Reprint from Schernewski G. & U. Schiewer (Eds) 2002. 'Baltic Coastal Ecosystems: Structure, Function and Coastal Zone Management'

Central and Eastern European Development Studies, Springer-Verlag, Berlin Heidelberg, pp. 253-275.

Harmful Non-Native Species in the Baltic Sea -An Ignored Problem

Erkki Leppäkoski Environmental and Marine Biology, Department of Biology, Åbo Akademi University, Turku/Åbo, Finland

ABSTRACT

A total of 100 nonindigenous species (NIS) have been recorded in the Baltic Sea (including the Katte-gat). Some 70 of these species have been able to establish reproducing populations. The Atlantic coast of North America and the Ponto-Caspian realm have been the most important source areas. Among the most successful and invasive species are the barnacle Balanus improvisus (1840s-), the bristle worm Maren-zelleria viridis (1985—) and the cladoceran Cercopagis pengoi (1992-). Seven species of the NIS com-monly occurring in the Baltic, inside the Danish Straits, have caused significant damage, namely three Ponto-Caspian species (the hydrozoan Cordylophora caspia, the predacious water flea C. pengoi and the zebra mussel Dreissena polymorpha), two North American species (B. improvisus and the American mink Mustela vison), the Japanese swim-bladder nematode Anquillicola crassus and the "ship worm" mollusc Teredo navalis.

The invasive status of the Baltic is briefly described with regard to harmful NIS. Altogether, 20 spe-cies are classified according to the type of harmful impacts they are known to cause in this area.

1 INTRODUCTION

This contribution is aimed to be a review of an environmental problem which is currently one of the most widely spread and important in the maritime world but least discussed until the mid-1990s. It is the tran-sport, intentional or most often unintentional, of non-native aquatic organisms from one coast or sea to another and their role as biopollutants. In some isolated areas, particularly on oceanic islands, current ra-tes of invasion may be more than 1 million times their natural levels (Bright 1998). This trend obviously also applies to semi-enclosed and enclosed seas, even if no comparable estimates are available. As with communities on islands, the structure of species assemblages in semi-enclosed seas is the product of immigration and extinction. No species are known to have become extinct in the Baltic in historical times, whereas immigration continues through both natural dispersal and human-mediated introduction of species.

The role of humans in altering the Baltic Sea has been studied and debated about for at least five deca-des. We became worried about the overall impacts of eutrophication, chemical pollution, overfishing, hunting and habitat destruction along coastal margins by dredging and construction works (e.g. Leppä-koski 1980; Melvasalo et al. 1981; Elmgren 1989; Rapport 1989; Thurow 1989; Rosenberg et al. 1990; Leppakoski and Mihnea 1996; Bonsdorff et al. 1997).

In the mid-1990s, at latest, we had to add human-mediated introductions of nonindigenous species to this list. The invasion of a North American polychaete (Marenzelleria viridis) in the mid-1980s and a Ponto-Caspian predacious water flea (Cercopagis pengoi) in the early 1990s, in addition to tens of historical and recent invaders already present in the Baltic, largely impacted both the benthic and pelagic subsystems with regard to their structure and function and increased both common and scientific awa-reness of bio-invasions (Leppakoski and Olenin 2000).

Non-native species such as crayfish pest, fish parasites, planktonic algae (often toxic) and bacteria can cause economic losses which may be greater than the losses caused by most individual chemical or oil discharges. These species, if invasive, may affect pristine ecosystems and native species, impact techno-

logical systems, impair or enhance economic resources and pose human health risks. In some extreme cases, they have caused severe discontinuities in the technological development of water-dependent hu-man activities, e.g. fisheries. For example, fisheries in the Black Sea collapsed in the early 1990s, when a comb jelly (Mnemiopsis leidyi) entered the sea with ships from North America (GESAMP 1997).

In historical times, many barriers to the natural spread of both animal and plant species have been weakened by human activities such as the opening up of inter-oceanic and intracontinental canals, the unintentional transportation of living organisms by ships from one area to another (fouling communities on the ships' hulls, solid ballast and ballast water used regularly since the 1880s), the transplantation of commercial species of fish and shellfish with associated organisms, etc. All these vectors contributed to what can be defined as biological pollution.

Since the 1400s (and much longer on an intraoceanic basis), ships have been an important transport vector for both aquatic and terrestrial species (Carlton 1999). Presently, most species travel in the ships' ballast tanks or attached to the hulls of ships. Globally, some 3,000-4,000 species are estimated to be in motion each day in ballast water "conveyor belts" around the world (Carlton and Geller 1993; Gollasch 1996). These artificial ballast currents are estimated to move some 10 billion m3 of water per year from sea to sea (Bright 1999). Recent revised estimates yielded a still higher number of species being in motion. There are more than 35,000 merchant vessels at sea at any given time, carrying more than 7,000 species during any given 24-hour period; if one adds all kinds of vessels, the number exceeds 10,000 (Carlton 1999).

