Decrease in number of harbour seals observed in the Wadden Sea
Harbour seals, the most well-known seal species in the Wadden Sea, are monitored annually through aerial surveys that cover the entire Wadden Sea World Heritage site and the island of Helgoland. The surveys are conducted during the pupping season in June and moulting season in August, when seals can be observed more frequently on the sandbanks during low tide. The 2022 count results show a decrease of 22% in pup numbers and a 12% decrease in seal counts during the moulting season compared to counts in 2021.
In June, 8,514 pups were counted in the Wadden Sea compared to the 2021 count of 10,903 pups. Decreasing numbers were observed in each of the Wadden Sea regions. In Denmark, pup numbers decreased by 18%, in Schleswig-Holstein by 25%, in Lower Saxony and Hamburg by 17%, and in the Netherlands a drop of 22% compared to 2021 was recorded. One pup was observed on Helgoland.
During the moulting season in August 2022 the number of seals counted decreased in all areas, except for Denmark, where an increase of 106% was recorded. In total, 23,652 harbour seals were spotted in the Wadden Sea area, making these the lowest numbers counted since 2011. In Schleswig-Holstein, 8,384 seals were counted (-5% compared to 2021) and at Helgoland, 98 seals were counted (-16%). With 7,548 seals recorded, the Netherlands reported a decrease of -8% from 2021. Numbers in Lower Saxony and Hamburg dropped sharply to 4,822 (-42% from 2021). In this area, change in survey methods in parts of the area that allowed a clearer distinction between harbour and grey seal may have resulted in lower numbers being counted, but this change can only partly explain the drop in numbers.
It is currently unclear what may have caused the decrease in counted numbers of both pups and moulting seals to decrease. It may be that the population is approaching the carrying capacity of the environment and is thus starting to be limited by resources, most likely food. “If the population is facing density dependence, we would expect pups to be most challenged to cope with limited resources. This would result in higher pup and juvenile mortality and stagnating population growth. After a period, this would affect the number of breeding females, leading to stagnation of pup number as well. This year’s counts are in line with such a development, but significantly lower than what we would expect. However, we cannot draw firm conclusions based on a single year’s observations” says Anders Galatius, lead author of the report. Overall, the results highlight the need for the ongoing trilateral monitoring, but also additional studies to better understand the underlying mechanisms of these changes.
Harbour seals, as well as grey seals, are the top marine predators in the Wadden Sea. As part of the monitoring work conducted by the Trilateral Wadden Sea Cooperation, the trilateral Expert Group Marine Mammals coordinates the counts and harmonizes 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).