Danish waters teeming with jellyfish and invasive comb jellies

Unusually high numbers of jellyfish and invasive comb jellies are filling Danish waters this year, appearing two months earlier than normal, reports DR.

Marine biologist Chris Sejerskilde, a member of the Aarhus Bay diving club Biodyk, described dense clusters of moon jellyfish and lion’s mane jellyfish near Kalø Vig, with some 20 to 30 per square metre forcing divers to cancel outings. On a recent dive, he encountered around ten American comb jellies—known locally as “killer comb jellies”—along with numerous stinging jellyfish.

Cornelia Jaspers, a biological oceanographer at the University of Copenhagen, attributes the surge to three factors: strong winds since March pushing saltwater from the Kattegat and North Sea into Danish waters, historically low Baltic Sea water levels allowing saltwater and jellyfish to enter, and record-high sea surface temperatures of 20.86°C in June, per Copernicus data.

In the Limfjord, densities have reached one comb jelly per litre. The invasive American comb jelly, first spotted in Denmark around 2005, reproduces rapidly—each can produce 13,000 eggs daily without needing a mate. While it does not sting humans like the lion’s mane jellyfish, it competes with fish such as herring and sprat for food.

The comb jellies are cylindrical with eight glowing rib-like stripes, visible in daylight and luminescent at night. Sejerskilde acknowledges their beauty but notes their ecological harm, saying, “We can’t remove them, so we might as well appreciate their appearance.”