Marine researchers find large comb jelly population in Baltic
Experts say herring, perch, and pike have avoided serious damage
A new invasive species, the Mnemiopsis leidyi, or comb jelly, has strengthened its foothold in the Baltic Sea. Fresh samples taken in January show that populations were especially high in the depths of the Åland Sea - up to 3,800 individuals per square metre.
Aranda, the research vessel of the Finnish Institute of Marine Research, returned to Helsinki from a one-month monitoring expedition in the Baltic Sea on Friday after taking samples in 78 locations.
In the southern Baltic, off the Danish island of Bornholm, there were 1,460 comb jellies per square metre were found. There were also small numbers of the tiny creatures in other areas. Many were also found in the Gulf of Finland, between Helsinki and the Estonian capital Tallinn.
"However, they were in poor shape - many were dead. The possible reason is the cold temperature and low salinity of the water, or the shortage of nutrition.", said researcher Maiju Lehtiniemi of the Finnish Institute of Marine Research.
The unusually mild winter has nevertheless benefitted the invaders.
No comb jellies were found in the northern part of the Gulf of Bothnia - possibly because of the low salinity.
The comb jellies are similar to jellyfish. They are indigenous to the east coasts of north and South America, appearing in water temperatures ranging from 4 to 32 degrees Celsius, with a salinity of 3-39 parts per mille.
They feed on fish eggs, newly-hatched fish, and zooplankton.
In the 1980s, fish stocks in the Black Sea collapsed after an invasion of the comb jelly, at a time of serious overfishing.
The individuals found in the Baltic are very small. The ones in the depths of the Åland Sea were mostly just one millimetre in length. In warm climates they can grow to about 10 centimetres.
Special researcher Eero Aro notes that the comb jellies found in the Baltic Sea appear to be prevalent in areas where cod breed. It is feared that cod populations could suffer if populations are large in August when the cod lay their eggs.
On the other hand, Atlantic cod have been known to eat comb jellies.
Baltic herring breed in shallow waters and along the coastline, which means that they are likely to be safe, as the comb jellies are found in deeper waters. Pike and perch are believed to be safe for the same reason.
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 leidyi (comb jelly)