Workshop on Management Strategy Evaluation of Mackerel (WKMSEMAC)

WKMSEMAC (Workshop on Management Strategy Evaluation of Mackerel) took place during the period January to July 2020 with a kick-off meeting at ICES HQ, Copenhagen from 7 to 9 January followed by a number of WebEx meetings. A second scheduled physical workshop was cancelled following the COVID-19 pandemic, necessitating completion of the work by correspondence. The workshop was chaired by Andrew Campbell (Ireland) and was attended by 26 participants including three reviewers. The purpose of the workshop was to evaluate a long-term management strategy for northeast Atlantic mackerel, following a request from the EU, Norway, and the Faroe Islands (EU, Norway, and the Faroe Islands request on the long-term management strategies for Northeast Atlantic mackerel - full feedback approach).

In line with the request, the approach adopted was to include an assessment and forecast in a full-feedback simulation. This approach was computationally challenging such that the simulations had to be executed in a high performance computing environment. In comparison with earlier iterations of this evaluation (which adopted a shortcut approach), this requirement lim-ited the number of scenarios that could be evaluated and led to significant resource pressure. However, sufficient scenarios were explored to identify a range of harvest rule parameters (F_target and B_trigger) that are both precautionary in the long term (risk to B_lim of < 5%) and maximize long term yield for management strategies both with and without stabilization measures. All scenarios considered are precautionary in the short term, due to the current high stock size.

Strategies incorporating stabilization measures were evaluated for a subset of the base case (without stabilization). Measures included TAC change limitation, 5% constant banking, and an extreme bank and borrow scenario (10% alternating bank/borrow), and were tested both in isolation and in combination. In terms of long term yield and risk, none of the stabilization measures had a notable impact compared to the base case. Limiting the interannual change in TAC mitigated extreme changes in TAC, particularly for HCRs with relatively low Btrigger values. Median IAV in TAC is only marginally reduced compared to the base case for which the IAV is generally lower than the limitation imposed by the stabilization measure.

Simulation of future recruitment within the HCR evaluations is based on assessment estimates of recruitment from the contemporary period (1998 onwards) which are strongly influenced by the recruitment index. These are considered to be reflective of the current situation and result in an increased perception of stock productivity compared to previous MSE evaluations. An evaluation with an alternative operating model based on recruitment estimated from the abundance of older (fully selected) fish indicated that although the maximum yield is associated with dif-ferent HCR parameter values, all combinations considered precautionary under the base case remained so under this alternative.Due to time constraints it was not possible to fully explore robustness to alternative operating models with respect to natural mortality and density dependent weight as included in the re-quest. The results are contingent on both recent data and assessment performance such that should the perception of recruitment, biological characteristics, or exploitation change, re-evaluation of the management strategies tested should be considered.

Reference points were reviewed and updated values proposed based on evaluation with the MSE framework software.