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Invasive species in the Baltic Sea

Erkki Leppдkoski, Environmental and Marine Biology, Еbo Akademi University, FIN-20500 Turku, Finland, e-mail

Being both a recipient and donor area for nonindigenous species (NIS), the Baltic Sea has become an important node in a global network of NIS transfers during the recent decades, thereby facilitating the process of homogenization of the world’s aquatic fauna and flora. During the last 150 years, about 100 alien species have been recorded in the Baltic Sea, most of them being introduced unintentionally by shipping in the ballast water tanks or by hull fouling, or spread from their primary sites of introduction in adjacent freshwater bodies. Only a few of the NIS recorded do behave invasively in the Baltic.attach_10.jpg  attach_10.jpg

There are few and perhaps no truly endemic species in the Baltic - its flora and fauna consist of species of varied ecological and biogeographical origin. These include euryhaline species that have experienced natural range expansion from the North Atlantic, relicts from previous periods of the history of the sea, brackish and freshwater species, as well as species recently introduced by man. Thus, being a former post-glacial lake, the Baltic has been subjected to both spontaneous and human-aided invasion of fauna and flora over the last 10,000 years.

The brackish nature of the Baltic Sea does not protect its waters from introductions of alien species. The biota of the Baltic are exposed to other brackish-water biotas of the world, owing to the breakdown of large-scale geographical barriers by ships’ traffic leading to an exchange of species. Most of the important harbors in the world are located at river mouths or in estuaries. The salinity range of these brackish habitats covers the oligo- and mesohaline conditions prevailing in the Baltic. The Baltic Sea is characterized by a complicated, three-dimensional (i.e. physical, chemical and biological) network of steep environmental gradients. This complexity can be expected to facilitate invasions of NIS by offering a wide range of hospitable conditions for a great variety of life forms.

In addition to its contact with the Atlantic Ocean through the Danish Straits, the Baltic and its drainage area are connected to the Ponto-Caspian brackish seas (Black, Azov and Caspian Seas) by rivers and canals, which were opened since the 1770s. Thus, several geographical invasion corridors, other than ship traffic, open into the Baltic Sea and its Gulfs. Several species of introduced crustaceans, notably Mysidacea and Amphipoda, have spread into the coastal lagoons of the Baltic from the adjacent freshwater bodies. Other species were introduced intentionally through acclimatization experiments performed in the former USSR during the 1960s and 1970s, especially in the Baltic Republics, in order to improve foraging conditions for fish; most of these species originate from the Ponto-Caspian region.

There is evidence of large-scale biological contamination by NIS in the zoobenthic and planktonic communities of the Baltic Sea. Introduction of NIS into the Baltic, and their further spread within the basin, has resulted in major changes in near-shore ecosystems especially in coastal lagoons and inlets. By contrast, hypolimnetic and pelagic regions of the Baltic have been practically free of NIS until the 1980s. Approximately 70 NIS have been able to establish reproducing populations in the Baltic Sea. Of the 58 unintentional introductions (including non-established species) with more or less known dispersal history, 38 are transoceanic, among them 19 trans-Atlantic ones of American origin. Another 18 NIS are of Ponto-Caspian origin. After their introduction many species and their drifting pelagic larvae invade new nearby habitats by natural dispersal (secondary introduction) via the transport by water currents and coastal traffic.

Baltic Sea estuaries have functioned as bridgeheads that have aided in the establishment of several NIS. The Curonian, Vistula and Szczecin Lagoons, German Boddens, and the Neva estuary are known to host a high number of well established NIS and can be identified as "centers of xenodiversity” along the Baltic coasts. Based on published first findings, the minimum rates of secondary, within-basin, spread were estimated as follows: the barnacle Balanus improvisus from Kцnigsberg (now Kaliningrad) (1844) to Turku (1868) 30 km·yr-1- ; the polychaete Marenzelleria viridis from German Boddens (1985) to Lithuania (1989) 170 km·yr-1, further to South Finland (1990) 480 km·yr-1, and finally to Northern Quark (1996) 90 km·yr-1; the gastropod Potamopyrgus antipodarum from Wismar Bight, Germany, to Gotland (1920) 20 km·yr-1, to the Еland Islands (1926) 50 km·yr-1, and further to the Bothnian Bay (1945) 30 km·yr-1.

Recent introductions into the Baltic Sea

Three recent introductions into the Baltic Sea serve as examples of the dispersal capacity of aquatic NIS. Marenzelleria viridis, a North American polychaete, spread from the southern Baltic coastal inlets up to the Bothnian Bay within 10 years. Hemimysis anomala, a Ponto-Caspian mysid, first found at the entrance to the Gulf of Finland in 1992, was detected in both the Stockholm archipelago and the inner part of the Gulf of Finland some years later. A new cladoceran species for the Baltic, Cercopagis pengoi, an introduction from the Black Sea area, spread from its bridgeheads in the Gulf of Finland and Gulf of Riga up to the Northern Quark and down to the Gulf of Gdansk in less than 10 years.