The number of ballast and fouling species being brought into the Baltic Sea remains unknown. The obvious increase of introductions during the last 50 years into the Baltic (Gollasch and Mecke 1996; Jansson 2000; Leppakoski and Olenin 2000) and elsewhere is due to increases in the number, size and speed of ships, the successive opening of new routes of commerce in the post-war era and the expansion of aquaculture. Solutions to the problem are being intensely searched for because it is not possible to put an end to shipping; rather, it will increase as national economies increasingly integrate into the global economy (Bright 1999).

1.1 Invasion Status of the Baltic Sea

Humans have moved thousands of aquatic species around the world, sometimes intentionally but most often accidentally. These nonindigenous species (hereafter NIS) are any species that enter, with man's direct or indirect aid, an ecosystem beyond its historic range. Since 1800, at least 100 NIS have been recorded in the Baltic Sea (inch the Kattegat; Baltic Sea Alien Species Database 2001). Approximately 70 of these species became established and maintain reproducing populations (Gollasch and Mecke 1996; Jansson 2000; Leppäkoski and Olenin 2001). The Atlantic coast of North America and the Ponto-Caspian realm have been the most important source areas. In addition, at least five species are listed as crypto-genic (taxa whose status as clearly native or introduced remains undetermined; Carlton 1996, 1999); so-me of these species may represent early but overlooked invasions.

Nonindigenous species are abundant and dominant throughout the shallow benthic and fouling com-munities of the Baltic Sea — no shallow-water habitat is now free of human-mediated invaders. Their number is lowest in the northernmost part of the Gulf of Bothnia and highest in the coastal lagoons of the south-eastern and southern Baltic (Leppäkoski and Olenin 2001). In these areas, vast amounts of energy and nutrients pass through the nonindigenous set-up of species; in the lagoons, newcomers dominate ma-ny of the bentho-pelagic food webs (Leppäkoski 1984; Olenin and Leppäkoski 1999).

Because of its low salinity and low winter temperatures, the Baltic Sea is often thought to be relatively well-protected against harmful species invasions. Species are more likely to establish themselves in envi-ronments that are similar to those of their origin. However, it is important to keep in mind that most of the major harbours, even along fully oceanic coasts, are situated at river mouths. Ballast water may be taken in from the brackish part of a harbour to be discharged somewhere in the brackish Baltic; thus, the risk of successful species introductions appears to be relatively high, in fact.

There are few estimates available of the proportion of NIS of the total number of species in different parts of the Baltic. In brackish waters of the German coast of the Baltic, some 450 macrozoobenthic species have been recorded, of which 15 species, i.e. 3 %, are nonindigenous (Nehring 1999, 2000). In a benthos study off the town of Rauma (E Bothnian Sea), 22 species were recorded, among them four

NIS (=18%). Marenzelleria was found in 82% of the samples (n=141) at < 15 m depth in 1994 (Jump-panen and Räisänen 1996). In the Curonian Lagoon (SE Baltic), 16 of the about 95 benthic animal species recorded (17 %) are nonindigenous; in the inner oligohaline part of the lagoon, the ratio would be 16 to 55 (29 %; Olenin 1987; Olenin and Leppäkoski 1999).

In the Gulf of Finland alone, five new species have been recorded during the last ten years (Panov et al. 1999). In estuaries and lagoons, land, freshwater and the sea meet and make them susceptible to invasions from all these environments (Fofonoff et al. 1998). High-risk recipient areas for NIS introductions in the Baltic are the Gulf of Finland, the Gulf of Riga, the coastal lagoons and the German boddens. All of them are known as centres of xenodiversity, i.e. areas that host many well-established NIS (Panov et al. 1999; Östman and Leppäkoski 1999; Olenin et al. 1999a; Leppäkoski and Olenin 2000). In these areas, introduced species even account for the majority of the species richness. These "hot spots" serve not only as the entrance gate for many invasions into the Baltic but also as bridgeheads for secondary introductions, carried towards the inner parts of the Baltic by coastal ship traffic and recreational craft to other sites along the coast.

Considering present intensive shipping activity, future development of new ports in the eastern Gulf of Finland, the creation of new international transport routes and, consequently, also new invasion corri-dors, the Gulf of Finland can be identified as a "hot spot" area, presently and in the future, in terms of openness to NIS invasions and high potential of established invaders for negatively affecting the ecosys-tems (Panov et al. 1999).

1.2 Vectors for Primary Introductions

The main vectors for the transportation of NIS from other brackish or freshwater sites into the Baltic Sea are: (1) intentional introduction of species into adjacent freshwater bodies for stocking and aquaculture purposes, (2) active or passive intra-continental dispersal via canals (opened from the 1710s to 1952) between the rivers that belong to the catchment areas of the Baltic, Black and Caspian Seas, and (3) un-intentional transport with ships' ballast water and hull fouling (Leppäkoski and Olenin 2000; Olenin et al. 2000). Presently, ship traffic is the most important vector for the spreading of aquatic organisms into north-western Europe, including the Baltic Sea (see above). Investments in waste water treatment plants have, since the 1960s and 1970s, improved the water quality in most harbours and urbanised coastal are-as. Reduced pollution is believed to have facilitated species introductions. The ameliorated donor areas have become richer in species and the recipient waters more hospitable for a great variety of invaders (Carlton 1995). On the other hand, NIS are generally expected to be common in habitats that are open and disturbed and, therefore, poor in natural enemies and competitors (Holdgate 1986; Moyle and Light 1996).

