Science Advisory Report 2021/030
Recovery Potential Assessment for 11 Designatable Units of Fraser River Chinook Salmon, Oncorhynchus tshawytscha, Part 2: Elements 12 to 22
- This is the second of two parts of a Recovery Potential Assessment (RPA) for 11 Designatable Units (DUs) of southern British Columbia (BC) Chinook Salmon that spawn in the Fraser River watershed. The primary focus of this portion of the RPA is to propose recovery targets, predict short-term population trends, evaluate mitigation options, and develop an allowable harm assessment.
- DUs were assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in November 2018 (COSEWIC 2019). Four were designated as Threatened and seven were designated as Endangered. Declining trends in abundance have continued for these DUs since the COSEWIC assessment. The assessed DUs are:
- Lower Fraser River Ocean Fall - Harrison (DU2)
- Lower Fraser River Stream Summer - Upper Pitt (DU4)
- Lower Fraser River Stream Summer (DU5)
- Middle Fraser River Stream Spring - Nahatlatch (DU7)
- Middle Fraser River Stream Fall - Portage (DU8)
- Middle Fraser River Stream Spring (DU9)
- Middle Fraser River Stream Summer (DU10)
- Upper Fraser River Stream Spring (DU11)
- South Thompson Stream Summer - Bessette (DU14)
- North Thompson Stream Spring (DU16)
- North Thompson Stream Summer (DU17)
- For each DU two recovery targets were proposed:
- A survival target that approximates conditions such that a DU would not be characterized as Endangered or Threatened by COSEWIC.
- A recovery target at which the DU’s long term persistence is secured.
- For DU2, a 30-year series of spawner and recruitment data (brood years 1984-2013) was used in a population model to estimate recent population parameters. That analysis suggested that population productivity has been variable, but has declined over time.
- For DU2, abundance benchmarks were based on average long-term population productivity. However, alternative approaches for estimating benchmarks when productivity is varying were discussed. For the other DUs a habitat method based on a meta-analysis of data from other populations was used to estimate abundance benchmarks; there is more uncertainty surrounding these benchmarks than those estimated for DU2.
- A projection model was used to simulate DU2 abundances over the next three generations (2020-2031) under the base case assumption that the productivity estimated for the most recent four years for which complete data were available (brood years 2010-2013) and recent United States (US) and Canadian salmon fishery harvest rates (catch years 2009-2015) will continue unchanged into the future. The modelled population was As Likely as Not (33%-66%) to meet the survival target, and Unlikely (10%-33%) to meet the recovery target under base case conditions.
- At base case harvest rates, the projection model predicts that an increase in productivity over 12 years of at least 40% is required for the model population (DU2) to be Likely (>66%) to reach the survival target; a productivity increase of at least 140% is needed to be Likely to reach the recovery target. Conversely, at base case productivity the projection model predicts that with US harvest rates held constant, a decrease in Canadian salmon harvest rates of at least 90% is required for the model population to be Likely (>66%) to reach the survival target. At the base case productivity with no Canadian salmon harvest, the probability of reaching the recovery target was 41%.
- There were insufficient data for the other ten DUs to conduct an analysis similar to that used for DU2. However, the rate of decline in abundance observed in most of these DUs and the threats identified in Part 1 of the RPA suggests long-term declines in productivity are occurring, and it was judged these DUs are unlikely to reach either the survival or recovery targets in three generations if current conditions continue. For DUs 9, 10, and 11, migration to spawning areas is currently impeded by the Big Bar landslide in the Fraser River and these DUs are likely to suffer greater declines in the short term than the other DUs. Mitigation measures are underway to alleviate the slide but their long-term effectiveness is unknown.
- A preliminary list of mitigation measures was developed that could address threats identified in Part 1 of the RPA. These measures may increase survival or productivity but information was not available to assess their effectiveness, nor their potential to increase the probability of meeting the recovery targets.
- For DU2, under base case values for harvest and productivity, human-induced mortality and other sources of harm identified in the threats assessment should be significantly reduced from base case mortality so as to not jeopardize recovery.
- Many stream-type DUs have experienced more severe declines in abundance than DU2 and some have small populations (<1000 spawners). For DUs 7, 8, and 14, the area of spawning habitat is limited and current populations are very small. As noted above, for DUs 9, 10, and 11, additional concern due to the Big Bar landslide will remain until the impacts from the slide are alleviated. Harm is likely to continue to jeopardize recovery. Therefore, to promote the survival and recovery of these DUs, it is advised that all future and ongoing human-induced harm should be prevented.
- Predicting future changes in salmon productivity and abundance is challenging in the current era of rapidly changing conditions, as there is significant uncertainty in both the future state of natural environments and the ability to mitigate anthropogenic effects. Spawner abundance and productivity should be monitored closely to determine if changes are occurring, and model assumptions, population projections, and science advice should be re-visited as warranted.
- Data to reliably monitor changes in population status exist for DU2, but for most of the other DUs, abundance estimates are less reliable, consistent estimates of exploitation rates are not available, and basic biological attributes of spawning populations are not well known. Increased understanding of these populations is needed for recovery planning.
- Each target was comprised of two benchmarks: generational average spawner abundance and the three-generation trend in spawner abundance.
This Science Advisory Report is from the 7-9 July 2020, 1 October 2020, and 11-12 March 2021 regional peer review on Recovery Potential Assessment – Fraser River Chinook Salmon (Oncorhynchus tshawytscha) – Eleven Designatable Units (Elements 12-22). Additional publications from these meetings will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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