Alien aquatic species

Signal crayfish, mink, lake trout, Canadian waterweed. Numerous alien species live in Finnish lakes, rivers and coastal areas, and some of them have made themselves far too much at home.

Alien species are animals or plants that have been introduced by humans, either deliberately or unknowingly. Some of the alien species in aquatic environments have been defined as invasive: for example, they may push out indigenous species, change ecosystems or hamper the recreational use of waters.

The signal crayfish, which comes from North America and was introduced extensively in Finnish waters in the late 20th century, is a good example. It competes with the European crayfish for living space and spreads the crayfish plague, to which the European crayfish is more susceptible. Consequently, the indigenous European crayfish has declined strongly.

Invasive alien plants include the Canadian waterweed, which grows under the surface and spreads easily from even small shoot fragments. It can grow into large and impenetrable jungles in lakes with nutrient-rich and calcareous waters.

The most recently arrived alien species include the magnificent bryozoan, which forms soft, spherical colonies. These slimy blobs of jelly may be fist-sized or even bigger.

You can report observations of alien species on the Vieraslajit.fi website.