Report of the Workshop on management strategy evaluation for the mackerel in subareas 1–7 and 14, and in divisions 8.a–e and 9.a (Northeast Atlantic) (WKMACMSE)

The workshop on management strategy evaluation for the mackerel in subareas 1–7 and 14, and in divisions 8.a–e and 9.a (Northeast Atlantic), WKMACMSE, chaired by Carmen Fernández, was convened to prepare the technical basis needed by ICES to respond to the request from Norway, the EU and the Faroe Islands concerning a long-term management strategy for mackerel. It met in Copenhagen, Denmark, during April 28–29 2017, and was attended by 20 participants, including two reviewers. The workshop addressed its terms of reference, with the following main outcomes:
The fishing mortality reference points were evaluated according to the ICES guide-lines and this resulted in their values being updated. The reference points were calculated using the same simulation tool used for the mackerel management strategy evaluation; this was to ensure there is consistency between the derivation of the fishing mortality reference points and the evaluation of harvest control rules that may form the basis of a future management strategy for this stock. 

The set of harvest control rules indicated in the request, based on (F_target, B_trigger) combinations, was evaluated. The evaluations included scenarios without and with densi-ty-dependence in the weights of mackerel, as this was part of the request. The workshop concluded that the scenario based on the weights from the last five years (2011–2015), which are low, should be the base case from which most conclusions for the advice should be drawn. This is also in line with the approach followed during the previous ICES evaluation of a management strategy for mackerel, in 2014. 

The results indicate increasing risk (meaning P(SSBlim) when going from the short term to the long term, which is expected given the currently high stock size. For any given level of risk, there are multiple (F_target, B_trigger) combinations that achieve that risk, with higher values of F_target associated with higher B_trigger values and vice versa. The workshop identified the precautionary combinations, defined as those that result in no more than 5% risk. Detailed results, including metrics for stock size, fishing mortality, yield, and interannual variability, are presented in the report, for rules without and with a constraint on interannual change in TAC. 

The scenario based on density-dependent weights was considered by the workshop as an exploration, indicating the likely direction of the effects that density-dependence could have on the performance of HCRs, rather than as a definitive modelling of the situation. The results indicate that there is likely some loss in yield if the stock is managed based on an HCR conditioned on the recent low weights but the actual weights are density-dependent. The loss in yield, in the medium and long terms, does however appear to be relatively minor. If, on the contrary, the weights stay at the recent low values but the stock is managed based on an HCR conditioned on an assumption of density-dependence, then the probability of the stock going below B_lim will be greater than 5% and the harvest control rule will not be precautionary. 

The workshop spent considerable time trying to elucidate if applying the manage-ment strategy evaluation (MSE) framework developed for the mackerel request was appropriate for this MSE. After substantial discussion, it was concluded that it could be used as the basis for the MSE. However, it was also concluded that more work is needed in ICES to investigate the most appropriate way to handle stocks assessed with the model SAM (used for mackerel) in the context of MSE. This refers in particular to the handling of process error, a feature of SAM, in MSE contexts. The extent to which this is an important issue may well depend on the stock, but it does seem im-portant for mackerel, given the large magnitude of the process error that was esti-mated for some ages and years in the assessment. The workshop also concluded that the processes followed in ICES to conduct management strategy evaluations should be reviewed and improved.