Invasive species

An increasing threat

Due to increasing shipping, more alien species are finding their way into the Baltic Sea than ever before. These non-indigenous invaders can induce considerable changes in the structure and dynamics of marine ecosystems. They may also hamper the economic use of the sea or even represent a risk to human health. About 100 non-native species have been recorded in the Baltic Sea, and almost 70 of them have been able to establish viably reproducing populations. Most of these invasive species originate from freshwater or brackish-water environments, particularly from North America or the Ponto-Caspian region.

In some cases, alien species have been deliberately introduced for fishing or aquaculture, but most have been brought in accidentally by ships, which can rapidly transport marine animals, plants and algae across the world in their bilge and ballast waters.

Non-native species can seriously disrupt ecosystems, and harm livelihoods. Fishermen in the Gulf of Riga and the Gulf of Finland remember the sudden arrival in 1992 of an alien water flea species. These tiny animals soon started to clog up the gills of fish and fishing nets, leading to serious economic losses. By 1998 the species had spread as far as Stockholm and Gotland.

Three invaders

Cercopagis pengoi is a predatory water flea which is native to the Ponto-Caspian region, the Caspian Sea, the Aral Sea and the Azov Sea. It was first observed in the Gulf of Riga and the Gulf of Finland in 1992, and in 1995 it was found in large quantities in samples from the eastern Gulf of Finland. This species clogs fishing nets and competes with herring for zooplankton prey. It was  exceptionally numerous in the Bothnian Bay during the warm summer of 2002.

Marenzelleria viridis is a polychaete worm that lives inside bottom sediments. It was observed in the southern Baltic Sea for the first time in 1985, but has subsequently spread all the way up to the Bothnian Bay. Its distribution has mainly been restricted to shallow coastal areas, but in 2000–2002 Marenzelleria started to colonize the  deeper waters of the Bothnian and Åland Seas, and it has also increased profusely in the Quark. Marenzelleria can out-compete the very few species that form the native benthic community in the northern Baltic Sea and thus alter the structure of the whole benthic ecosystem.

Prorocentrum minimum is a phytoplankton species of the open seas, which may originally have been brought into the Baltic Sea by currents or in ships’ ballast water. It is well established as a common species in the southern Baltic Sea, but in summer 2002 it formed blooms in the Archipelago Sea along the Finnish coast, and it has also been found in the Gulf of Finland. There are no records of toxic Prorocentrum blooms in the Baltic Sea.

HELCOM Actions

To help prevent the spread of alien species HELCOM is also supporting a proposed International Convention for the Control and Management of Ships' Ballast Water and Sediments.

Links

Marine alien species of Estonia

Database on Alien Species in the Baltic Sea