Initial interrogations have ascertained that the light fuel oil was discharged from the ship in Naantali harbour and in the nearby Airisto Strait. The captain of the offending vessel has denied the discharge of oil, police investigators in Turku say. On Tuesday the Finnish tv channel MTV3 reported that the ship in question was Finnlines' Finneagle, a cargo ship that also carries some passengers between Naantali and Kapellskär, Sweden.
Witnesses have also been interviewed in the initial investigation being carried out in collaboration with the Coast Guard and other officials.
Observations of the oil were made as far away as Ledsund in the Åland Islands. The largest concentrations are in Naantali harbour and in the Airisto Strait, a popular holiday area.
"The spilled oil is light fuel oil, which spreads as a film over a large area on contact with water. This type of oil dissipates quickly. The biggest threat is to birdlife in the archipelago," says WWF Finland's Project Secretary Toni Jokinen, who heads the environmental group's oil spill response efforts.
"There is not much that can be done about a light-oil spill. Rescue Services set up floating booms to contain the oil within the Naantali port area. But we will just have to wait for the fuel that is drifting around various parts of the archipelago to evaporate," he says, adding that sunshine accelerates the process.
Ice in the region also makes it nearly impossible to collect the oil.
Jokinen notes that the situation would have been far more serious if the spill had involved heavy oil, and/or if it had occurred later in the spring during the main waterfowl nesting season. As it is, though, migrating birds may become contaminated by the oil, as they seek smooth areas of water to land on.