Natural mortality-at-age vectors from the hake predation model to be used in the Reference Set Operating models for the OMP-2022 revision

The 2021 Reference Case (RC) model uses mortality-at age vectors from the 2018 version of the hake predation model (Ross-Gillespie and Butterworth, 2018). This predation model has since been updated, and corresponding updated mortality-at-age vectors needed to be used in the Operating Models for the 2022 OMP revision. Use of these updated vectors resulted in a very low estimate of 𝐵𝑀𝑆𝑌/𝐾𝑠𝑝 of 0.10 for M. paradoxus, but it was found that a simple increase of the basal mortality rate for older fish from 0.20 to 0.25 was sufficient to increase this 𝐵𝑀𝑆𝑌/𝐾𝑠𝑝 estimate to 0.17. This document provides the details of the mortality-at-age vectors that are being used as the base case assumption for the OMs being for testing OMP-2022. It shows the impact of using these vectors in the RC model, both on the assessment results and on OMP-2018 projections. The updated RC has an improved negative log-likelihood score, higher absolute values for the M. paradoxus spawning biomass and slightly lower estimates of 𝐵𝑠𝑝/𝐵𝑀𝑆𝑌 trajectories for M. paradoxus, although this ratio is still well above one at present. The projections of the two models under use of the OMP-2018 rules to set future TACs are not very different, apart from the Update with the new mortality-at age vectors resulting in slightly wider probability envelopes. An earlier version of this document was presented to the hake Task Team, and as there was no obvious reason to reject the proposed new BC mortality at-age vectors, these have been accepted for use for the conditioning of the Reference Set models and OMP-2022 development. Sensitivity to alternative mortality-at-age vectors will be considered in robustness tests.