The role of aquaculture in distributing NIS, other than fish diseases and parasites, into and within the Baltic Sea is negligible. The business of oyster culture, globally a major vector for NIS introductions, is of no importance in brackish water. Successful dispersal through releases of NIS into watersheds by aquarium hobbyists or from research laboratories seldom occurs in northern latitudes.

1.3 Expansion within the Baltic Sea

There are four successive stages of biological invasions: (1) arrival, (2) establishment, (3) spread and (4) persistence (Mollison 1986). Many disperse but few establish. Not all introduced species can persist and behave invasively in the Baltic - less than a dozen of them have a basin-wide distribution nowadays. Approximately two-thirds of the 46 species of bottom-living invertebrate NIS discovered in the Baltic Sea have a pelagic larval stage enabling their within-basin spread by currents or by regional or local ship traffic (Leppäkoski and Olenin 2000).

One of the most aggressive NIS in the Baltic is the bay barnacle Balanus improvisus that became es-tablished in western Europe in the 19th century. Probably introduced to Europe by hull fouling of ships from North America, it was first recorded in the Baltic Sea in 1844 at Königsberg (presently Kalinin-grad) (Gislén 1950; Luther 1950). From this likely dispersal centre, it spread rapidly and became com-mon, especially in ports. It may have invaded most of its present new range in the 1870s or, at the latest, before 1900. However, there were few records of it from the Swedish east coast before the 1920s (Gislen

1950). It was not found north of the Aland Islands before 1950 (Luther 1950), while in the 1990s, B. im-provisus has been recorded up to the Northern Quark (64°N; Leppäkoski 1994). Today, it occurs from the Gulf of Bothnia and Gulf of Finland to the west coast of Sweden (Jansson 1994, 2000; Leppäkoski 1999).

B. improvisus is the most important fouling organism and the only barnacle species living in the coas-tal waters of the Baltic proper and the gulfs. It causes marked habitat alteration through the construction of dense crusts on hard surfaces and secondary hard substrates (e.g. Olenin and Leppakoski 1999). This barnacle can survive in freshwater - it was already found in the late 1860s in south-western Finland in the lower Aura River in the middle of the city of Turku, associated with obligate freshwater organisms (Luther 1950). The approximate (minimum) rate of spread for B. improvisus from Königsberg (1844) to Turku (1868) was 30 km yr-1 (Leppäkoski and Olenin 2000).

Other rapidly dispersing NIS in the Baltic Sea are the fish-hook water flea Cercopagis pengoi (see be-low) and the North American spionid polychaete Marenzelleria viridis that was first recorded in the sout-hern Baltic in 1985 (Bick and Burckhardt 1989). Marenzelleria was recognised as a potential invader in the whole Baltic. Its pelagic larvae can achieve an abundance of up to >21xl06 ind m-3 (Bochert and Bick 1995); all developmental stages can tolerate salinities of < 1 PSU (adults even down to 0.03 PSU); even if successful settling is restricted to areas with a salinity of > 5 PSU (Bochert et al. 1996; Kube et al. 1996; Daunys et al. 2000), the distribution range follows the 15 PSU isohaline; the benthic stages are highly mobile; adults can cope with short-term hypoxia (Kube et al. 1996; Daunys et al. 2000). By 1990-1995, Marenzelleria expectedly expanded its distribution into the eastern Gulf of Finland and southern Bothnian Bay and became a major faunal element in several coastal areas (Kube et al. 1996; Zmudzinski 1996; Stigzelius et al. 1997; Daunys et al. 1999; Gruszka 1999).

2 RESOURCES AT RISK

2.1 Nuisance Species

Nonindigenous species presently or potentially threaten ecological processes and natural resources. Cha-racteristics of a species in its native community do not necessarily predict its performance in and impacts on the invaded novel community. In urgent situations, such as with rapidly spreading new invaders, there is no local information on likely impacts available. Extrapolations from what is known about their beha-viour in their native range may be risky (Fofonoff et al. 1998; Ruiz et al. 1999). Thus, any release of bal-last water or a single NIS can be considered as "ecological roulette" (Carlton and Geller 1993). Several of the NIS recorded in north-western European waters are known to adversely affect human life, causing economic impacts and (small-scale) health effects. Therefore, the pervasive feeling that "it won't happen here because we are different" (Cairns and Bidwell 1996) is false and misleading. There is scientific proof for inoculation events to take place, at a particular site, along the whole salinity gradient of the Baltic Sea, from the Kattegat (> 20 PSU) in the west to the diluted, innermost parts of the Baltic (< 1 PSU).

