human-induced mortality. Our simulation results indicate first, that close to full restoration of the historic rates of natural recruitment will be necessary to achieve recovery objectives, and second, that restoration of historic rates of natural recruitment would be sufficient to achieve abundance objectives within 100 years, but not within 50 years. Hatchery supplementation will also be necessary to achieve abundance objectives, but would not be sufficient by itself. Hatchery supplementation should be viewed as experimental, but supported as a calculated risk to reduce the serious risk of genetic bottlenecks in natural spawning expected over the next 30 years. Given that the very feasibility of recovery depends upon successful human interventions to increase natural recruitment, it might be reasonable to allow some continuing incidental harm contingent on a commitment to engage in habitat restoration that is deemed sufficient to increase natural recruitment to historic levels, and to hatchery supplementation that is deemed sufficient to avoid future genetic bottlenecks. Simulated scenarios with habitat restoration to fully restore historic rates of natural recruitment combined with low level, short-term hatchery releases, indicate that recovery objectives could likely be achieved in the face of continuing incidental mortality not exceeding twice the current estimated level in each of the three non-recruiting populations. Sensitivity analyses suggest that this conclusion is robust over plausible ranges of parameter values and levels of annual variability. Our analyses were designed to demonstrate the necessary and sufficient conditions for achieving recovery objectives, not to determine the best options for recovery. We acknowledge that other scenarios involving different trade-offs might achieve recovery objectives with better socioeconomic outcomes. |
