Finescale distribution and seasonality of harbour porpoises (Phocoena phocoena) investigated with citizen science

Information on the distribution of harbour porpoises (Phocoena phocoena) in the Western Baltic has been obtained by passive acoustic monitoring, visual surveying and acoustic surveying. Here, we collected complementary data on the presence of porpoises around the Danish Island of Funen using citizen science. Porpoise sightings were reported with the custom‐made mobile app ‘Marine Tracker’, disseminated to the public through community meetings, newspaper articles, radio interviews and posters. App users could choose between three types of reports: ‘single porpoise’, ‘multiple porpoises’ and ‘mother & calf porpoises’. A total of 7,755 credible porpoise sightings were reported from Funen between 2019 and 2022. These observations were primarily concentrated around larger harbour towns, but also in less populated parts of southern Funen, which previously had not been recognised as a significant porpoise habitat. A high proportion of ‘mother & calf’ observations in this area may indicate its importance for future conservation efforts. Citizen science data provided insights into the spatial and temporal distribution of porpoises, particularly the timing and spatial distribution of ‘mother & calf’ pairs during summer months. Our study highlights the possibilities of citizen science as a tool to complement traditional surveying techniques to understand marine mammal distribution.