Pacific Salmon Foundation: Salmon Watersheds Program

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Estimates of a Biologically-Based Spawning Goal and Biological Benchmarks for the Canadian-Origin Taku River Coho Stock Aggregate

author Fisheries and Oceans Canada
published year 2015
species coho
location Taku River
subjects Transboundary River, stock assessment, biological benchmarks
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Taku River Coho Salmon (Oncorhynchus kisutch) are a transboundary stock managed cooperatively by Canada and the United States under the Pacific Salmon Treaty (PST). The Transboundary Chapter of Annex IV of the treaty requires Parties to “develop a joint technical report and submit it through the various Parties’ review mechanisms with the aim of identifying and establishing a bilaterally agreed to MSY goal for Taku coho”. It was agreed by the Transboundary Panel of the PST that Canada would take the lead on addressing this obligation, and this assessment is the first milestone in this bilateral process. The purpose of this assessment is to estimate biological benchmarks, including the spawner abundance that maximizes sustainable yield over the long-term in average conditions (SMSY) for Taku River Coho Salmon stock aggregate, which can then be used to evaluate status and set management goals.

Eighteen model-data combinations were tested, covering six alternative data sets and three alternative SR model forms. Model fits and SMSY 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 SMSY estimates is less than 40%. Model fits using juvenile data were much poorer than the adult fits, and produced SMSY estimates that were much higher and much more uncertain (i.e. wider posterior distributions).

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.