Research Document 2020/069
Evaluation of potential rebuilding strategies for Outside Yelloweye Rockfish in British Columbia
By Cox, S.P., Doherty, B., Benson, A.J., Johnson, S.D.N., and Haggarty, D.R.
Abstract
This paper aims to provide advice on rebuilding Outside Yelloweye Rockfish (OYE) using a combination of stakeholder-manager-science consultations and closed-loop simulation modelling to test performance of a set of candidate management procedures (MPs) against specific quantitative objectives. The overall approach aims to expose the ecological and fishery consequences of specific analytical (e.g., data collection, assessment methods) and management choices (e.g., harvest control rules, target fishing mortality rates) for Yelloweye rebuilding. The key components of this work are: (i) development of two-stock hierarchical age-structured operating models for OYE that represent a range of hypotheses about natural mortality and exploitation history, (ii) testing MPs comprised of monitoring data, assessments, and harvest control rules (HCR) used to implement rebuilding policies, and (iii) evaluating performance measures that are used in determining the expected conservation performance of alternative MPs relative to stated rebuilding objectives.
Alternative data scenarios produced a wide range of estimated stock status, as well as biological and management parameters, from which 4 representative operating models (OMs) (using a 1960 or 1918 start year and alternative catch scenarios) were selected for simulation testing MPs. The 4 OMs ranged in current biomass from approximately 2,600 to 8,200 t in the North (groundfish management areas 5BCDE) and 1,900 to 4,400 t in the South (groundfish management areas 3CD5A). This range is considerably wider than the statistical uncertainty within any particular OM. No single factor clearly explains the range of biomasses because natural mortality, absolute catch levels, and historical recruitments all affect biomass and recruitment estimates either directly or indirectly. The 1960 start year generally has the higher unfished and current biomass, while the lower bound commercial catch scenario leads to the lower unfished and current biomass. None of the 4 OMs indicate that either OYE stock area has been fished to less than 20% of the unfished level or below 40% of BMSY (Biomass at Maximum Sustainable Yield), as inferred in previous assessments. Model estimates of spawning biomass depletion relative to unfished levels range from 29–51% in the North, 21–43% in the South, and 27–48% coastwide. These correspond to 111–185% of BMSY in the North, 75–154% in the South, and 96–173% coastwide.
The candidate MPs evaluated include three different assessment methods: i) a catch-at-age (CAA) assessment model, ii) a surplus production (SP) assessment model, and iii) an empirical rule (IDX) using survey index trends.
The three assessment methods were used in combination with different harvest control rules or implementation error scenarios to create a set of candidate MPs that were simulation tested for each of the 4 OMs for North and South areas independently. Performance statistics were evaluated using combined outputs across OMs via a 50%–16.67%–16.67%–16.67% weighting scheme. Simulations of MP performance for setting future OYE total allowable catches (TACs) generally showed robust, or potentially robust, performance to a wide range of OM scenarios. The CAA MPs were tuned to achieve a target fishing mortality rate that would provide relatively stable OYE biomass over the projection period and biomass in both the North and South areas. Management procedures based on SP models or survey index trends (IDX) produced a range of increases or stable trends in future OYE biomass. The IDX MPs were tuned to avoid biomass declines in the first 10 years, which produced long-term increases or stable trends in biomass with high inter-annual catch variability. Although the SP models generally led to biomass increases, they did so because of under-estimation biases and often showed erratic patterns in TACs. It is likely that undesirable properties of IDX and SP MPs could be improved via further tuning.
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