Larger and earlier than usual: jellyfish spread in the Baltic Sea

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The reaction of most people to sight or even contact with jellyfish is disgust. This year it will not be that easy to get around the slippery animals while bathing in the Baltic Sea, because there are unusually large specimens on the way. And they’re early.

This year they spread in large quantities much earlier than usual in the Baltic Sea and they are particularly large: jellyfish. “This is a very good jellyfish year,” said organic oceanographer Cornelia Jaspers. “Three weeks ago, especially in Eckernförde Bay, we observed a very dense accumulation of ear jellyfish, isolated fire jellyfish and introduced rib jellyfish.” The latter usually appear here in late summer, but this year in May.

“In winter, a lot of salt-rich water flowed from the North Sea and the Kattegat into the southwestern Baltic Sea,” explained Jaspers, who does research at the Geomar Helmholtz Center for Ocean Research in Kiel and at the Technical University in Copenhagen. The high salinity has apparently led to the strong emergence of the jellyfish, which was first observed in these regions in 2006. “If the salinity is low, the rib jellyfish Mnemiopsis Leidyi, meaning sea walnut, cannot reproduce.” This jellyfish is not poisonous and therefore harmless to humans – but it eats food from domestic fish.

Warm winter has promoted its existence

In addition, the warm winter promoted the existence of the rib jellyfish and also the domestic jellyfish, said Jaspers. Instead of two to three degrees – as usual in the past 40 years – the water was around five degrees this time. Because jellyfish also eat floating organisms (zooplankton) in the sea, they can also contribute to oxygen loss. The introduced rib jellyfish had disappeared from the Baltic Sea for three years, from 2011 to 2013 after severe winters, and returned in 2014 after a very mild winter with the salty water from the North Sea.

And what does the jellyfish called Blackfordia virginica do, which was first seen in the Kiel Canal in 2014? “We saw them there in great numbers and earlier than usual,” said Jaspers. The brackish water-loving jellyfish species can also live well with a low salt content and consequently become an additional food competitor for fish in the Baltic Sea if it should establish itself there, and thus become a real problem. “So far we have only discovered the Blackfordia sporadically in the Bay of Kiel,” said Jaspers. An expedition in September should clarify more about the current distribution area.

“Jellyfish are part of normal life,” said Jaspers of the marine animals that are not very popular with holidaymakers and some of which are also very toxic and dangerous to humans. “They have been around for 550 million years, they are very primal creatures.” For example, the endangered European eel in the Sargasso Sea is dependent on eating jellyfish. “The key role jellyfish have in the ecosystem is an exciting question that is still largely unresolved.”

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