Benchmark workshop on Baltic pelagic stocks (WKBBALTPEL)

Three pelagic stocks in the Baltic Sea, Central Baltic Herring (CBH; her.27.25-2932), Sprat (spr.27.22-32) and Gulf of Riga Herring (GOR, her 27.28) were examined during this benchmark. Work was prepared well in advance and 3 preparatory online meetings took place between the Data Evaluation Workshop and the actual benchmark to follow up progress. The benchmark meeting advanced really well until reference points were discussed. There were follow up online meetings on 3rd and 13th March to further discuss, and agree, on reference points for the three stocks.

All three stocks that was benchmarked were previously assessed with XSA, and hence a main point with the benchmark was to change the assessment model. Two of the stocks changed to SAM (GOR and sprat) whereas the CBH changed to stock synthesis (SS3). For the GOR and sprat the reference points were estimated following the ICES procedure using EQSIM. For the CBH the reference points were obtained using a Management strategy evaluations (MSE), a method previously used for one of the Northern shrimps stocks in ICES (pra.27.3a4a).

There was an extensive issue list for the CBH and although not all of the issues were addressed, significant progress was made. Additional survey indices, maturity at age, weight in catch, adding age reading error, landings corrected for misreporting (Danish landings only) were all investigated and updated time series were included in the new assessment. SS3 was chosen as main assessment method and there was extensive model development work carried out in advance and presented at the meeting.

An ensemble model was developed to incorporate uncertainty in natural mortality. Three scenarios of natural mortalities were developed from rescaling the natural mortalities from a multispecies model for the Baltic (SMS) with life history parameters. An objective weighing approach was used based on model performance statistics diagnostics to develop the final assessment.

The reference points were estimated using Management Strategy Evaluations (MSE). Blim for this stock was set at 15% of B0. The group agreed that, according to the results of the MSE, a combination of an FMSY proxy at F35% combined with a Btrigger at 60% of B35% satisfied the criterion of being above Blim with 95% probability, achieves a realised target F smaller than FMSY and generates catches higher than MSY. It is important to note that several combination of F target and Btrigger achieve long-term catches equal to those under FMSY and result in a median B that is significantly larger than BMSY despite a lower F. However, these scenarios do not match with the ICES interpretation of MSY, for which the scenario with the highest F (which fulfils the MSE performance and ICES precautionary criteria) is selected.

This option results in an FMSY proxy of FB35% and a MSYBtrigger reference point of 60% of B35%.

At the meeting a roadmap towards the next benchmark for CBH was set up in order to deal with issues that were not covered at WKBBALTPEL including, stock identity, multiarea assessment, catch data corrected from misreporting from more countries.

For the Sprat in the Baltic, the issue list was almost as extensive as for the CBH. Similarly, to the CBH stock, significant progress was made during the Benchmark, the progress includes:

· The natural mortalities were updated with new M values from the updated SMS runs.

· Include the Danish data that was corrected for misreporting

· Additional survey indices

· Moving from an XSA assessment model to a SAM model.

The reference points were calculated using EQSIM. Blim was set to the biomass that produces 50% of maximal recruitment. There is one thing left on the issue list for the next benchmark also for this stock; to include catch data corrected for misreporting from countries other than Denmark.

For the GOR herring significant achievements were performed during the Benchmark meeting. The assessment model was changed from XSA to SAM, a trap net tuning series previously used in the assessment was investigated and excluded, maturity at age values were updated and the ages considered for Fbar were updated.

The reference points were calculated using EQSIM. There was no evident SR relationship, hence Blim was defined based on Bpa.