One of the main goals of the science of invasion biology is to understand and minimise, if possible, economic and ecological impacts of nonindigenous nuisance species that become established. Aquatic nuisance species (ANS) are defined as "nonindigenous species that threaten the diversity or abundance of native species, the ecological stability of infested waters or commercial, agricultural, aquacultural or rec-reational activities dependent on such waters" (Aquatic Nuisance Species Task Force 2000).

2.2 Economic Impacts

In the Baltic Sea, approximately 70 NIS are more or less naturalised. Out of this number, less than 20 species (i.e. less than 30 %) can be classified as nuisance species (Table 1) that cause damage to under-water constructions, fisheries, shores and embankments or target species for hunting. Several species of phytoplanktonic algae can cause hygienic problems or risks (toxic blooms). For the sake of comparison, 10 % of NIS have caused significant economic or ecological damage in the North American Great Lakes (Cairns and Bidwell 1996). In 1993, approximately 15 % of the 4,500 NIS, both aquatic and terrestrial, established in the US were thought to be nuisance species that have significant ecological or economic impact (Ruiz et al. 1997).

How to define a "significant" impact? In my opinion, seven species of the NIS commonly occurring in the Baltic, inside the Danish Straits, have caused significant damage, namely three Ponto-Caspian spe-cies (the hydrozoan Cordylophora caspia, the predacious water flea Cercopagis pengoi and the zebra

mussel Dreissena polymorpha), two North-American species (the barnacle Balanus improvisus and the American mink Mustela vison), the Japanese swim-bladder nematode Anquillicola crassus and the "ship worm" mollusk Teredo navalis, believed to be of Indo-Pacific origin (the status of T. navalis as a NIS in Europe is uncertain, however; Nehring 1999, 2000). What is important from the human point of view is that the economic damages caused by them are irreversible and most often not manageable. NIS are liv-ing organisms that tend to reproduce and spread in contrast to chemical pollutants and most other forms of environmental degradation that cannot reproduce and tend to dissipate over time.

On the other hand, several alien species have appeared to be beneficial, in some way or other, in man's economy in this area (muskrat and Canada goose as hunting objects, rainbow trout, peled whitefish and round goby of some interest for local fishermen, Cercopagis and planktonic larvae of benthic species as food items for the Baltic herring and other commercial fish).

In economic terms, attempts to evaluate the damages and benefits related to NIS can yield (cf. Turner et al. 1996) (1) direct use values (outputs measurable as fish yield, recreational values, transport); (2) indirect use values (functional values, e.g. the value of changes in productivity, preventive expenditu-res); (3) non-use values (value of passing on the Baltic Sea in an intact way to future generations). Many of the ecosystem services or damages related to NIS as well as the novel functions provided by them in the Baltic (Olenin and Leppäkoski 1999) are typically non-market goods. Indirect economic losses or be-nefits resulting from NIS are therefore not possible to assess. Valuing costs or benefits for direct uses (fishery, industrial water use, sea traffic and other assets potentially at risk) is needed to consider policy options of crucial importance for coastal zone management.

Our knowledge of the costs and benefits related to NIS in the Baltic Sea is still very fragmentary or non-existent. Therefore, only some few examples can be given here. Most of the NIS do not, however, have any influence on man's economic or societal interests.

Water-Based Technology

The technological impact of aquatic NIS is mainly the result of the sessile mode of life of some of the most common invaders and thus their ability to foul intake pipes for municipal and industrial water inta-kes. Such installations provide ideal habitats for biofouling species by providing a continuous supply of food carried by currents, protection for predators and often higher temperatures (Cairns and Bidwell 1996). Biofouling causes security risks and a considerable drop in cooling capacity even in the brackish Baltic. Of the NIS occurring in the Baltic coastal waters, mainly three fouling species, the hydrozoan Cordylophora caspia, the barnacle Balanus improvisus, and the bivalve Dreissena polymorpha, cause economic damage to industries and power plants where cooling water from the sea is used (Laihonen and Vuorinen 1981; Gollasch and Mecke 1996).

Some exotics interfere with boating, resulting from various impacts of hull fouling, as summarised by Cohen and Carlton (1995): increased frictional resistance of boat hulls, resulting in slower speeds, incre-ased transit times, increased fuel costs, and reduced manoeuvrability. A 1 mm thick coating of slime on a boat hull can increase skin friction up to 80 % and reduce speed up to 15 % (Gordon and Mawatari 1992; quoted in Cohen and Carlton 1995). Additional environmental costs result from antifouling coat-ings and costs of pollution from the use of antifouling compounds formulated with, e.g., tri-butyl-tin and copper. In the Baltic, increased maintenance requirements and the common use of antifouling paints are due not only to the occurrence of filamentous green algae but mostly to the mass occurrence of barnacles attached to boat and ship hulls.

The ship worm Teredo navalis was likely brought to Europe from East Asia several centuries ago. It tolerates much fresher water than do most marine wood borers (Cohen and Carlton 1995) and is now ful-ly established in the south-western Baltic region (Gollasch and Riedel-Lorje 2000; Gollasch and Rosent-hal 2000). This boring species has major direct economic impacts in this part of the Baltic. For example, it has caused approximately $ 15 to 25 million damage to submerged wooden installations along the German east coast alone since 1995 (K. Hoppe, pers. comm.; Gollasch and Riedel-Lorje 2000). The ship worm also causes damage to marine archaeological objects.

