Research Document 2020/035
Estimates of a Biologically-Based Spawning Goal and Management Benchmarks for the Canadian-Origin Taku River Sockeye Salmon Stock Aggregate
By Miller, S.E., and Pestal, G.
Abstract
The purpose of this paper is to identify a range of spawning escapements that would likely result in maximum sustained yields and identify the appropriate biological benchmarks (management reference points) for management of the Canadian-origin Taku River Sockeye salmon Oncorhynchus nerka stock aggregate. A Bayesian state-space Ricker model that included age-structure and a one year-lag autoregressive component was fit to 1980–2018 data for Taku River Sockeye salmon greater than 349 mm mid eye to fork length. Data for the state-space model included:
- estimates of harvest of naturally spawned and enhanced (hatchery-produced) Sockeye salmon above and below the U.S./Canada border in the lower Taku River;
- pooled Petersen capture-recapture estimates of above-border abundance; 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.
Coefficients of variation were also associated with these data sources. Historical annual terminal run abundance and inriver run abundance, spawning abundance, stock-recruitment parameters, and biological benchmarks were estimated from this model. The median estimate of spawner abundance that maximizes sustained yield, SMSY, was 43,857 fish. A sensitivity analysis on the beta prior of the Ricker model concluded that a uniform distribution produced similar median estimates of key model outputs and biological reference points as the normal distribution prior, although the computation time was greatly increased with the uniform prior. Likewise, a normal prior on beta that was not constrained to be greater than 1.00 x 10-6 greatly reduced the precision on the reference points, but produced similar median estimates of key model outputs. A sensitivity analysis on the early years (1980–1983) concluded that uncertainty in the early years of model data may bias the estimate of SMSY low. Based on the analyses from the state space model, consideration for the uncertainty in the stock recruit curve, and the minimal contrast within the time series, the recommendation from the Taku River Sockeye Salmon Working Group is a biological escapement goal range of 40,000–75,000 fish. This range has a greater than 50% probability of achieving at least 70% of maximum sustained yield at the lower and upper bounds, and minimizes the risk of overfishing (less than a 10% probability of overfishing the stock at lower bound if optimal yield based on 80% or more of maximum sustained yield).
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