Science Advisory Report 2020/011

Recovery Potential Assessment – Cultus Lake Sockeye Salmon (Oncorhynchus nerka) (2019)

Summary

• Cultus Lake Sockeye Salmon are the Designatable Unit (DU) of Sockeye Salmon (Oncorhynchus nerka) that spawn in Cultus Lake, British Columbia (BC). This DU was first assessed as Endangered in an emergency assessment by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2002 and confirmed as Endangered by COSEWIC in 2003 and again in 2017.
• Historically (1921-1970), the 4-year average abundance was 19,890 spawners but in the mid-1970s the population began to decline in abundance. Since 2006 the spawning population has been augmented by hatchery supplementation. The most recent (2015-2018) generational average of adults spawners entering the lake was 254 natural-origin and 941 hatchery-origin fish.
• Since 2010, the poor status of the population can be primarily attributed to very low rates of smolt production from the lake. Smolt-to-adult survival has also declined since the 1990s.
• Cultus Lake is undergoing cultural eutrophication, mainly resulting from excess anthropogenic nutrient loadings to the lake from the watershed and atmospheric deposition from the nutrient-contaminated regional airshed. These potentially reversible changes are generating conditions in the lake that are unsuitable for all Sockeye Salmon freshwater life stages occurring there.
• Redds, the spawning nests constructed by Pacific salmon and other species, meet the definition of a "residence" under the Species at Risk Act (SARA).
• The main anthropogenic threats and limiting factors identified for Cultus Lake Sockeye Salmon include: lake eutrophication, adult mortality associated with change in migration timing, fisheries interceptions, and climatically-mediated variability and change in freshwater habitat conditions.
• A recovery target based upon a 4-year mean of 7,000 adults entering the lake is proposed. A survival target of 2,500 adults entering the lake is also proposed. These targets may include hatchery fish if the hatchery program is designed to minimize risks to the wild population. Quantitative guidelines for hatchery supplementation are provided.
• An empirically-based population model was used to estimate the probability that the population would reach the survival and recovery targets under scenarios that evaluated key mitigation measures: hatchery supplementation, limits to fishing mortality and improving freshwater population productivity. The results showed that the probability of reaching either the survival or recovery target in 12 years (3 generations) under current conditions is unlikely, although some scenarios that included hatchery supplementation and freshwater mitigation resulted in population growth that could result in recovery over a longer period.
• Recovery or survival of a natural, self-sustaining, population will require successful mitigation of the cause of low smolt production in the lake. This could include measures to mitigate nutrient inputs to reduce cultural eutrophication that has increased over the last decade. Otherwise, the natural-origin population is predicted to continue to decline.
• Given the negative population growth rate there is no allowable harm for this population. If threats to freshwater productivity can be mitigated, and ocean survival does not decline further, it is possible that recovery could occur under limited allowable harm.
• Key uncertainties that may impact the potential for recovery include the effectiveness of measures to improve conditions in Cultus Lake for Sockeye Salmon, future changes in the survival of smolts in the ocean and rates of prespawning mortality of adults, and long-term effects of hatchery supplementation on the fitness of the natural-origin population.

This Science Advisory Report is from the October 7-10, 2019 regional peer review on Recovery Potential Assessment – Fraser River Sockeye Salmon (Oncorhynchus nerka) – Ten Designatable Units. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2020/011 (PDF, 1,095 KB)

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