Fisheries

Impacts of NIS on commercial and recreational fishing and aquaculture occur through both direct and indirect effects. Many pelagic and benthic invertebrate NIS form part of the trophic webs that support recreationally and commercially important fish. Their indirect "bottom-up" impacts on commercial and sport fishery are difficult to assess. In coastal lagoons, the impact may come from zebra mussel filtering, resulting in a reduction in phytoplankton and plankton-eating fish.

A still more important recent impact on the Baltic pelagic ecosystem is the obvious dietary overlap of the strongly expanding cladoceran Cercopagis pengoi with planktivorous fish. This may result in a dec-lined food basis for important commercial fish (herring and sprat) and decreased planktivorous fish production (Ojaveer et al. 2000). Cercopagis was most probably transferred to the Baltic Sea from its Ponto-Caspian area of origin with ballast water. The species was first discovered in the Gulf of Riga and the Gulf of Finland in 1992 (Ojaveer et al. 2000). In the Gulf of Riga, it comprised 25 % of the total zoo-plankton biomass in 1995 (Ojaveer and Lumberg 1995; Ojaveer et al. 1998). Secondary within-basin in-troductions are assisted by currents. By 1995, Cercopagis colonised the whole Gulf of Finland. In 1997, the species was reported from the Stockholm archipelago and appeared in the Baltic proper (the Gotland Basin; Gorokhova et al. 2000). Due to the exceptionally warm summer of 1999, the expansion of its distribution area further north (Gulf of Bothnia) and south (Gulf of Gdansk) took place (Uitto et al. 1999; Zmudzinski 1999; Ojaveer et al. 2000; K.-E. Storberg, pers. comm.). Cercopagis was recorded in high densities in both the Curonian and Vistula Lagoons as well as in the adjacent open sea (Hornatkiewicz-Zbik 1999; Gasiunaite 2000; Naumenko 2000).

Cercopagis pengoi is a euryhaline species - the wide range of optimal salinity (up to 10 PSU) does not restrict its spreading throughout most of the Baltic Sea (Ojaveer et al. 2000). The average density of Cercopagis off Kotka, eastern Gulf of Finland, was estimated at 300 ind m-3 in 1997; maximum abun-dances of 1,800 ind m-3 have been reported from the Gulf of Finland (Uitto et al. 1999) and 800 ind m3 from the Gulf of Riga (Ojaveer et al. 1998). The species usually appears in the plankton community at water temperatures over 15 °C and starts to disappear when the temperature falls below 8 °C (Ojaveer et al. 2000).

Direct impacts on fishing gear, such as clogging of reels and fouling of nets, make Cercopagis a nui-sance species in invaded waters. This may cause substantial economic losses in fisheries. The estimated loss in one fishery enterprise in the eastern Gulf of Finland averaged a minimum $ 50,000 in the 1996-1998 period; these losses were caused by the drastic decline in fish catches in the coastal zone due to the fouling of fishing equipment by Cercopagis (Panov et al. 1999). This figure does provide some quanti-fication of the scale of potential economic impact of a single introduced organism. By 1999, biofouling (clogging) of fishing equipment by Cercopagis became a serious problem and caused problems, espe-cially to whitefish fisheries, in the eastern Gulf of Finland (Panov et al. 1999; this volume), inner parts of the Archipelago Sea, northern Bothnian Sea and in Lithuania (K. Hakkila, K.-E. Storberg, and I. Ole-nina, pers. comm.).

Cercopagis also became an important food item for plankton-eating fish (e.g. Antsulevich and Vali-pakka 2000). In the Gulf of Riga, the mean percent contribution of Cercopagis in herring stomachs bet-ween 1994 and 1998 varied from 0-0.1 % in June and July to 11-17 % in September and August, respec-tively. In July 1999, Cercopagis made up 59 % (wet weight) of herring diet; stomachs of 66 % of her-rings contained this species (Ojaveer et al. 2000). The most abundant pelagic fish (herring, stickleback and smelt) can prey on Cercopagis, potentially reducing its abundance. In unfavourable years, Cercopa-gis remains a rare prey item in fish diet, accounting for an average of 3-7 % of recovered prey items, whereas the proportion of it in the herring diet can be about one half during the peak periods of abun-dance of Cercopagis (Antsulevich and Välipakka 2000; Ojaveer et al. 2000).

Cercopagis appears to be a successful invader in the Baltic Sea. Within 5 to 7 years after its first ap-pearance, it colonised the sea from the Gulf of Gdansk (54°N) to the northern Bothnian Sea (62°N). Further spread of Cercopagis will be monitored carefully (Gorokhova et al. 2000), the species being one of the few recent introductions and obviously the most important one in both ecological and economic terms into the pelagic subsystem of the Baltic Sea. Ongoing (e.g. Uitto et al. 1999; Ojaveer et al. 2000) and future research will help us to understand the ecological role of this invasive species (especially the potential impacts that Cercopagis will have on the Baltic food web) and assess its economic impact in

the Baltic Sea. If the density of Cercopagis does not decrease significantly in the near future, it may seriously affect the commercial fisheries.

