Benchmark Workshop on Herring (Clupea harengus) in the Gulf of Bothnia (WKCLuB 2021)

The assessment for the Gulf of Bothnia herring (SD 3031) in 2019 was not accepted by the Advice Drafting Group and was changed from category 1 to 3. The assessment was not accepted based on the poor retrospective diagnostics where the Mohn’s rho values were above 20% for SSB, F and recruitment. The aim for the Benchmark was to evaluate a new model, Stock Synthesis (SS3) as a candidate for the assessment of Gulf of Bothnia Herring SD30–31 in order to minimize the retrospective pattern previously observed.
Following the path of the Benchmark data related issues were revealed. This led to a situation that the benchmark was prolonged one year in order to correct the data related issues. Apart from a misspecification in the model about how the trapnet index was used (abundance index vs. biomass index), the data problem had been related to the acoustic survey. The acoustic survey index used in the assessment was thoroughly examined by the Baltic international fish survey working group (WGBIFS) in a meeting in December 2020. A number of model runs (six prior to meeting, and an additional 16 during the benchmark) were conducted for evaluation at this benchmark. The analysis presented extensive diagnostic tests including the standard ICES criterion related to retrospective patterns. This was considered an enhancement over using one method for accepting or rejecting an assessment. It was noted that the final retrospective pattern had low and acceptable values of Mohn’s rho.
In general, the benchmark using the stock synthesis platform with the settings specified during the benchmark are considered acceptable for assessment and advice and have features that should ensure stability as new data are added (e.g. selectivity is assumed to be constant over time).