High numbers of harbour seal pup in the Wadden Sea despite long-term population decline
The latest harbour seal surveys conducted across the Wadden Sea and on Helgoland reveal a long-term decline of seals counted during the moulting season in August, while the number of pups born in June are exceptional high this year. The 2025 survey recorded a total of 23,954 moulting harbour seals and 10,044 harbour seal pups. These are the findings of the “Survey results of Harbour Seals in the Wadden Sea in 2025”, published under the Trilateral Cooperation on the Protection of the Wadden Sea.
Researchers from the trilateral Expert Group on Marine Mammals – comprising specialists and site managers from Denmark, Germany, and the Netherlands – are faced with an ongoing puzzling trend: Overall, the harbour seal abundance has shown decline over the last decade, while pup production relative to the total number of seals continues to increase. “The numbers might indicate a higher proportion of breeding females and at the same time a higher pup mortality, leading to fewer recruits joining the adult population,” says Anders Galatius, the report’s lead author and senior researcher at the Department of Ecoscience of Aarhus University. “Another or an additional explanation may be a change in haul-out behaviour during the moult, leading to fewer animals on the sandbanks during the survey flights. These are only two hypotheses, and we need further studies to uncover the mechanisms driving this phenomenon.”
The total of 23,954 harbour seals counted in August 2025 represents a 1% increase compared to 2024. However, as with the 2022 and 2024 counts, this figure remains below all the counts from 2012 to 2021, indicating a stabilisation at lower overall abundance. Regional trends varied significantly: In Denmark, counts decreased by 20% from 2024 to 1,721 seals. In Schleswig-Holstein, they decreased by 8% to 7,806. In Lower Saxony and Hamburg, they increased by 9% to 7,042. In the Netherlands they increased by 10% to 7,285. At Helgoland, 100 seals were counted compared to 56 in 2024.
Following relatively low pup counts from 2022 to 2024, the 2025 surveys recorded the second-highest pup count as well as the highest proportion of pups to moulting seals ever observed. A total of 10,044 pups were counted – an increase of 22% compared to 2024. In Denmark, 727 pups were counted – a decrease of 4% relative to 2024. Pup numbers saw an increase of 25% both in Schleswig-Holstein (4,375) and in Lower Saxony and Hamburg (2,131). The highest increase – 44% – was noted in the Netherlands with 2,809 pups, where the count of 2024 was unusually low. Two pups were observed on Helgoland.
Harbour seals are one of the top marine predators in the Wadden Sea. As part of the monitoring work conducted as part of the Trilateral Monitoring and Assessment Programme, the trilateral Expert Group Marine Mammals coordinates the counts and harmonises the data from across the Wadden Sea region. The harbour seal is trilaterally protected under the Agreement on the Conservation of Seals in the Wadden Sea (WSSA) concluded under the auspices of the UN Convention on the Conservation of Migratory Species of Wild Animals (CMS).