EU request on support for the implementation of the Action Plan – harbour porpoise in the Baltic Sea (Baltic Proper)

Due to lack of information from the Baltic Proper, ICES drew on evidence on fishing métiers most likely to cause bycatch of the Baltic Proper harbour porpoise from other areas in the Northeast Atlantic (including the Belt Sea). Métiers with a substantial risk of bycatch identified were set gillnets and trammel nets.

By using data from all sizes of vessels, ICES mapped the fishing effort by quarter of set gillnets and trammel nets for 2021 and 2022. Fishing effort is mapped at the available spatial resolution of an ICES rectangle (0.5 × 1 degrees), equalling an area of approximatively 60 × 60 km.

An existing method for bycatch risk assessment was used as a proxy to estimate the relative bycatch risk for Baltic Proper harbour porpoise. The relative bycatch risk was determined from the temporal and spatial overlap between the harbour porpoise detection probability data, which served as a proxy for habitat suitability, and the fishing effort data.

The relative bycatch risk of harbour porpoise, based on relative ranks, is mapped for each quarter of the year for ICES rectangles in the Baltic Sea. High bycatch risk is driven by high harbour porpoise habitat suitability and consistently high fishing effort, specifically set gillnets with medium and large mesh sizes.

ICES recommends improvements in the monitoring of the population and fishing effort data, in the case of the Baltic Proper harbour porpoise. However, a sustainable population could not be achieved without the implementation of the management measures previously advised (ICES, 2020a).