Research Document 2019/001
Evaluation framework for assessing potential Pacific Salmon Commission reference points for population status and associated allowable exploitation rates for Strait of Georgia and Fraser River Coho Salmon Management Units
By Korman, J., Sawada, J., Bradford, M.J.
Abstract
The work presented here continues the development and implementation of a bilaterally-reviewed methodology to establish reference points and associated allowable exploitation rates for Coho Salmon management units detailed in the Pacific Salmon Treaty (PST) Southern Coho Agreement (Annex IV, Chapter 5). Marine survival has been identified as a major factor influencing escapement. However, accurate escapement estimates for many Coho Salmon populations in British Columbia are limited. This project therefore focuses on evaluating effects of exploitation rate across a range of marine survival rates that can be used to define PST abundance classes. We fit a variety of stock-recruitment models with a hatchery-based smolt-adult marine survival covariate to data from the five conservation units (CUs) in the Interior Fraser Coho Salmon management unit (IFC MU) (brood years 1998-2012), and to Black Creek (1986-2012), the sole monitored population representing the Strait of Georgia (SOG) MU. We used a simulation framework based on posterior distributions of stock-recruit parameters to determine the probability of meeting previously established conservation benchmarks over a range of exploitation and smolt-adult marine survival rates.
Based on best fit Ricker models, productivity (adult recruits/spawner at low stock size) of IFC CU’s ranged from about 2.2-2.6 recruits/spawner at the geometric mean hatchery smolt-adult marine survival rate since 1998 (1.1%). Exploitation rates that maximized yield (Umsy) at these productivities ranged from 0.36 to 0.42. The range in Umsy declined to 0.27-0.32 based on a Ricker model with an assumed higher carrying capacity, which eliminated overcompensatory dynamics over the range of stock sizes, but reduced stock productivity. Productivity of Black Creek based on the average smolt-adult marine survival for the SOG hatchery indicator stock since 1998 (0.84%) was 2.3 recruits/spawner, resulting in a Umsy of 0.37, and these values declined to 2.0 and 0.32, when using a Ricker model with higher carrying capacity, respectively.
A retrospective simulation analysis of IFC stock-recruit data demonstrated that the majority of variation in escapement to IFC CUs since 1998 has been driven by variation in smolt-adult marine survival rate. Historical simulations indicated that increasing exploitation from 10% (geometric average since 1998 was 11%) to 30% decreased the probability of achieving the short-term MU conservation benchmark (20,000 spawners) from 71% to 43%. However a fixed exploitation rate of 20% resulted in a 50% probability of exceeding the short-term benchmark, which was similar to the observed probability when hatchery-origin fish are not included in the conservation statistic. The modest effects of slight increases in exploitation rate occurred because the increase in escapement associated with lower exploitation did not produce a proportional increase in recruitment in the next generation owing to density-dependence. Forward simulations quantified conservation performance over a wide range of exploitation and smolt-adult marine survival rates and included effects of harvest rate implementation error. This information can be used by decision-makers to set exploitation rates for the IFC MU for marine-survival based PST status categories. However, there is not sufficient information to make these determinations for SOG and lower Fraser River (LFR) MUs.
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