Alien species travel with humans

Alien species are those that have spread from their original distribution range due to human activity, sometimes over very long distances. A good example of this is alien species being transported in the ballast water of ocean-going vessels.

Invasive species, in turn, can spread to new areas independently of humans and without help. As a result, the propagation distances are shorter than in alien 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.

Alien species can pose a threat to the original native species

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.

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. 

Two alien plant species have been detected on Finnish shores

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. 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 lush growth of Canadian pondweed.
Although the Canadian pondweed is originally a North American species, it has spread with humans to a wide area.

Many invertebrate alien species have become established in the Baltic Sea

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.

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.

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, mud crabs (Rhithropanopeus harrisii), which occur on muddy and rocky bottoms, have now become food for perch.

Bay barnacles among red- and green gutweed algae.
The bay barnacle is the first recognised alien species of the Baltic Sea. It had already arrived in the Baltic Sea by the early 19 th century, most likely via ship traffic.

The vertebrate alien species of marine areas consist of about ten fish species

Vertebrate alien species in Finnish marine areas include about ten species of fish. Some species have been dispersed passively through shipping, others through active transfer, such as breeding programmes for common- and Prussian carp.

The round goby has spread rapidly

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, this fish species has been found near most of Finland’s busy ports, from where it has spread along the coast throughout the entire archipelago and beyond. Fortunately, no species have yet been found in Finnish inland waters.

Although the belly of the round goby is usually pale brown or grey, it has darker spots or patches on its flanks and back. The fish can be easily identified by the black spot on the back of the anterior dorsal fin. Individuals are able to change their colours very quickly, according to their environment. In addition, males become very dark in colour during the spawning period.

The round goby can replace indigenous species

The round goby is most active at dusk and at night. It usually resides in crevices, swimming near the bottom, or lying on the seafloor on its conjoined ventral fins. In the summer, the females lay their oval eggs in holes on rock surfaces. The male remains to guard and care for the eggs he has fertilised. In the winter, the round goby usually moves to slightly deeper water.

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.

A glaring piebald fish with a prominent black spot on the dorsal fin.
The round goby is identified by the black spot on the front dorsal fin.

The round goby is suitable as a food fish

The spread of this species to inland waters should be prevented so that the populations of the indigenous species are not endangered. However, it may enter the Vuoksi River waterways with ships, as the implementation of the Ballast Water Management Convention, which entered into effect in 2017, is likely to take several years.

Preventing the spread of the round goby is made more difficult by the fact that it can migrate not only with ballast water, but also by attaching eggs to the hulls of boats or ships. It is also capable of climbing upwards along surfaces, so that the possibility of it actively ascending cannot be ruled out either.

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.