Benchmark workshop on mackerel and Norwegian spring-spawning herring (WKBMACNSSH)

WKBMACNSSH benchmark workshop on Mackerel (Scomber scombrus) in subareas 1–8 and 14 and Division 9.a (the Northeast Atlantic and adjacent waters) and Herring (Clupea harengus) in subareas 1, 2, 5 and divisions 4.a and 14.a, Norwegian spring-spawning herring (the Northeast Atlantic and Arctic Ocean) was conducted to refine input data, stock assessments and fisheries reference points. For mackerel, the benchmark group performed a comprehensive review and update of input data, including catch numbers and weight at age, as well as stock weights. The maturity ogive and natural mortality rates were revised, and RFID tagging data was reclassified as an abundance-at-age index. The egg survey data was investigated, though the DEPM estimates were deemed too short for reliable use at this point. The new assessment starts with catch data from 1998 and includes ages from 2 to 12+. The stock trends remain largely similar to previous assessment, but the revised stock estimates show a 60% increase in spawning stock biomass (SSB) and a 26% decrease in fishing mortality (Fbar) compared to previous assessments. These changes are largely due to revision of natural mortality parameters. The reference points were updated using EqSIM. For herring, the benchmark group incorporated an additional recruitment index and the RFID mark-recapture dataset, and refined survey data handling. The Norwegian spawning survey on NSS herring (NASF) was split into two time-series with different catchability and observation variance parameters. The MSY and precautionary approach reference points were revised, with Blim estimated using stock-recruitment pairs from 1988 to present. A custom-coded shortcut Management Strategy Evaluation (MSE) framework tailored to the biology of the stock, including density-dependent weight and maturity, was used for estimating FMSY. Advice based on the existing management plan for the Norwegian Spring-Spawning Herring is still considered precautionary, but the benchmark group strongly encourage conducting an MSE for this herring stock, including considerations on whether further investigations of accounting for implementation variability would be useful, as the existing management plan was calculated in the absence of overfishing, and therefore any fishing above advice based on this would be unprecautionary in ICES terms. Priorities for future work involve investigations on recruitment variability in forecasts for both stocks. For mackerel, focus should also be put on understanding the assessment model’s overfitting to catch data, and natural mortality estimates should be revisited in connection with an MSE. Continued research on appropriate methods for egg production for estimating an index from the egg survey should also be prioritized. For herring, future work should include developing the methodology for specifying and including density-dependent maturity and weight-at-age.