Workshop on guidelines and methods for the design and evaluation of rebuilding plans for category 1-2 stocks (WKREBUILD2)

A first workshop on guidelines and methods for the design and evaluation of rebuilding plans for category 1-2 stocks, WKREBUILD, took place in 2020. WKREBUILD generated a guidance on best practices for evaluation of rebuilding plans against potential criteria of acceptability. However, it did not propose specific rebuilding plans or harvest control rules (HCRs). Instead, the workshop re-commended that a follow-up workshop be organized for testing the guidelines with actual test cases, with the aim of defining more specific criteria and guidelines. Thus, the main objective of WKREBUILD2 was to propose performance indicators and thresholds for the implementation and acceptability of rebuilding plans and a framework for the integration of rebuilding advice rules within the ICES advice framework.

To facilitate the evaluation of rebuilding plans a simulation tool was developed. The tool is based on FLR libraries and the application to specific cases is facilitated through a template consistent with TAF (Transparent Assessment Framework) and ensures an easy integration of the analysis into the ICES TAF system. The tool was tested in three specific case studies, Celtic Sea Whiting, Western Horse Mackerel and Western Baltic Herring. The application to the case studies served to validate the tool and to provide the science base to propose operational performance thresholds and criteria for the evaluation and implementation of rebuilding strategies in ICES.

WKREBUILD2 proposes to use B_PA as the entry point to the rebuilding phase and MSY B_trigger as the exit point. Once a stock is estimated to be below B_PA in the last assessment year, stock specific rebuilding strategies should be tested by means of simulations, preferably using the WKREBUILD2 simulation tool as the performance of HCRs are dependent on depletion rate and life history. The harvest control rule that fulfils the rebuilding criteria could then be selected to provide headline catch advice. Two criteria to be simultaneously fulfilled are proposed for the selection of the HCR: 1) the time of recovery (i.e., time to rebuild the stock) should not exceed twice the time required to recover the stock in the absence of fishing; 2) cumulative catches are maximised over a common rebuilding period. A stock is considered to be rebuilt when it has been above MSY B_trigger for at least three consecutive years.

Future work should be focused on defining suitable MSY B_trigger reference points that are linked to a target biomass level as for example B_MSY and not to B_PA, replacing the three consecutive year criteria by a time criteria based on the biological characteristics of the stock as for example generation time or time to sexual maturity, revising the proposed guidelines based on the implementation of rebuilding strategies in practice and considering augmenting the rebuilding criteria with indicators of the demographic structure of the stock. The acceptable time to recovery (twice the time needed in the absence of fishing) could be shortened and the maximum cumulative catch criteria could be replaced according to management objectives agreed by relevant management parties. Recommendations for the improvement and further development of the simulation tool were made and they are already being implemented.