Comb jelly poses serious threat to Baltic Sea ecosystem
Invasive species spread through ships' ballast water
Alarming numbers of comb jellies (Mnemiopsis ledyi) have been detected in parts of the Baltic Sea. It is feared that the invasive species could have disastrous consequences for the entire ecosystem in the Baltic.
The species is a very efficient predator, which consumes young fish and edible zooplankton.
Juha Flinkman, a special researcher at the Finnish Marine Research Institute, says that the arrival of the comb jelly has been both expected and feared. Scattered individuals were found in the Baltic already last winter.
According to Flinkman, scientists on the institute's research vessel Aranda noticed that there were large amounts of the jellies in the depths of the Åland Sea and in the southern part of the Gulf of Bothnia. The greatest numbers were found at depths of more than 200 metres.
"There the density was as much as 600 individuals per square metre, which is about the same as at the worst times in the Black Sea", Flinkman says.
The species has not yet been detected in the Bay of Bothnia, but individuals in the larval stage were found in the northeast edges of the main basin of the Baltic, and at the western end of the Gulf of Finland.
"Such a wide distribution and density sound very bad. The ecosystem and the array of species will change. The impact could be seen already next year, and the year after that at the latest", says Anita Mäkinen, head of the marine programme of WWF.
Mäkinen says that the first fish species to disappear could be cod, whose populations are on the decline anyway.
"The species eats zooplankton, which the fish use as nutrition, as well eating as the eggs of the fish", Anita Mäkinen says.
The comb jelly is native to the east coast of North America, from where it spread around the world.
The Aranda researchers found one individual, about a millimetre in diameter, which was just hatching. This indicates that the comb jelly actually multiplies in the Baltic.
Comb jellies were also found off the coast of Norway. Tone Falkenhaug of the Norwegian Marine Research Institute is not sure how well they can adapt to cold water. The species managed to survive last winter at least in the southern part of the Baltic.
"It would seem that even if the surface of the sea is covered with ice, the comb jelly can descend to deeper layers of water and make it through the winter", Falkenhaug told Helsingin Sanomat over the telephone.
The comb jelly faces few predators or competitors in the Baltic Sea.
However, it is difficult to predict the impact that the new invader will ultimately have on the marine environment. Scientists are not sure exactly what happened in the Black Sea. When the species was spotted in the Black Sea, it was a time of serious overfishing and depletion of fish stocks.
It remains uncertain if the comb jelly was primarily responsible for the lower catches, or if the overfishing contributed to the spread.
Eeva-Liisa Poutanen, director-general of the Marine Research Institute, says that the Aranda will make a new voyage in the Baltic in early September to examine the situation then.
Global invasive species database: Mnemiopsis ledyi (comb jelly)