Until the establishment of permanent populations of M. viridis and C. pengoi, scientific interest in the Baltic Sea area was mainly directed towards abundance assessment of nonindigenous species and their invasion history, whereas their fundamental role as consumer members in and regulators of brackish-water ecosystems, as well as their realized niches in the invaded communities, still remain to be studied in more detail. The appearance of M. viridis and C. pengoi contributed widely to the scientific awareness of aquatic bioinvasions.

Recent research and development efforts

The first risk assessment study within the Baltic Sea area was published in 1999, including risk profiles for five northwest European harbours along the salinity gradient from St. Petersburg (Gulf of Finland) to Bergen (Atlantic coast of Norway). The first shipping study in the Baltic Sea was undertaken as a part of a recently completed European research initiative, to quantify the survival of organisms in ballast tanks during ship voyages, in 1998 onboard the Russian Navy hydrographic ship "Sibiryakov" on its voyage from St. Petersburg to Lisbon (Portugal) and back. With one exception, both phytoplankton and zooplankton species were found in lower numbers at the end of the voyage than at the beginning.

Cooperation and exchange of information on NIS in the Baltic is mainly through the Working Group on Nonindigenous Estuarine and Marine Organisms (NEMO), established in 1994 by the Baltic Marine Biologists (a non-governmental organisation, founded in 1968). Members of this Working Group permanently communicate with each other, mostly by e-mail, and meet for workshops once per year.

This group with some 15 active members works in close co-operation with representatives from intergovernmental and non-governmental bodies such as the International Council for the Exploration of the Sea (ICES), especially with the Working Group on Introductions and Transfers of Marine Organisms (WGITMO), and the IMO. A database contains a list of ca. 100 introduced species recorded in the Baltic, including information on taxonomic and common names (in different languages), ecological group, year of first record, likely vector of introduction, origin and references to information sources. The database is maintained by Coastal Research and Planning Institute, Klaipeda University, Lithuania.

A Ph.D. course on Ecology of Marine and Estuarine Introductions and Invasions was held in summer 1997 at Еbo Akademi University, Finland, for participants from the Nordic and Baltic countries and NW Russia, with. profs. James T. Carlton (USA) and Inger Wallentinus (Sweden), among others, as lecturers.

Harmful or beneficial invaders?

There have been a few ecological and economic problems with NIS already established in the Baltic Sea. Most of them are relatively benign and appear to have served to increase both species and functional diversity in a variety of habitats. Of the NIS occurring in the coastal waters, mainly four fouling species, the hydrozoan Cordylophora caspia, the barnacle Balanus improvisus, the cladoceran Cercopagis pengoi and the bivalve Dreissena polymorpha cause economic damage to fisheries, shipping, boating or fish farming, as well as industries and power plants where cooling water from the sea is used (Table 1).

Table 1. Nuisance species among the 100 nonindigenous species recorded in the Baltic Sea (the Kattegat included) by April 2001. Data on origin, date of introduction and vectors mainly from Leppдkoski and Olenin (2000a). Modified from Leppдkoski 2001.

Type of nuisance

Common name


Time of introduction into the Baltic

1. Fouling of industrial installations, water supply systems, boats, fishing gear
Coscinodiscus wailesii
Centric diatom
Indo-Pacific? 1980s Shipping or associated
Cordylophora caspia
Brakish-water hydroid
Ponto-Caspian early 1800s Shipping, canals
Styela clava
Leathery sea squirt
NW Pacific 1990s Shipping
Ficopomatus enigmaticus*
A tubeworm
S Hemi-sphere 1950s Shipping
Cercopagis pengoi
Fish hook water flea
Ponto-Caspian 1990s Shipping
Balanus improvisus
Bay barnacle
N America 1840s Shipping
Dreissena polymorpha
Zebra mussel
Ponto-Caspian early 1800s Shipping, canals
2. Preventive (antifouling) measures needed
B. improvisus
C. caspia
3. Impacts on fisheries, boating and recreational qualities
Sargassum muticum
Jap weed
SE Asia 1980s Associated
Elodea canadensis*
Canadian waterweed
N America 1870s Ornamental
C. pengoi
Eriocheir sinensis
Chinese mitten crab
SE Asia 1920s Shipping
4. Parasites or pests on fish and shellfish
Pseudodactylogyrus spp.
Gill parasitic monogeneans
Pacific 1980s Associated
Anguillicola crassus
Swim-bladder nematode
SE Asia 1980s Associated
Crepidula fornicata*
Slipper limpet
N America 1940s Associated
5. Damage caused to wooden objects (boring)
Teredo navalis
Ship worm
SE Asia? 1700s Shipping
6. Impacts on water quality; hygienic (toxic) risks
Alexandrium tamarense
Unknown Unknown Shipping
Gyrodinium spp.
Unknown 1980s Shipping
Gymnodinium spp.
Unknown 1990s Shipping
Branta canadensis
Canada goose
N America 1930s Stocking
7.Damage caused to agriculture from overgrazing
8. Damage caused to shores (burrowing)
E. sinensis*
Ondatra zibethicus
N America 1920s Stocking
9. Damage caused to target species for hunting
Mustela vison
American mink
N America 1920s Escapee

*) negligible in brackish water

With the recent invasions and mass occurrences of M. viridis and C. pengoi in the 1990s, the Baltic Sea entered into a new era in its invasion history. However, ecological impacts of these species are not well understood and require further investigation. An essential component of this new era of invasion biology in the Baltic ought to incorporate a process-oriented approach consisting in parallel of field observations and experimental studies. As well, further information is required to fully document economic consequences of biological invasions, including direct and indirect loss of fisheries production and inpacts on tourism business.

