Workshop on Supporting the EU Action Plan to restore marine ecosystems; harbour porpoise in the Baltic Proper (WKSUP)

In the context of the ongoing management effort to ensure the conservation of the Critically Endangered harbour porpoise (Phocoena phocoena) in the Baltic Proper, The Workshop on SUPporting the EU Action Plan to restore marine ecosystems; harbour porpoise in the Baltic Proper (WKSUP) aimed to spatially identify higher risk métiers and develop a bycatch risk assessment. This was a follow-up of an earlier ICES process from 2020 concerning the introduction of emergency measures to mitigate bycatch of harbour porpoises in the Baltic Proper.

The first step was to identify fishing métiers with a higher bycatch risk for Baltic Proper harbour porpoises. In the Baltic Sea, the fishing is dominated by bottom-set gillnets and trammel nets (GNS, GTR) in terms of vessel numbers. As harbour porpoises are much more often taken in static nets (GNS, GTR) compared to the other gears (a difference of a order of magnitude), WKSUP focused on these métiers. Fishing effort data were aggregated by year and quarter for the years 2021 and 2022, grouping the gears into Gillnets and Trammel nets and large, medium and small mesh sizes nets. To follow a precautionary approach, the maximum fishing days per ICES statis-tical rectangle by year and by quarter from both years was chosen as the fishing effort metric to be used in this study of bycatch risk.

Data on harbour porpoise occurrence came from the SAMBAH study, which used passive acoustic monitoring at approximately 300 monitoring stations between 2011-2013, the only finalized Baltic porpoise regional abundance survey to date.

WKSUP adapted the Bycatch Risk Assessment (ByRA) method, to assess bycatch risks for the Baltic Proper harbour porpoise. ByRA identified risk hotspots across the region, particularly between the 13th and 17th degree East in Danish, German, and Polish waters, and extending north near Öland, Gotland, and the Swedish East coast, varying by season. Notably, the highest levels of relative risk are distributed evenly along the Baltic Proper coast but vary between quarters. Towards winter, hotspots are detected further south between October and March.

Compared to the context of the earlier ICES advice on this matter (ICES, 2020), the main difference in terms of scientific knowledge was that the uncertainty on the genetic status of the Baltic harbour porpoise population is now resolved. In a comparative study on this species looking at ~6 million high quality Single Nucleotide Polymorphisms (SNPs) from eight locations across the North Atlantic and adjacent waters, Celemín and colleagues (2023), indubitably identified the critically endangered Proper Baltic Sea porpoises as a separate population. Hence, given the critically endangered status of the Baltic harbour porpoise, WKSUP reiterated the need for EU Member States to fully implement existing regulations and consider extending seasonal closures of static net fishing in high-risk areas. While no new data on harbour porpoise abundance will be available until 2027, immediate bycatch mitigation measures are crucial.

Unless substantial improvement of effort data of fishing vessels of all sizes can be reached, bycatch risk can be estimated only with very coarse spatial resolution, which limits the precision to inform management measures. A more accurate bycatch risk assessment as requested by the EU Commission will depend on improved fishing effort data, including gear dimensions, soaking times, and finer scale spatial and temporal resolution of fishing positions. In this context, it would be very helpful if these fishing effort parameters were given special consideration in the implementation of the latest relevant regulations.

Regular Baltic-wide surveys of harbour porpoise abundance, like the SAMBAH project, are essential for adaptive management, ensuring measures are applied where and when needed, thereby reducing unnecessary burdens on fishers and managers.