Terms of Reference

Development of a Biological Escapement Goal for Taku River Sockeye Salmon

Regional Peer Review – Pacific Region

November 5-6, 2019
Nanaimo, BC

Chairperson: John Candy

Context

The Taku River is a transboundary river system that originates in the Stikine plateau of northwestern British Columbia and terminates in Taku Inlet in Southeast Alaska. The river produces one of the largest runs of Sockeye Salmon in the region; this is jointly managed by the Department of Fisheries and Oceans Canada (DFO), the Alaska Department of Fish and Game (ADF&G) and the Taku River Tlingit First Nation (TRTFN). The Canada/U.S. Pacific Salmon Treaty (PST) of 1985, and subsequent additions to the original treaty, established conservation and harvest sharing objectives for the Taku River Sockeye Salmon run. Based on current assessment methodologies, in-river run size has averaged 128,000 over the past decade.  Terminal run size, which includes marine harvest (U.S. District 111), has averaged 190,000. Average harvest rate is 44%, which results in average spawning escapements of 106,000 fish. Sockeye spawn throughout the Taku River drainage in both river and lake habitats.

Taku River Sockeye Salmon are managed as an aggregate under provisions of Chapter 1, Annex IV of the PST. The spawning objective has been considered an “interim” objective since it was established in 1985.  This is because it was based on the very limited harvest and escapement data available at the time. The parameters of this interim objective were a range from 71,000 to 80,000 fish with a point goal of 75,000 fish. A revised interim objective was set for the 2019 season, specifically a range of 55,000 to 62,000 fish with a management target of 59,000 fish. The most recent provisions of Chapter 1, Annex IV call for the development of a bilaterally agreed maximum sustainable yield (MSY) escapement goal prior to the 2020 fishing season. Paragraph 3(b)(i) states:

“(B) The Parties shall develop a joint technical report and submit it through the Parties’ respective review mechanisms with the aim of establishing a bilaterally approved maximum sustainable yield (MSY) goal for Taku River Sockeye Salmon prior to the 2020 fishing season...”

The Transboundary Panel requires escapement goal recommendations at the aggregate level to support the management and stock assessment regime that has been developed by the Parties through the joint Transboundary Technical Committee.

In-river abundance has been estimated annually since 1984. The methodology used to date has been mark-recapture, conducted jointly by DFO, ADF&G and the TRTFN. Marks are applied at Canyon Island, Alaska and recoveries are made upstream in the Canadian commercial fishery and, to a lesser extent, test fisheries directed at other species as well as First Nation fisheries. The mark-recapture methodology and past estimates are currently under review, and the results of this review will form the basis for development of a MSY-based escapement goal for the stock aggregate. Review of stock status and escapement goal development are directives of policies in both Canada (Precautionary Approach, Wild Salmon Policy [Fisheries and Oceans Canada 2009, 2005]) and the U.S. (Alaska’s Sustainable Salmon Fisheries Policy, and Policy for Statewide Escapement Goals [Alaska Board of Fisheries 2000, 2001]).

DFO Treaties and Fisheries Unit, Yukon-Transboundary Rivers Area (YTRA) has requested that Science Branch provide advice respecting a biologically-based escapement goal for Taku River Sockeye Salmon and the establishment of biological benchmarks that are consistent with DFO’s Precautionary Approach (Fisheries and Oceans Canada 2009). This assessment will present an analysis and advice respecting of both MSY-based escapement goal and biological benchmarks. Methods utilized for biological benchmark assessment will reference approaches and criteria previously developed (see Holt 2009a, Holt 2009b, Grant 2011).  Advice respecting biological benchmarks will contribute to a future assessment of status to meet Precautionary Approach commitments.

The advice arising from this Canadian Science Advisory Secretariat (CSAS) Regional Peer Review (RPR) will be used to inform fisheries management decisions and meet treaty/international obligations.

Objective

The following working paper will be reviewed and provide the basis for discussion and advice on the specific objectives outlined below.

Pestal, G. and S. Miller. Development of a Biological Escapement Goal for Taku River Sockeye Salmon. CSAP Working Paper 2018SAL04

The specific objectives of this review are to:

1. Identify the spawning escapements that would produce maximum sustainable yields for the Taku River Sockeye stock aggregate.
2. Identify the appropriate biological benchmarks for the management of the Taku River Sockeye stock aggregate.