Jellyfishes in the Baltic Sea

The jellyfish fauna of the Baltic is poor in species. In the Baltic proper (surface salinity 6-8 PSU), three species only of Scyphozoa and Ctenophora can be found (Table 2). In addition, the hydromedusa Maeotias inexpectata (M. marginata), native to the Black Sea basin, appeared in the western Estonian waters, NE Baltic proper, in 1999. Maeotias is known as an invader also from the Loire estuary in France, and from both the Atlantic and Pacific coasts of North America.

With regard to its salinity and temperature range, introduction of Mnemiopsis leidyi into the Baltic is likely. The probability of mass occurrence is low due to the low summer temparatures of the surface waters of the Baltic proper, normally not exceeding 17 C. However, the eutrophicated and warmer coastal lagoons of the southern Baltic are at obvious risk.

Table 2. Jellyfishes in the Baltic Sea from the entrance area (Kattegat, approx. 20 PSU) to the Gulf of Bothnia (5 to 3 PSU).

Species Katte-gat SW Baltic Baltic proper N Baltic Gulf of Bothnia
Haliclystus auricula



Lucernaria quadricornis



Aurelia aurita






Cyanea capillata





C. lamarckii



Chrysaora hysoscella


Rhizostoma cf. pulmo




Maeotias marginata


Beroe cucumis



B. gracilis


Bolinopsis infundibulum


Pleurobrachia pileus






Most of the NIS of brackish- or freshwater origin in the Baltic originate from warmer areas. Therefore, even slight changes in the temperature and salinity conditions (e.g. due to changes in inflow and outflow through the Danish Straits) may influence the invasion pattern and population dynamics of non-native species. If the process of global warming continues, the risks that warm water species become established in the Baltic Sea will increase.

Further reading

Baltic Sea Alien Species Database 2001. Baltic Marine Biologists’ Working Group on Non-indigenous Estuarine and Marine Organisms. Olenin, S. & E. Leppдkoski (eds).

Gollasch, S. & E. Leppдkoski 1999. Initial Risk Assessment of Alien Species in Nordic Coastal Waters. Nord 1999:8. Nordic Council of Ministers, Copenhagen. 244 pp.

Gollasch, S. & R. Mecke 1996. Eingeschleppte Organismen. In: Warnsignale aus der Ostsee. J. L. Lozan, R. Lampe, W. Matthдus, E. Rachor, H. Rumohr & H. v. Westernhagen (eds). Parey Buchverlag, Berlin, pp. 146-150.

Jansson, K. 1994. Alien species in the marine environment. Swedish Environmental Protection Agency. Report 4357, 67 pp.

Jansson, K. 2000. The marine environment. In: Weidema, I.R. (ed) Introduced Species in the Nordic Countries. Nord 2000:13. Nordic Council of Ministers, Copenhagen, pp 43-86.

Leppдkoski, E. 2001. Harmful non-native species in the Baltic Sea - an ignored problem. In: 'Baltic Coastal Ecosystems: Structure, Function and Coastal Zone Management’. Central and Eastern European Development Studies, Springer Verlag (in press).

Leppдkoski, E. & S. Olenin 2000a. Non-native species and rates of spread: lessons from the brackish Baltic Sea. Biological Invasions 2, 151-163.

Leppдkoski, E. & S. Olenin 2000b. Xenodiversity of the European brackish water seas: the North American contribution. In: Pederson, J. (ed) Marine Bioinvasions:

Proceedings of the First National Conference, Massachusetts Institute of Technology, Cambridge, Massachusetts, January 24-27, 1999, pp. 107-119.

Olenin, S. & E. Leppдkoski 1999. Non-native animals in the Baltic Sea: alteration of benthic habitats in coastal inlets and lagoons. Hydrobiologia 393, 23-243.

Olenin, S., S. Gollasch, S. Jonusas & I. Rimkute 2000.En-route investigations of plankton in ballast water on a shipsґ voyage from the Baltic Sea to the open Atlantic coast of Europe. International Review of Hydrobiology 85, 577-596.

Regional Biological Invasions Center - Group on Aquatic Alien Species (GAAS)

Vдinцlд, R. & P. Oulasvirta 2001. The first record of Maeotias marginata (Cnidaria, Hydrozoa) from the Baltic Sea: a Pontocaspian invader. Sarsia (in press).

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