Wadden Sea wide counts: Long-term growth in grey seal numbers may be slowing down

Since their recolonization of the Wadden Sea in the mid 20th-century, the grey seal population has considerably increased. Seal experts found that the grey seal pup numbers in the Wadden Sea have grown at an average annual rate of 12% over the past five years. Over the same period, the grey seal numbers counted during the moult have also grown at an annual rate of 10%. However, the recent data show a slight decrease of 1 % in the total number of grey seals counted during moult compared to last year.

On an annual basis, the trilateral Expert Group Marine Mammals, with scientists and managers from Denmark, Germany and the Netherlands, undertakes coordinated aerial surveys that cover the whole Wadden Sea World Heritage site. Ground-based surveys are conducted on the island of Helgoland. During the winter’s pupping season 2021-2022, 2,214 pups were counted in the Wadden Sea and on Helgoland, 15% more than the previous year. Most newborns were recorded in the Dutch Wadden Sea where 1,168 pups were counted, 14% up compared to the previous count. On Helgoland 611 grey seal pups were observed and in Lower Saxony 432 pups (+9% and +27%, respectively). Three pups were counted in Schleswig-Holstein and one in Denmark, the latter outside the coordinated survey dates.

During the annual moult season in March-April, Wadden Sea grey seals spend more time hauled out on sandbanks, where they are joined by migrating seals from the UK. This year, seal experts counted a total of 8,948 grey seals, 1% less than last year. With 6,500 grey seals recorded, the Dutch part of the Wadden Sea presents 73% of all grey seals counted in the Wadden Sea area. Compared to the previous count, the Dutch experts registered 4% less animals. “This may be a true decline, or possibly a change in the influx from the much larger grey seal colony in the UK” explained Jessica Schop, main author of the report. “We could also have missed the moulting peak; hence, it is important in the future to understand the timing of the peak in the different Wadden Sea regions.“ Helgoland and Lower Saxony recorded an increase by 5% to 1,090 individuals and 19% to 1,086 individuals, respectively. In the Schleswig-Holstein and Danish parts of the Wadden Sea, 120 and 152 grey seals were counted, respectively.

Grey seals are the largest predators along the Wadden Sea coast and, like harbour seals, one of the iconic species of the region. Although the Wadden Sea Seal Agreement (WSSA) under the auspices of the Convention of Migratory Species in Bonn does not cover the grey seals, they benefit from their inclusion in the related Seal Management Plan, which is updated every 5 years. All the agreement’s protection measures and the monitoring and research activities which are named in the Seal Management Plan also cover the grey seals. This includes the annual counts, which are required to identify trends in seal numbers and variations in their geographical distribution across the Wadden sea countries. These data help inform comprehensive conservation and management of the grey seal population. CWSS acts as the secretariat of WSSA.

Full report "Grey Seal Numbers in the Wadden Sea and on Helgoland in 2021-2022"