Altogether 23 alien species can be found from the Finnish part of the Baltic Sea. These species have arrived in the Baltic Sea assisted - intentionally or unintentionally - by man. Five of the species may be considered clearly harmful for original biodiversity and are classified as invasive alien species. Seven species are potentially invasive and 11 more alien species are not invasive or their impact is not known.
The number of alien species in the Finnish part of the Baltic Sea increased steadily troughout the 20th century until the 1980s. On average one new alien species was found per decade. Three new species were discovered during the 1990s and no less than 11 during the first ten years of the new Millennium .
The greatest number of alien species can be found in the southern sea areas where water is relavitely warm and salty. These circumstances are most suitable for southern salt water species. However, more than ten alien species can be found even in the northernmost parts on the Baltic Sea.