German Federal Agency for Nature Conservation (BfN)


Non-indigenous species

Chinese mitten crab (Eriocheir sinensis), Photo: Christian Buschbaum (AWI)
Chinese mitten crab (Eriocheir sinensis), Photo: Christian Buschbaum (AWI)

Changes in species composition are a natural part of evolutionary development in any ecosystem. A case in point is the Baltic Sea. Ever since its birth only a few thousand years ago and right through to the present day, the Baltic has always undergone colonisation by flora and fauna from the saline North Sea to the west and freshwater river systems to the east. Yet human seafaring with the discovery of new continents and the onset of global grade created new opportunities for plants and animals to colonise new regions. Today, our seas face a growing influx of such newcomers, also known as neobiota.

Not all non-indigenous species that are introduced become successfully established in their new home. But other organisms that succeed in building stable populations and spreading can be a severe threat to the natural environment and people. Non-indigenous species of this kind are referred to as invasive alien species. Currently, three new benthic species are discovered in German marine waters about every two years, with a higher introduction rate in the North Sea than in the Baltic Sea.


In marine aquatic ecosystems, the most common route of entry is via ballast water used to stabilise ships when travelling empty. Countless organisms, and most of all their free-swimming larvae, are carried in ship ballast water to far locations where they are discharged into the sea. The faster speeds at which modern ships traverse the oceans mean that such organisms have a better chance of surviving. If they encounter similar ecological and climatic conditions to the waters where they originate, then they are likely to become established in their new home. In the North Sea and the Baltic Sea, this relates first and foremost to species that have high tolerance to variations in salinity and temperature and that have high reproduction rates.

Fouling organisms that attach themselves to ships’ hulls can likewise easily traverse great distances as ‘stowaways’. Pleasure craft also play a part here in the onward spread of non-indigenous species.

A further pathway is aquaculture. Introducing and farming imported fish, algae and mussels can result in the spread of organisms that attach to the farmed species, the larvae of such organisms, and the larvae of the farmed species themselves.

Organisms can also spread along artificial waterways. This is the pathway by which the round goby, a species of Ponto-Caspian origin, is thought to have reached the Baltic Sea.