Science Advisory Report 2023/008

Biological Benchmarks And Building Blocks For Aggregate-Level Management Targets For Skeena And Nass Sockeye Salmon (Oncorhynchus nerka)

Summary

• This research focuses on comparing methods for developing alternative aggregate-level biological reference points (including aggregate escapement goals) for Skeena and Nass Sockeye Salmon and evaluate advantages and disadvantages of each.
• This research provides an analytical framework to develop aggregate biological reference points for wild Skeena and Nass Sockeye Salmon, including both a single stock and Hierarchical Bayesian Model (HBM) spawner-recruit models and alternative approaches for developing aggregate biological reference points.
• This framework is intended to support the development of escapement goals for implementation of the Pacific Salmon Treaty, Wild Salmon Policy (WSP), Section 35, Part II of the Constitution Act, 1982, fisheries, and Indigenous treaties.
• This work does NOT provide recommendations for setting formal escapement goals. Formal recommendations for aggregate escapement goals are expected to be developed based on subsequent work and engagement processes that explicitly take management objectives, trade-offs between aggregate yield, and stock level conservation objectives into consideration.
• The analysis of spawner-recruit relationships focuses on wild-origin spawners, but also explores model fits for enhanced stocks in Babine Lake. While relevant biological information on enhanced (e.g., artificial spawning channels) returns is provided, further research is required to address the interaction between enhanced and wild stocks.
• Both the single stock and HBM analysis show overall productivity decline for many stocks in recent years (including the largest wild stocks).
• Changes in productivity for component stocks within each aggregate appeared to differ substantially based on the single stock analyses.
• Setting aggregate reference points should consider information on differential productivity of wild stocks across different life history types and trends in productivity over time (e.g., habitat change and declining fecundity based on available data [i.e., lower egg production as size of spawning females declines]).
• The analyses explore alternative approaches to develop biologically based aggregate reference points for Skeena and Nass stocks, with results summarized in Table 4.
• A clear positive relationship exists between fry production and seaward migrating smolts from Babine Lake; however, the benefits of increasing smolt production to adult returns are less clear, with high variability in smolt to adult survival.
• The relative contribution of non-Babine stocks to the aggregate Skeena abundance had already declined before enhancement and continued to decline after the start of the Babine Lake Development Project.
• This analysis is not meant to provide a formal stock status assessment; however, one of the illustrated aggregation approaches depends on multi-criteria status assessment applied under the Wild Salmon Policy.
• Although some preliminary simulation analyses were provided, a Management Strategy Evaluation is recommended, which would evaluate trade-offs between different objectives under potential productivity regimes.
• The simulation analyses provide examples of candidate management objectives and associated results based on potential productivity scenarios.
• Although more thorough evaluation is needed that includes observation and outcome uncertainties and covariation among stocks, the results of a simple forward simulation suggests that a steady decline in the largest stocks (i.e., Meziadin and Babine Late Wild) is likely for fixed exploitation rates greater than 50% based on recent productivity levels.
• Even a simple forward simulation helps identify potential interactions between the component stocks of the aggregate. For example, these analytical results suggest that under a fixed aggregate escapement goal, with exploitation rates responding to abundance, the individual stocks within the Skeena and Nass aggregates are more likely to meet conservation objectives under recent low productivity (compared with historical higher productivity) because lower aggregate run sizes result in reduced aggregate exploitation rates.
• Several required improvements to the proposed analytical framework were identified: (1) exploration of the sensitivity of spawner-recruit model fits to alternative prior assumptions (e.g., capacity priors and common shared year effect within the HBM), (2) assessment of model sensitivity to estimation biases, and (3) exploration of the effect of using longer time periods to define the recent productivity scenario (e.g., 10+ years).
• Several key priorities for future work were identified, including: Formal simulation testing to explore sensitivity to alternative spawner recruit model forms (e.g. capacity priors, HBM with common shared year effects), include additional data treatment steps (e.g. uncertainty on infilled estimate), assess model biases, explore integration of a shared year effect that accounts for separation between wild and enhanced stocks, account for changes in fecundity related to decreasing body size.
• This data preparation and analysis provide the groundwork to support a formal multi-criteria status assessment as applied under the Wild Salmon Policy.

This Science Advisory Report is from the April 26-28, 2022 regional peer review on Biological benchmarks and building blocks for developing aggregate-level management targets for Skeena and Nass Sockeye Salmon, British Columbia. 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 2023/008 (PDF, 1,119 KB)

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