Inter-Benchmark Process on Baltic Sprat (Sprattus sprattus) and Herring (Clupea harengus)

The Inter-Benchmark Process (IBP) on BAltic Sprat (Sprattus sprattus) and Herring (Clupea harengus) (IBPBASH) was carried out by three WebEx meetings in March 2020. The process had two purposes;

1. to evaluate the appropriateness of the use of the natural mortality estimates derived from the most recent (2019) multispecies SMS keyrun for the Baltic in the stock assessments for herring and sprat;

2. to re-examine and update MSY and PA reference points for both stocks according to ICES guidelines.

It was found appropriate to use unsmoothed values of natural mortality from the SMS keyrun in the assessments for both stocks. Future M values should be predicted using a model which includes cod spawning stock biomass.

Both MSY and PA reference points were re-estimated for both stocks. For sprat, the biomass reference points were unchanged, while all fishing mortality reference points increased. For herring, the biomass reference points were lowered by about 25%. FMSY and the corresponding range were practically unchanged, while F_lim and F_pa increased slightly. The regime shift in the Baltic around 1990 is a challenge when deciding which part of the time-series to be used, this applies to both stocks.