Comb jellies threaten fish stocks off Helsinki
Treaty to prevent introduction of non-indigenous species through ballast water expected in 2011
Fish stocks in the Gulf of Finland in waters near the capital Helsinki are threatened by the fairly recent introduction of a new invasive species, the comb jelly (Mnemiopsis leidyi).
The small jelly-like plankton, which compete with fish for nutrition, have been found for the first time in waters near the Helsinki shore.
"Comb jelly larvae have been found less than ten kilometres away from the shoreline", says Jari-Pekka Pääkkönen of the Helsinki Environment Centre.
The numbers are still fairly small - from 5 to 20 individual animals per square metre at a depth of 40 to 50 metres.
The small invertebrate, which feeds on zooplankton, could be a threat to fish that spawn in shoreline waters, says Maiju Lehtiniemi, a researcher at the Finnish Institute of Marine Research. In addition to plankton, the largest comb jellies can also feed on young fish.
The comb jellies were introduced to the waters of the Baltic Sea in the ballast water of ships. The first observations were made in January 2006. In the autumn of 2007, scientists on the marine research vessel, the Aranda found that the species had acclimatised well to a number of parts of the Baltic Sea.
They have not yet been found very near the shore, but they have been observed in the outer parts of the archipelago.
The comb jelly is capable of multiplying in the Baltic Sea. Locally there population densities of up to 600 individuals per square metres have been found, which is about the same level as in the Black Sea, where the species has caused extensive damage to local fisheries.
Maiju Lehtiniemi says that it is not yet sure if the largest individuals can survive the winter in these northern seas. Those that have been found up to now are very small - less than a millimetre in size. The comb jellies do not have any natural predators in the Baltic Sea, although some Fish might start eating them.
"They are not very good nutrition, as more than 90 per cent of their mass is water", Lehtiniemi explains.
Eradication of the small invader is no longer possible, in Lehtiniemi's view, and she feels that the important task at hand is to prevent the introduction of of other non-indigenous species to the Baltic.
The International Maritime Organisation has drawn up a treaty aimed at cleaning up the ballast water in ships to prevent organisms from travelling to places where they are not supposed to go. The aim is for all new ships built from 2011 onwards to have in-built ballast water cleaning systems.
The treaty has been approved by only two European countries so far - Spain and Norway - but more countries are expected to sign on soon.
Previously in HS International Edition:
Invasive comb jellies now found throughout Baltic Sea (17.12.2007)
Comb jelly poses serious threat to Baltic Sea ecosystem (27.8.2007)
Global invasive species database: Mnemiopsis ledyi (comb jelly)