2024 annual survey reveals decline in Wadden Sea harbour seal population
The trilateral Expert Group Marine Mammals has released its latest findings from the 2024 annual harbour seal survey. The results reiterate previous findings, indicating a change in population trend in the past decade, with harbour seal numbers in the Wadden Sea now being lower than 10 years ago. The harbour seal, a Wadden Sea icon, is annually monitored in the transboundary Wadden Sea World Heritage Site and the island of Helgoland.
In June 2024, the survey recorded a total of 8,230 harbour seal pups, marking a 12% decline compared to the 2023 count of 9,334 pups. This reduction was observed across most regions, with Schleswig-Holstein experiencing the largest decrease of 19%, while Denmark saw an increase of 14% in pup numbers. In the Netherlands, the number of pups fell by 15%, and in Lower Saxony and Hamburg, the drop was 2%. Again, no pups were recorded on Helgoland. The total number of pups in 2024 is now lower than it was ten years ago in 2014, reflecting a drop in the number of reproducing females, following decreased pup survival rates.
During the August moult season, a total of 23,772 harbour seals were counted across the Wadden Sea area, representing a 5% increase compared to 2023. Despite this slight uptick, the 2024 count remains lower than those from the period 2012-2020, signalling a long-term decrease in overall seal abundance. While Schleswig-Holstein and Lower Saxony & Hamburg saw increases of 7% and 14% respectively, both Denmark and the Netherlands recorded declines, with Denmark experiencing a 6% decrease and the Netherlands showing a 2% drop. On Helgoland, 56 seals were counted, a decrease from 72 the previous year (-22%).
The results represent the fourth consecutive year of lower counts compared to the period of stagnation observed from 2012 to 2020. Several potential causes of the decline are being explored. While migration and diseases have been ruled out as a major factor, other challenges affecting harbour seal numbers may be increased competition, or the intensified human use of the North Sea, where harbour seals forage. The full impact of these changes is not yet clear, as more research is needed to draw clear conclusion for management purposes.
“We need information on survival and behaviour of individual seals to better understand the mechanisms behind the population changes and to develop effective conservation strategies”, says the report’s lead author Anders Galatius, University of Aarhus. “Annual variations are normal, but the long-term decline in numbers of harbour seal points to broader ecological pressures that must be addressed to effectively protect harbour seals in the Wadden Sea.”
A workshop with members of the Expert Group Marine Mammals and partners from Wadden Sea seal centres was organised by the Common Wadden Sea Secretariat to discuss if and how data from rehabilitated seals could be used to obtain information on the current changes in the population. In a first step, the participants formulated an approach to ensure the availability of new data for future analysis.
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).
The full report can be downloaded here.