Boaters report extensive blooms of jellyfish in Finland’s coastal waters

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This autumn numerous sightings of swarms of transparent common jellyfish (Aurelia aurita) have been made in Finland’s southwestern sea areas. Since the end of August, Finland’s Environmental Administration SYKE has received around 50 reports of jellyfish sightings from yachtsmen and recreational boaters.
     
Dense blooms of jellyfish have been observed particularly outside Hanko and Tammisaari, but also in the Southwestern Archipelago. Jellyfish sightings have also been made in the sea areas outside Helsinki, in the Sea of Åland, and in the southern parts of the Bothnian Sea.
      Jellyfish sightings have not previously been systematically collected in Finland, and very little is known of the life of these creatures in the Baltic Sea.
      The variety found in Finnish waters is the so-called moon jelly or common jellyfish, which is easily recognised by its four horseshoe-shaped gonads that can be seen through the top of the bell.
      The moon jelly is harmless to humans for its ability to sting is weak.
      “Jellyfish exist in the open sea only during the jellyfish or medusa phase of their life cycle, which is the phase when they have their customary free-swimming large bell-shaped, jelly-like bodies”, explains special researcher Maiju Lehtiniemi from the SYKE Marine Research Centre. Hence jellyfish are usually observed towards the end of summer or in the early autumn.
      Lehtiniemi hopes that the reports from boaters will in time also provide information as to in which water temperatures jellyfish no longer thrive.
      During the winter months jellyfish are not seen, even if the sea is open, because they are in the other phase of their life cycle, namely as polyps attached to the sea floor.
     
In large blooms, jellyfish can block power station cooling systems that rely on the use of sea water. for this reason, industries in Finland have requested more information with regard to the prevalence of the species.
      Jellyfish can also cause harm to fishermen by getting attached to their nets. According to Lehtiniemi, some professional0 fishermen have pondered whether jellyfish in the net act as a warning sign to fish, thereby reducing the size of catches.


Previously in HS International Edition:
  Comb jelly poses serious threat to Baltic Sea ecosystem (27.8.2007)

See also:
  Comb jellies threaten fish stocks off Helsinki (21.4.2008)

Links:
  Common jellyfish – Aurelia aurita (Wikipedia)

Helsingin Sanomat