Science Advisory Report 2018/016

Framework for Determination of Pacific Salmon Commission Reference Points for Status Determination and Associated Allowable Exploitation Rates for Select Canadian Southern Coho Salmon Management Units

Summary

• In order to fulfill obligations to the bilateral Canada/US Southern Coho Management Plan (Pacific Salmon Treaty, Annex IV, Chapter 5), DFO Fisheries Management branch requested that DFO Science Branch provide biologically-based benchmarks across a range of status categories to inform the determination of management reference points and associated allowable exploitation rate targets for all Canadian Southern BC Coho management units (MUs).
• Biological benchmarks demarcate categories of population status based on conservation and production considerations.  For example, biological benchmarks used to assess population status of Interior Fraser Coho conservation units were based on several of the conservation objectives outlined in appendix A.
• Management reference points (also called operational control points) incorporate biological, economic and policy considerations to demarcate population levels triggering specified management actions. In the current context, management reference points could be derived from this process to define Pacific Salmon Treaty status categories (low, moderate and abundant) and propose associated allowable exploitation rates to guide management of the Canadian Southern BC Coho MUs.
• To date, there is no commonly accepted method available to derive status benchmarks for MUs that lack reliable escapement data time series. Presently, Interior Fraser is the only Canadian Coho Salmon MU where extensive escapement data have been collected and biological abundance-based benchmarks have been established based on the work of the Interior Fraser Coho Recovery Team (IFCRT) and Wild Salmon Policy integrated biological status assessment processes. It is well documented that a considerable number of potential biases and uncertainties are associated with these data.
• This assessment focuses on evaluating the effects of exploitation rate changes on meeting conservation objectives across a range of smolt-to-adult survival rates that can be used to define status across three Pacific Salmon Treaty (PST) status categories: low, moderate, abundant. To do this, a range of stock-recruitment (S-R) models were fit using Bayesian estimation procedures that incorporated hatchery smolt-adult survival as a surrogate for population productivity. The resulting parameter estimates are then used to conduct both retrospective and forward simulations.
• Retrospective simulations were used to examine the historical pattern in escapement to each Interior Fraser Coho (IFC) conservation unit (CU) under different assumed exploitation histories and assumed S-R dynamics. Subsequently, forward simulations populated with estimates derived from the Bayesian S-R analyses were used to determine expected conservation performance (i.e., the mean probability of meeting previously established conservation benchmarks) over a 50-year time frame, and across a range of exploitation and smolt-to-adult survival rates.
• S-R parameters were derived for the individual IFC CUs, the IFC MU as a whole and the Strait of Georgia (SOG) MU, as represented by the Black Creek indicator stock.  No S-R analysis was conducted for the lower Fraser River (LFR) MU due to a lack of suitable data.  Retrospective and forward simulations were completed for the IFC MU, resulting in a set of decision tables that can be used to inform the selection of status benchmarks and management reference points using hatchery survival as an index of productivity. Due to lack of data, comparable analyses for the SOG and LFR MUs are not possible at this time.
• Examples are provided to illustrate how the results from this framework could be used to inform the development of management reference points for setting low, moderate and abundant status categories under the PST; formal recommendations on reference points to use for management purposes are not provided.  The selection of these reference points will require input from government, First Nations, and stakeholders on acceptable probabilities of achieving conservation outcomes given the uncertainties.
• Given the uncertainties associated with the S-R data, there are several model forms that might be used to represent true underlying Coho population dynamics.  In the context of this analysis, the choice of S-R model would influence the estimation of exploitation and smolt-to-adult survival rates, as well as the resulting assessments of future conservation performance.
• Inherent uncertainties and potential inter-annual biases in the available data ultimately leads to wide credible intervals around the estimated probabilities of achieving stated conservation objectives obtained through the forward simulations. For example, errors in estimation of realized exploitation rates (as a result of under- or unreported catch) and annual escapement estimates, along with necessary assumptions about underlying population dynamics all contribute considerable uncertainty and potential inter-annual biases to the estimates.
• The present simulation results are based on natural production only. If the current levels of CU-specific hatchery production were included in the model, hatchery fish spawning in the wild would contribute to the production of natural-origin fish, resulting in an increase to the modeled estimate of conservation performance.
• To capitalize on opportunities to update and improve this analysis as identified through this review (e.g, adding a freshwater survival covariate to the S-R models to account for sources of inter-annual and inter-population variation), periodic review and update of this assessment is recommended as new information and data becomes available.

This Science Advisory Report is from the September 20-21, 2017 regional peer review and November 6, 2017 editorial board follow-up meeting on the determination of reference points for status determination and associated allowable exploitation rates for Canadian PST Southern Coho Management Units. 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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