The work will be conducted by a working group led by DFO and ADF&G personnel with expertise in the stock assessment and management of Taku River Sockeye Salmon.  The working group will analyze Taku River Sockeye Salmon spawner-recruit data using a Bayesian age-structured state-space model. State-space models are time series models that feature both observed variables and unobserved states. Use of a Bayesian age-structured state-space model will allow for consideration of process variation (natural fluctuations) in stock productivity, recruitment, and age-at-maturation independently from observation error (uncertainty in measurements of observed data) in run size, harvest, and age composition. By correctly specifying annual age-structure in the Bayesian state-space model, missing data, common to salmon stock assessment data sets, can be represented as unknown quantities for which posterior samples are generated. Additional uncertainty then flows through to the remaining model parameters as appropriate (Fleischman et al. 2013). This provides a powerful advantage over traditional spawner-recruit analysis, in which independence of individual quantities of spawners (S) and recruits (R) is assumed and missing data must be imputed before the model is run. Another advantage of the Bayesian age-structured state space model over traditional spawner-recruit methods is on obtaining good quality estimates of spawning abundance at maximum sustained yield in regards to bias reduction and interval coverage. As a result, Bayesian age-structured state-space models have been used with increasing frequency in place of traditional methods in spawner-recruit analysis of Pacific salmon. Other methods may be examined in cooperation between DFO / ADFG staff and consultants.

Data inputs for the state-space model relate directly to the stock assessment program, and the corresponding review of that program may provide insights into how to use and/or adjust inputs to the escapement goal analysis. Data that will be required for the escapement goal review include directed marine harvests by age, in-river harvest by age for commercial, indigenous, test, and personal use fisheries, and escapement estimates by age. Other data may be considered and incorporated into the model. The R code for fully reproducible estimates of biological benchmarks will be incorporated as an appendix in the working paper developed.

Expected Publications

• Science Advisory Report
• Proceedings
• Research Document

Expected Participation

• Fisheries and Oceans Canada (DFO) - Treaties and Fisheries, YTRA
• Alaska Department of Fish and Game (ADFG) – Commercial Fisheries Division
• Academia or Academics  - Simon Fraser University
• Indigenous communities/organizations - Taku River Tlingit First Nation

References

Alaska Board of Fisheries. 2000. Policy for the Management of Sustainable Salmon Fisheries. 5 AAC 39.222.

Alaska Board of Fisheries. 2001. Policy for Statewide Salmon Escapement Goals. 5 AAC 39.223.

Fisheries and Oceans Canada. 2005. Canada's Policy for Conservation of Wild Pacific Salmon. Fisheries and Oceans Canada. Vancouver, BC. 34 pp.

Fisheries and Oceans Canada. 2009. A fishery decision-making framework incorporating the precautionary approach.

Fleischman, S. J., and T. R. McKinley. 2013.  Run reconstruction, spawner-recruit analysis, and escapement goal recommendation for late-run Chinook salmon in the Kenai River.  Alaska Department of Fish and Game, Fishery Manuscript Series No. 13-02, Anchorage.

Grant, S.C.H., MacDonald, B.L., Cone, T.E., Holt, C.A., Cass, A., Porszt, E.J., Hume, J.M.B., and Pon, L.B. 2011. Evaluation of uncertainty in Fraser Sockeye (Oncorhynchus nerka) Wild Salmon Policy status using abundance and trends in abundance metrics. DFO. Can. Sci. Advis. Sec. Res. Doc. 2011/087. viii + 183 pp. 

Holt, C.A. 2009a. Evaluation of benchmarks for Conservation Units in Canada's Wild Salmon Policy: technical documentation. DFO. Can. Sci. Advis. Sec. Res. Doc. 2009/059. xii + 50 pp.

Holt, C.A., Cass, A., Holtby, B., and Riddell, B. 2009b. Indicators of status and benchmarks for Conservation Units in Canada's Wild Salmon Policy. DFO. Can. Sci. Advis. Sec. Res. Doc. 2009/058. vii + 74 pp.