Joint request from the Faroe Islands, Greenland, Iceland, Norway and United Kingdom to ICES for evaluation of different rebuilding Harvest Control Rules for Atlantic mackerel

Just below B_lim, the fishing mortality from these HCRs is lower than the F from ICES AR. Therefore, these HCRs all contain a step change in the advice at B_lim in the HCR.</p><p dir="ltr">HCRb is identical to the ICES AR when SSB is above MSY B_trigger. Below MSY B_trigger, the advised F reduces linearly and more rapidly than in ICES AR.

Therefore, when the stock is below MSY B_trigger, HCRb always advises a lower F than ICES AR. When the HCR reaches F_min, advised F is kept constant and equal to F_min up to SSB = 0.</p><p dir="ltr">All tested HCRs led to spawning-stock biomass (SSB) increases in the short term (five years), but the speed and terminal level depended on the value of F_min used in the HCR. HCRs with higher F_min values resulted in less/slower rebuilding and less catch following initial big reductions in the first year.

HCRb, which applies a faster reduction of F when SSB falls below MSY B_trigger, leads to slightly healthier SSB in 10 years at the cost of slightly lower catches over this period. All HCR options led to a higher probability of being above B_lim compared to the simplified ICES AR (assuming the slope of the HCR is followed) because, in the specific simulations carried out, the SSB was always in the range where the targeted F from the evaluated HCRs is either the same or lower than ICES AR.</p><p dir="ltr">In response to the three objectives specified in the request, all HCRs avoid further reductions in the stock size in the short term with a high probability.

However, the ability to recover the stock above B_lim in the short to medium term with a high probability is limited, given the low recruitment level assumed in the simulations. Most HCRs get the stock above B_lim with 50% probability within two years, but none achieve this with 95% probability over the simulated period. Similarly, none achieve recovery above MSY B_trigger in 10 years.</p><p dir="ltr">Shortcut MSEs have some inherent issues due to the simplified assumptions in the observation of the population.

There are also uncertainties about modelling productivity realistically for this stock, which ultimately should be explored more thoroughly through a full reference set of operating models to find an HCR that is robust to this uncertainty. The results from these evaluations should not be considered predictions for the future. Rather, it is an intermediate step between a short-term forecast (as done by ICES to provide annual catch advice) and full MSE.

Results are sufficient to draw comparisons between alternative HCRs and can be used to guide decisions on interim measures until a full MSE can be conducted.</p>