Science Advisory Report  2015/008

Estimates of a biologically-based spawning goal and biological benchmarks for the Canadian-origin Taku River Coho stock aggregate

Summary

• Taku River Coho Salmon are a transboundary stock, managed cooperatively by Canada and the United States under provisions of Chapter 1, Annex IV of the Pacific Salmon Treaty (PST). The most recent provisions of the PST call for the development of a bilaterally-agreed spawning goal for Canadian-origin Taku River Coho Salmon set at the spawner level that maximizes sustainable yield (S_MSY).
• In addition to PST obligations, as part of implementing Strategy 1 of the Wild Salmon Policy (WSP), Fisheries and Oceans Canada (DFO) is required to identify biological benchmarks to assess the status of WSP Conservation Units (CUs) for Pacific salmon. Prior to this work, WSP benchmarks had not been estimated for Taku River Coho Salmon.
• To address PST commitments and support progress towards a WSP status assessment for Taku River Coho, an assessment to estimate biological benchmarks for spawner abundances of the Taku River Coho Salmon stock aggregate was undertaken.
• Data are available for Taku River Coho Salmon at the aggregate level, including spawner estimates based on a consistent mark-recapture survey covering the period 1987-2013, and recruitment estimates for the 1987 to 2009 brood years based on coded-wire tag recoveries in Alaskan and Canadian fisheries and surveys. Annual age and sex composition estimates are also available. These data are sufficient to estimate biological benchmarks based on spawner-recruit (SR) models.
• Eighteen model-data combinations were tested, covering six alternative data sets and three alternative SR model forms. Model fits and Spawner level that Maximizes Sustainable Yield (S_MSY) estimates for Taku River Coho Salmon are remarkably consistent across all three alternative model forms and four variations of adult data, with medians ranging from 59,000 to 81,000 fish, so that the largest difference between any 2 of the 12 alternative S_MSY estimates is less than 40%. Model fits using juvenile data were much poorer than the adult fits, and produced S_MSY estimates that were much higher and much more uncertain (i.e. wider posterior distributions).
• Based on statistical and practical considerations, the DFO’s regional peer review process chose one of the 18 model-data combinations as the most appropriate basis for advice regarding management goals for Taku River Coho Salmon: the Ricker AR1 model, which corrects for observed time-series patterns in residuals, fitted to estimates of total spawners and total adult recruits based on the age composition in the Canyon Island survey.  For this model-data combination, biological benchmarks based on data for the 1987-2009 brood years are an estimated spawner level supporting maximum sustainable yield (S_MSY) of 69,000 fish [59,000-89,000 fish]; a spawner level that maximizes adult recruits (SMAX) of 107,000 fish [82,000-154,000 fish]; and an equilibrium spawner level in the absence of fishing (S_EQ) of 183,000 fish [158,000-226,000 fish]. Values listed are the median and, in square brackets, the 10th percentile and 90th percentiles of the posterior distributions.
• Across all 18 model-data combinations, estimates of S_MSY are substantially higher than the spawning goal range of 27,500 to 35,000 fish used under the PST through 2012. However, the interim goal of 70,000 fish, adopted by the Transboundary Panel for 2014, falls in the middle of the range of S_MSY estimates across four adult data sets and three alternative models (59,000 to 81,000 spawners), and is essentially equal to the median S_MSY for the recommended model-data combination (69,000 spawners).
• Taku River Coho Salmon are currently delineated as a single CU under Canada’s WSP (DFO 2005). Integrated WSP CU status assessments have used a suite of standardized metrics. For the recommended data-model combination, the Upper and Lower Benchmarks (BM) for the Relative Abundance metric are 71,000 spawners (90% probability of meeting or exceeding 80% of S_MSY) and 23,000 spawners (90% probability of rebuilding to S_MSY in one generation in the absence of fishing).
• Available data are sufficient to assess the other formal WSP metrics (i.e. extent of decline, short-term trend, probability of decline), but some of the supplementary information used in recent WSP status assessments is not available for Taku River Coho Salmon at this time (e.g. changing relative contribution of sub-populations). If future investigations indicate more than one CU exists for the Taku aggregate, SR-based assessment of WSP status may not be possible and other status assessment approaches would have to be explored.

This Science Advisory Report is from the November 3-4, 2014 Estimates of a Biologically-Based Spawning Goal and Biological Benchmarks for the Canadian-origin Taku River Coho Stock Aggregate. 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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