Science Advisory Report 2018/033

Units 1+2 Redfish Management Strategy Evaluation

Summary

• A Management Strategy Evaluation (MSE) was initiated for Unit 1+2 Redfish in December 2016 by DFO Resource Management and Science in collaboration with the Unit 1+2 Redfish Rebuilding Plan Working Group (the WG). The WG developed and refined a final list of 7 objectives, 12 performance metrics and 18 operating models (OMs) representing key areas of uncertainty.
• Five candidate Management Procedures (MPs) of an initial 21 were selected for further consideration by the WG in March 2018. All candidate MPs used the same Harvest Control Rule (HCR), but differed in the year in which the HCR was first implemented, limits on the magnitude of interannual changes in total allowable catch (TAC) and with respect to the presence or absence of: maximum TAC caps, adjustment of HCR catch limits (TACs) by a factor of 0.8 and the use of fixed TACs in early years.
• The spawning stock biomass (SSB) of the two redfish populations in Units 1+2 is growing. Presently, modelled estimates of SSB from the base OM suggest that the Sebastes mentella and S. fasciatus populations are respectively in the Healthy and Cautious zones of the Precautionary Approach, based on reference points estimated as part of the MSE.
• The strong 2011-2013 cohorts resulted in small differences in performance of candidate MPs in the simulations. However, under base OM conditions, there was a trade-off between average annual catches retained 10-20 years in the future and the duration of high TACs, interannual TAC stability, years during which the redfish Small Fish Protocol is met and the abundance of large fish in the catch.
• The presence or absence of maximum caps and HCR TAC adjustments mattered more to candidate MP performance than the year in which the HCR was first implemented. Projected median TACs in the next five years from most MP were sufficiently low as to constitute low conservation risk to the stocks.
• Stock conservation objectives that aim to reach in 10 years and then maintain the SSB of both species in the Healthy Zone were met with a high probability in all tested MPs. Under OMs assuming catch splits favouring S. mentella, only one MP failed to maintain sustainable exploitation rates for S. fasciatus with 50% probability.
• The abundant small fish associated with the 2011-2013 cohorts results in predicted catches from all candidate MPs will fail to meet the Small Fish Protocol in 2018 and 2019. Small fish (< 22 cm) are expected to remain abundant in the catch until 2020. Nonetheless, the MSE concluded that these predicted outcomes would not compromise the achievement of the other conservation objectives. However, adequate monitoring of small fish removals is required to ensure full accounting of fishing mortality for the stocks.
• Exploratory analysis indicates that the performance of MPs with respect to three conservation objectives was improved and total allowable catches were increased in simulations that assumed that the two species of redfish were perfectly distinguished in fishery catches, enabling species-specific TACs. The implementation of fishery sampling aimed at estimating the species composition of redfish catches should be a high monitoring priority.
• An implementation period of up to five years is recommended for this MSE, following which a retrospective analysis should take place in which the actual performance of the MP in the fishery are evaluated, and any updates to MP design, OMs, simulations, objectives, and exceptional circumstances are re-evaluated in a new MSE process.

This Science Advisory Report is from the April 25-26, 2018 Unit 1+2 Redfish Management Strategy Evaluation. 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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