Common seal numbers have gone down for the third year in a row in the Dutch Wadden Sea area and scientists are at a loss as to why this is happening.

The Wadden Sea is home to grey seals and common seals. Common seals have a more defined habitat than grey seals and are therefore easier to keep track of.

The seals moult in August on dry land, which is when marine scientists in Denmark, Germany and the Netherlands count them. This year’s combined tally was 22,621 of which 6,706 were in the Netherlands, down 4% on last year.

“This is clearly a trend,” Wageningen marine researcher Sophie Brasseur told broadcaster NOS. “The population grew when seal hunting was banned and the number of common seals stabilised from 2013. But in the last three years we are seeing a decline, first by 4% in 2021, then 10% in 2022 and this year we are looking at another 4% fewer common seals.”

Although the number of pups has been consistently higher in the last few years that is not going to turn the tide, Brasseur said. Some 9,334 common seal pups were counted this year in the whole of the Wadden Sea area.

“This is the worrying aspect – more pups but fewer adult seals. This means over 10,000 seals died this year. The females are capable of finding enough food to produce pups but I wonder how many of them survive,” she said.

An international research team studying the problem has said possible explanations may be the deterioration of the seals’ habitat, or the presence of wind farms and increased marine traffic but that more research will be needed to find the exact cause.