In this paper, we use simulations based on operating models using the most recent (2023) toothfish stock assessments in the Ross Sea region, Subarea 48.3, Division 58.5.1 and Division 58.5.2 to evaluate a range of harvest control rules (HCRs) which were based on harvest rates U. For each HCR, we first determined the target harvest rates that would be consistent with the CCAMLR Decision Rules for toothfish and ensured a target level of 50% spawning stock biomass (B0), and then evaluated the robustness of these HRCs to a range of assumed future recruitment patterns. In contrast to the constant catch HCR which are used as part of the current CCAMLR Decision Rules, U-based HCRs do not rely on any assumptions about future recruitment patterns.
All evaluated constant or ramp U-based HCRs achieved the target level (50% B0) and avoided the depletion level (20% B0) when historical and future recruitment conditions were similar. Ramp rule, as opposed to constant harvest rate rules, were more precautionary under low recruitment conditions, at the cost of lower catches and higher catch variability.
When future recruitment was lower than the historical average, all evaluated HCRs resulted in long-term spawning stock status below the target level, but the ramp rules also had a much lower probability of the stock being at levels below 20% B0. These results are likely to be more negative than what would be expected in reality, since the biomass value at the reference points remained unchanged in the simulations over the entire 135-year projection period. In reality, B0 and related reference points would be re-estimated over time with each stock assessment, and therefore reflect any reduction in average recruitment.
We recommend that:
1. WG-SAM consider suitable U-based HCRs for use by CCAMLR to determine catch limits in assessed toothfish fisheries.
2. WG-SAM consider relevant performance indicators (PIs), and possible approaches for trade-offs, to evaluate the performance of HCRs.
3. WG-SAM consider what HCRs, conditions and questions should be investigated in a management strategy evaluation (MSE) to evaluate HCRs and their combined effects in decision rules, e.g.
- What types of HRCs and decision rules should be tested
- What historical and future population and productivity scenarios should be tested
- What type of stock assessment features and misspecifications should be investigated
- Should other constraints (e.g. on changes to catch limits) be evaluated
4. WG-SAM consider how U-based HCRs could be integrated into the CCAMLR decision rules.
5. WG-SAM develop a workplan to complete these tasks.