Dreaded comb jellies survive winter in Baltic Sea, but do not grow much

Aranda voyage shows spread of oxygen depletion

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The marine research vessel Aranda had good news and bad news when it returned from a month-long research voyage in the Baltic Sea on Friday.
     “The new imported species, the comb jelly, which was first found in the Baltic in 2006, has survived the winter”, said researcher Maiju Lehtiniemi of the Finnish Institute of Marine Research.
     Considered an intrusive species, the comb jelly (Mnemiopsis leidyi), which was introduced into the Baltic in the ballast water of ships, has caused extensive destruction in the Black Sea, where it has wreaked havoc on vast populations of anchovies by eating their food.
     However, in the Baltic, the comb jellies do not appear to be growing very large.
      “The individuals that survived the winter are a couple of millimetres in size. In the Black Sea they can grow to be as long as 15 centimetres”, Lehtiniemi noted.
     
The densest populations are still in the depths of the southwest and west of the Åland Islands, and in central areas of the Gulf of Bothnia.
     “All over the Baltic Sea, the density of comb jelly populations have declined since they were measured in January, which is a good thing”, Lehtiniemi emphasises. None have been found in the northernmost part of the Baltic, the Bay of Bothnia.
     
Areas of the sea bottom with complete depletion of oxygen have increased from last summer.
     It has been a few years since a surge of salt water through the Straits of Denmark has brought oxygen-rich water to the depths of the Baltic Sea.
     “If there is no oxygen at the bottom of the sea, animals living in the bottom cannot live”, says Alf Norkko, special researcher at the Marine Research Institute.
     Animals living at the bottom have an important significance for the marine ecosystem, as they act as a kind of natural cleanup mechanism by processing organic material on the bottom.
     The good news is that the oxygen situation has improved in the Bay of Bothnia and the Gulf of Bothnia.


Previously in HS International Edition:
  Marine researchers find large comb jelly population in Baltic (4.2.2008)
  Invasive comb jellies now found throughout Baltic Sea (17.12.2007)
  Comb jelly poses serious threat to Baltic Sea ecosystem (27.8.2007)

Links:
  Global invasive species database: Mnemiopsis ledyi (comb jelly)

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