Science Advisory Report 2020/006
Estimates of a biologically-based spawning goal and management benchmarks for the Canadian-origin Taku River Sockeye salmon stock aggregate
Summary
- The Transboundary Panel requires a maximum sustainable yield (MSY)-based escapement goal recommendation at the aggregate level to support the management and stock assessment regime that has been developed by the Parties’ joint Transboundary Technical Committee for implementation prior to the 2020 fishing season.
- The specific objectives of this review are to:
- Identify the spawning escapements that would produce maximum sustainable yields for the Canadian-origin Taku River Sockeye salmon Oncorhynchus nerka stock aggregate.
- Identify the appropriate biological benchmarks for the management of the Canadian-origin Taku River Sockeye salmon stock aggregate.
- A Bayesian state-space Ricker model that included age-structure and a one year-lag autoregressive component was fit to 1980–2014 brood years for Canadian-origin Taku River Sockeye salmon greater than 349 mm (mid eye to fork length).
- Data for the state-space model included:
- harvest estimates of Canadian-origin Taku River Sockeye salmon in terminal areas;
- capture-recapture estimates of above-border abundance Canadian-origin Taku River Sockeye salmon; and
- weighted age composition estimates of Taku River Sockeye salmon harvested in the U.S. District 111 traditional commercial drift gillnet fishery and Sockeye salmon captured in the Canyon Island fish wheels in the lower Taku River.
- Capture-recapture abundance estimates were calculated with the BTSPAS package (Bonner and Schwarz 2020; Schwarz et al. 2009) which generates both Bayesian time-stratified Petersen estimates and pooled Petersen estimates. Bayesian time-stratified estimates explicitly account for several common sources of potential bias (e.g., tags missed while the fishery is closed) by extrapolating a run-timing curve from the tag data. However, Bayesian estimates are computationally complex, and can be sensitive to prior assumptions. The simple pooled Petersen and variations of the Bayesian time-stratified Petersen were generally very close. Based on this observation, the bilateral review process chose to use pooled Petersen estimates for the state-space model inputs.
- Pooled Petersen capture recapture abundance estimates for Canadian-origin Taku River Sockeye salmon were adjusted for dropout rate and for size bias. A dropout adjustment of 25.5% was used for 1984 to 2016, which was based on a weighted average dropout observed in radiotelemetry studies from 1984, 2015, 2017 and 2018. Year-specific dropout rate estimates were applied for 2017 and 2018. A size-bias adjustment of 6.4% (based on the 2003 to 2018 average) was applied to 1984 to 2002 estimates and adjustments for 2003 to 2018 were year-specific size-stratified estimates.
- Historical annual terminal run abundance and inriver run abundance, spawning abundance, stock-recruitment parameters, and biological benchmarks were estimated from this model.
- Specific biological benchmarks estimated from the model are as follows: SMSY = 43,857 (90% CI from 30,422 to 99,699) fish, 80% SMSY = 35,086 (24,337 to 79,760) fish, SMAX = 59,145 (35,843 to 164,901) fish, SEQ = 124,106 (97,418 to 252,655) fish, and SGEN = 5,873 (1,967 to 25,146) fish.
- Limited contrast in spawning escapement estimates resulted in large uncertainty in biological benchmarks. Sensitivity tests of the prior distribution on the density-dependence parameter (β) resulted in similar median estimates of key model outputs.
Estimates of uncertainty were included in these data sources. These data were reviewed and updated in a bilateral process starting in 2018.
This Science Advisory Report is from the November 5-6, 2019 regional peer review on the Development of a Biological Escapement Goal for Taku River Sockeye Salmon. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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