Scandinavian coastal waters are experiencing an unusually early and dense proliferation of jellyfish species in the summer of 2026, driven by elevated sea temperatures, strong winds, and the Baltic Sea's lowest water level in 140 years.
Danish waters have seen an influx including lion's mane, common jellyfish, and the invasive American comb jelly, with marine biologist Cornelia Jaspers of the University of Copenhagen noting the bloom arrived roughly two months earlier than usual. In the Limfjord, densities reached one jellyfish per litre of water, while a dive at Kalø Vig was cancelled due to 20 to 30 lion's mane jellyfish per square metre. The invasive comb jelly, a hermaphrodite capable of producing 13,000 eggs daily, poses no sting risk to humans but threatens fish stocks.
Sweden's west coast reports abundant lion's mane jellyfish from Skagerrak to Bergen, while declining moon jellyfish populations have benefited both blue and red varieties. New species such as the compass jellyfish have appeared in recent years, and Tjärnö Marine Laboratory's Christin Appelqvist observed a light jellyfish in 2025 that stings more severely than common varieties.