Introductions of Pacific salmonids and Siberian sturgeons in the 1960s failed and did not result in any enhancement of commercial fisheries (Koli 1990). In the Gulf of Gdansk, since the mid-1990s, the Ponto-Caspian round goby Neogobius melanostomus supports the Baltic's only recreational small-scale fishery based on an introduced species (Skora 1997). There are no other economic benefits that derive from introductions of fish into the Baltic.

Parasites and Pests on Fish and Shellfish

It can be predicted that introduced species harbour poor communities of ecto- and endoparasites: they are not readily susceptible to invasions of specialised parasites of native host species and, on the other hand, they have not had sufficient time to acquire generalist parasites from native hosts (Kennedy and Pojman-ska 1996). Among introduced parasites on fish, the eel swim-bladder nematode Anguillicola crassus, native to Southeast Asia, was first observed in Denmark in 1986. In Sweden, it was first discovered in areas affected by the discharge of cooling water from power plants (Höglund and Thomas 1992). It now occurs in the Baltic at salinities up to 8-10 PSU, where native fish such as ruffe, smelt and black goby serve as intermediate hosts (Jansson 2000). In Germany, up to 90 % of the eels caught are infested (Gol-lasch and Rosenthal 2000). In the outer parts of the Baltic, some NIS occur as pests on shellfish.

Damage Caused to Target Species for Hunting

Predation from the American mink has caused considerable damage among, e.g., crevice-nesting seabirds in the outer archipelagos and contributed to declines in populations of black guillemot Cepphus grylle and razorbill Alca torda, whereas eider ducks Somateria mollissima are less vulnerable to mink predation (Nummi 1996).

Damage Caused to Shores

Some NIS burrow in banks and man-made embankments. Muskrat in the sheltered northern Baltic coastal areas and the Chinese mitten crab along the German rivers cause limited damage only to the shorelines, walls of ditches and river banks. The crab is known to dig holes up to 80 cm deep in river banks (Nehr-ing and Leuchs 1999). In some places, muskrats have been burrowing into muddy shores inside the reed belt for over 50 years and created holes along the shoreline.

Interference with Research and Monitoring

Inevitably, every single species introduction and its establishment in a novel region and ecosystem opens new opportunities for ecological research. These most often unintentional "transplantation experiments" can be used for the studies of concepts such as adaptive strategies, niche dimensions, interspecific rela-tionships, dispersal mechanisms etc. However, they also result in reduced research possibilities in bio-geography (it is difficult to explain causalities behind the present distribution of species) and population genetics.

In the Baltic Sea, benthic communities have been monitored by quantitative methods since the 1910s. The results of this long-term effort, based on international sampling programmes, may be invalidated by the introduction of any successful NIS that becomes dominating, utilises the space and energy resources in different manner and rate and restructures the food webs. For example, the soft-bottom community was totally changed by the polychaete Marenzelleria viridis in the Vistula Lagoon, where it became a biomass dominant over sandy and muddy habitats in the mid-1990s, reaching 216 g m-2 and making up to 95 % of total community biomass (Zmudzinski 1996).

2.3 Human Health

NIS invasions can also pose health risks, mainly due to the global transfer of dinoflagellates and other potentially toxic planktonic species in the ships' ballast tanks. The ship-mediated spread of viruses and bacteria, among them human pathogens (e.g. Vibrio cholerae), is common (Ruiz et al. 1997, 2000). Several species of phytoplankton that are known or believed to be nonindigenous have been recorded from the western Baltic region. The potentially PST (Paralytic Shellfish Poisoning) inducing species Ale-xandrium tamarense and A. minutum (Dinophyceae) have been found along the Swedish west coast (Godhe and Wallentinus 1999). Other potential PST producers or cysts of them, such as Gymnodinium and Gyrodinium, have been found in Danish waters, in the Kiel Bight, Germany (Gollasch and Mecke 1996; Nehring 1996), and on the Swedish west coast (Godhe and Wallentinus 1999). The invasive status of several phytoplankton species such as Coscinodiscus and Thalassiosira spp. is still unclear - several of them are species with a wide distribution. Generally, the appearance of a new species in phytoplank-ton samples does not necessarily indicate a recent introduction; there is always the possibility that a spe-cies has been overlooked in previous studies or that its taxonomic status is uncertain or has been changed (Nehring 1998b).

Introduced pathogenic organisms that affect the human population are not known from the Baltic Sea. Canada geese (Branta canadensis) droppings along beaches and shorelines contain bacteria that may cause diseases; the aesthetic problems may hinder free utilisation of beaches (Weidema 2000).

