Inter-Benchmark process on Western Baltic cod

The assessment of Western Baltic cod had serious issues, with very large retrospective patternobserved during the assessment of WGBFAS in 2021. This led the group to not accept the assessmentand triggered an inter-benchmark process. Therefore, the overall objective of the WesternBaltic cod Inter-benchmark process (IBPWEB) was to resolve the issues with the assessment ofthe stock. Online meetings were held in June 2021 and had two external reviewers.Three different avenues were investigated during the process that aimed to resolve the assessmentissues: i) natural mortality and maturity, ii) survey indices, iii) assessment model options.The natural mortality was updated based on a meta-analysis method and contemporary markrecapturedata from SD22. The maturity ogives that showed large changes in recent years weredeemed not trustworthy by the group who decided to use a constant maturity ogive equal to the1998-2021 average. The survey index model was updated: a Delta-Lognormal GAM with timeinvariantspatial effect, no ship effects and last age group 4+. Most important changes in theassessment model, was the use of independent F processes between age groups instead of thepreviously used dependent ones, and the downscaling of the last data-year’s catch estimate to1/10, as there were indications of age group sampling issues potentially due to Covid disruptions.Finally, the reference points for the stock were updated and accepted. The external reviewersagreed and accepted all proposed changes.