Species that have spread from their original habitat to a new area due to human activity are called alien species. A good example of this is alien species being transported in the ballast water of ocean-going vessels. When adapting to a new environment, invasive species may proliferate rapidly and endanger the functioning of local species and ecosystems.
As a brackish water area, the Baltic Sea is a difficult habitat for both true marine and freshwater species, and thus few alien species survive.
After entering a new habitat, alien species often lose their competition with native species. Moreover, they often are unable to adapt to different local environmental conditions. In some cases, however, they adapt well to their new environment and can reproduce. At worst, they may also endanger the functioning of native species and ecosystems.
By 2024, over 30 alien species had been observed in Finland’s marine areas. The newest arrivals include the Atlantic rangia (Rangia cuneata) and the Japanese skeleton shrimp (Nippoleucon hinumensis) observed in the Gulf of Finland in 2021, as well as the tubenose goby (Proterorhinus semipellucidus) observed in the Eastern Gulf of Finland.
Along the shores of Finland, invertebrate alien species can be found living on and within the seafloor, as well as in the water column. These species have spread to Finland via waterborne traffic and many have become an established part of the fauna of the Baltic Sea. With climate change, sea water is warming, which may facilitate the establishment of alien species adapted to warm sea water.
For example, one alien invertebrate living in the bottom sediments is the red-gilled mud worm, i.e. Marenzelleria spp., which has spread along the entire Finnish coast within ten years. These mud worms survive well in bottom sediments which other native species, e.g. the amphipod (Monoporeia affinis) would have to avoid due to low oxygen levels. In 2021, over 90% of polychaetes on the seabed were alien species.
The bay barnacle, i.e. Amphibalanus improvisus, is an alien species that attaches to the hard seafloor and has been first observed in the Baltic Sea since the 1840s. The barnacle is the only cirripede crustacean on the Finnish coast. Fun fact! It might look like a clam, but it is actually a crustacean with feathery legs modified to catch food. It is not found in the Bay of Bothnia or in the eastern parts of the Gulf of Finland because the salinity is too low.
The aquatic fishhook water flea (Cercopagis pengoi) is a nightmare for net fishermen. When fishhook water fleas occur in abundance, they form a transparent mass of jelly which completely chokes the fishing nets.
Although alien species are often considered harmful, they can also become part of their new ecosystem. For example, small mud crabs (Rhithropanopeus harrisii), which occur on muddy and rocky bottoms, have now become food for perch. However, mud crabs have increased rapidly, and there are concerns about their impact on native species. For example, they eat crustaceans and snails living on bladderwrack, which keep the bladderwrack clean from other algae.
The alien vertebrate species in Finland’s marine area include at least five fish species. Some species have been dispersed passively through shipping, others through active transfer, such as breeding programmes for common- and Prussian carp. In addition to shipping, fish species also end up in the Baltic Sea due to intentional releases and fish farming.
The round goby, i.e. Neogobius melanostomus, which is native to the Black Sea and Caspian Sea areas, entered the Baltic Sea via shipping, apparently through a canal network of rivers. It was detected in 1990 on the coast of Poland and in 2005 in the Archipelago Sea. Since then, the species has spread from the eastern Gulf of Finland to as far north as Oulu. Fortunately, no species have yet been found in Finnish inland waters.
The round goby feeds mainly on bivalve molluscs and snails, but also other benthic animals, as well as fish eggs and juvenile fish. Within five years, it can grow to 25 centimetres, or twice the size of the black goby, i.e. Gobius niger, which is a native species of Finnish marine areas. Therefore, the high individual densities for which it is known can weaken the populations or nutritional status of Finland’s native species. Along the coast, the round goby has become so abundant in just a few years, that in some places both the black goby and the European bullhead, i.e. Cottus gobio, have had to withdraw.
The round goby can be caught with traditional fishing gear, such as angling or ice fishing. It can also be caught using nets and traps. Its fishing and use have already increased in Denmark, Poland, and the Baltic countries. The annual catches of the species in these countries have been as high as 60 to 90 tonnes.
In the nature, predatory fish and seals, as well as cormorants and other fish-eating birds, have also benefited from this alien goby species. It is said that the light-coloured flesh of the round goby has a mild flavour and tastes similar to perch.
Alien species migrate to the Finnish coast in the same way as other alien organisms. The most distant species are transported via ship ballast water and even by birds. Currents and catchment run-off waters help to spread alien species already established in other parts of the Baltic Sea.
Recently, two alien plant species, Canadian pondweed (Elodea canadiensis) and the soft hornwort (Ceratophyllum submersum) have been detected off the coast of Finland. Both are freshwater species that have reached the sea from the Finnish Lake District region via migrating birds.
These species benefit from eutrophication and aggressively invade the habitats of other species. In particular, Canadian pondweed is considered to be extremely harmful in waterways. Both species are much more common in the Finnish Lake District than on the coast.
Since the identification of algae is inadequate for many species, it may be possible that alien invasive algae species are living as yet undetected on the Finnish coast.
A large part of alien species spread through shipping. Therefore, in 2017, the Ballast Water Management Convention was drafted, requiring ships to install ballast water treatment systems by 2024. This helps prevent the spread of harmful species – such as the round goby – not only to marine areas but also from marine areas to inland waters.
Citizen observations are an important part of controlling alien species. They help determine how widely alien species have spread and guide the appropriate measures to combat them.
Report any alien species you observe on the website vieraslajit.fi.(siirryt toiseen palveluun)
Finland has a drafted a National Strategy on Invasive Alien Species, as well as a management plan by the Finnish government, which lays down provisions on how the EU Regulation on Invasive Alien Species is to be implemented in Finland. The aim is to prevent the damage and risks caused by alien species to Finnish nature and the well-being of society and humans.
A species is called an alien species if it has spread to a new habitat with human assistance – either intentionally or accidentally. In contrast, invasive species spread to new areas independently of humans, on their own. As a result, the spread distances of invasive species are shorter than those of invasive species. Human-induced climate change is causing widespread changes in the distribution ranges of species. In this case, making the distinction between invasive and alien species can be challenging.