2.4 Ecological Impacts

In most cases, the ecological impacts of NIS on the Baltic Sea ecosystem have been more or less un-observable. Several introductions into the Baltic have, however, resulted in both structural and functional changes at the ecosystem level; in fact, food chains and whole bottom-living communities can be based upon introduced species in the most heavily invaded coastal lagoons of the Baltic (Leppäkoski 1984; Olenin and Leppäkoski 1999).

Ecological impacts of any individual species can be assigned to at least one of nine impact categories (Ruiz et al. 1999): competition, habitat change, food-prey, predation, herbivory, hybridisation, parasi-tism, toxicity or bioturbation. There are examples of all (except hybridisation?) of these impacts on spe-cies interactions and ecological processes among the established NIS in the Baltic (Olenin and Leppä-koski 1999; Jansson 2000). In most cases, however, impact information is limited to qualitative data whereas quantitative studies of ecological effects are rare, especially for marine invasions (Ruiz et al. 1997; Fofonoff et al. 1998). It is possible to distinguish between types of impact but not to assess scales of impact; much remains to be explored in this field of invasive biology.

In the Baltic Sea, NIS are known to (1) modify rocky bottom or sediment substrate, (2) provide refu-ges from predators, (3) trap and accumulate particles, (4) affect macrophyte canopy, (5) redirect energy from pelagic to benthic subsystems or vice versa, (6) provide prey for planktivorous and/or benthivorous fish, (7) provide food for waterfowl and (8) exclude competing species (Stewart et al. 1998, 1999; Olenin and Leppäkoski 1999).

3 WHAT CAN BE DONE?

3.1 Actions Considered to Address the Problem

What measures can be taken to prevent the unintentional introduction of NIS into the Baltic and other semi-enclosed seas of Europe? In addition to monitoring the further spread of NIS and studying their biology and ecology in the invaded parts of the sea, no actions have been undertaken to address the prob-lem specifically in the Baltic Sea area. As a basis for future work, impact evaluations and checklists comparing pros and cons that might arise from species introductions should be prepared in advance (Ro-senfield 1992), especially for the most likely target species to become established in the future (Gollasch and Leppäkoski 1999).

Recent Research Efforts

In co-operation with Finland, Ireland, Lithuania, Norway, Sweden, the United Kingdom and the IMO, Germany coordinated a two-year (1998-1999) Concerted Action (within the EU MAST programme) en-titled "Testing Monitoring Systems for Risk Assessment of Harmful Introductions by Ships to European Waters". The Concerted Action (2000) focused on comparing and harmonising sampling methods for ballast water, describing state-of-the-art ballast water studies, case histories of selected NIS (Gollasch et al. 1999), assessing potential treatment options of ballast water, developing public awareness material and assessing European waters as potential donor areas. The first shipping study, during which the sur-vival of planktonic organisms was followed up en route from St. Petersburg to Lisbon, was carried out in 1998 (Olenin et al. 2000).

International conventions. With no way to eradicate the well-established populations of NIS such as the zebra mussel, Marenzelleria and Cercopagis or to control their further spread in the Baltic Sea, one can only try to prevent their spread to adjacent freshwater bodies or to other brackish-water areas of the world and prevent further unwanted introductions into the Baltic (Gollasch and Leppäkoski 1999).

Internationally, there is an increasing interest in finding technical and legislative solutions for the problem of unwanted introductions via shipping. The risks associated with uncontrolled species transfers have been recognised since the 1970s, even if the research into preventive and combating methods is of more recent date. The International Maritime Organisation (IMO) and the International Council for the Exploration of the Sea (ICES) co-operate in developing globally applicable guidelines to reduce the risk of transferring marine NIS.

One of the key documents is the IMO Assembly Resolution A.868 (20) (1997) "Guidelines for the Control and Management of Ships' Ballast Water to Minimise the Transfer of Harmful Aquatic Orga-nisms and Pathogens". According to this resolution, ships shall be provided with a ballast water manage-ment plan, identifying appropriate ballast water treatment options. IMO has ratified protocols and (vo-luntary) guidelines for handling ballast water, according to which (1) protocols should be kept, listing when and where ballast water is taken on board; (2) ballast water should be exchanged at sea; (3) taking on board ballast water from shallow areas where the water is turbid should be avoided; (4) ballast water should not be taken on board during algal bloom periods; and (5) sediment from tanks should be depo-sited in approved places on arrival.

The ICES approved the 1973 Code of Practice on the Introductions and Transfers of Marine Orga-nisms (updated in 1994). The code is based on quarantine measures and has to be used on every planned species introduction to control the intentional introductions of target species and to minimise the unin-tentional introduction of potentially harmful species.

