Maritime traffic alters distribution of the harbour porpoise in the North Sea

The North Sea is one of the most industrialised marine regions, but impacts of maritime traffic on marine mammals has yet to be understood. We analysed 81,511 km of aerial surveys (2015–2022), during which 8 597 harbour porpoises were sighted, with environmental data and AIS ship positions to assess their effects on harbour porpoise distribution. Porpoises avoided high-traffic areas, with disturbance extending up to 9 km. The number of ships and proximity of maritime traffic over 2 hours were better predictors than modelled noise levels. Reasons for this are unclear, but modelled sound levels may be less accurate over large areas due to gaps in AIS data (e. g. type of vessel). These short-term effects highlight the need to consider maritime traffic in marine spatial planning, especially as offshore wind service traffic grows.