3.2 Technical Solutions for Handling Ballast Water

A big tanker ship can carry up to 200,000 m3 of ballast water - enough to fill 2,000 Olympic-sized swim-ming pools (Bright 1998) - and a species-rich assemblage of organisms in it. Several methods for the kil-ling of organisms in ballast water have been proposed (e.g. Gollasch 1997), such as the use of chemical control agents, heating, oxygen depletion, alternating current fields at the ballast water intake, treatment plants on board (filtration, ultraviolet light, ultrasound) and land-based plants or specialised ships (tan-kers) to which the ballast water is transferred to for treatment in the harbour. Presently, mid-ocean bal-last water exchange represents a practicable, even if not fully effective, method for reducing the risk of further introductions of freshwater and brackish water organisms (e.g. Gollasch and Mecke 1996; Gol-lasch and Leppäkoski 1999). Vessels destined for any Baltic Sea harbour have to pass through marine waters with oceanic salinities. However, this method is of limited value for ships moving along coastli-nes and not diverting to the open ocean (Carlton 1999), e.g. on voyages between the major harbours on the North Sea coast and the Baltic (Olenin et al. 2000).

Several control strategies against biofouling NIS are available for industrial applications. Chemical methods include the use of coatings that are toxic or inhibit settling and intermittent short-time chlori-nation (Laihonen et al. 1985; Cairns and Bidwell 1996 and references therein). Non-chemical methods for mitigation of fouling problems are generally based on the design of water intake structures such as deeper water intakes, a flow velocity high enough to prevent settling in all parts of the water supply sys-

tem and parallel pipelines for cooling water intake, one of which is kept closed for 3 to 4 weeks to cause oxygen deficiency (Vuorinen et al. 1983; Laihonen et al. 1985).

3.3 Biocontrol - a Realistic Option?

The introduction of predators, competitors, parasites and disease agents for pest control is a risky option: there are several examples from terrestrial ecosystems of how the control species became a pest itself (Cairns and Bidwell 1996; Lafferty and Kuris 1996; Simberloff and Stiling 1996). Marine environments differ from terrestrial ones in some important respects. The eradication of NIS by mechanical or chemi-cal (pesticides) measures is normally not possible once a marine NIS has become established. Therefore, taking preventive measures and, in a few cases, early detection and immediate response remain the only realistic options to control invasions of harmful species. Another difference is the open recruitment in marine systems; larvae disperse widely, and offspring rarely settle near their parents (Lafferty and Kuris 1996). Therefore, local actions taken against NIS have minor or no impact on future recruitment and spread of invaders.

4 PERSPECTIVES FOR THE FUTURE

Even if we are certain that there will be further invasions in the Baltic, we are not able to predict exactly which species (or trophic guilds of invaders) will invade and when they will invade. An attempt was ma-de in the late 1990s to develop a semi-quantitative model for risk assessment (low-medium-high risk) for Nordic coastal waters (Gollasch and Leppakoski 1999). In this model, 13 different parameters were inc-luded and assessed for five harbours along the salinity gradient from St. Petersburg to the Atlantic coast of Norway: ship arrivals, duration of voyage, volume of ballast water released, number of ship yards, matching climate, matching salinity, salinity gradient, number of estuaries in the area, aquaculture activi-ties, degree of eutrophication, number of macrozoobenthos species, and previous primary and secondary introductions.

Many, if not all, NIS presently occurring in the Baltic Sea are native to warmer climatic conditions than those prevailing in the recent Baltic area. Therefore, the coastal Baltic is vulnerable to the range of possible impacts from global warming. Even a slight elevation of water temperature should favour the further spread of NIS (cf. Nehring 1998a, 2000; Dukes and Mooney 1999; Ruiz et al. 1999), support a further increase in abundance, increase the probability of new species introductions and increase the risk of invasions from the sea into the lakes of Finland, Sweden, Estonia and north-western Russia (La-doga) connected with the invaded parts of the Baltic by sea traffic. There is recent evidence of species introductions from the Baltic to the Finnish lake district where the mitten crab (Eriocheir sinensis) was first found in 1998 (Valovirta and Eronen 2000).

In a great majority of cases, the Baltic ecosystem has been able to assimilate NIS and continue to fun-ction in an altered state. Three of the most recent introductions (the predatory water flea Cercopagis, the round goby Neogobius and the bristle worm Marenzelleria) can, however, be expected to cause large-scale changes in the structure and function of the Baltic Sea ecosystem on a basin-wide scale. More knowledge is needed about negative economic impacts of these and other NIS in brackish and freshwater environments as well as on species which may behave invasively and become harmful in new areas. The-se goals can only be achieved by training students and scientists and informing decision makers as well as the public about the risks related to NIS and ballast water issues. An equally important goal is to in-volve NIS into monitoring programmes and guarantee proper exchange of information.

The majority of known harmful NIS were not pests in their area of origin. There is no way to predict how a NIS is going to behave in a new habitat with an area-specific set of abiotic and biotic factors that tend to regulate (or do not) its population growth. Therefore, treating all invasive species as guilty until proven innocent is the only environmentally sound approach. Once an aquatic invader is present in high enough numbers to be recorded, it is too late to attempt to eliminate it. The prevention of further arrivals is the only way to go and thus a crucial issue for the European marine biosecurity strategy.

ACKNOWLEDGEMENTS

This is a contribution of the project "Initial Risk Assessment of Alien Species in Nordic Coastal Waters", supported by a funding of the Nordic Council of Ministers.