Economic impacts of alien speciesIntroduced marine organisms can inflict economic losses on fisheries. The warty comb jelly Mnemiopsis leidyi, native to America, brought about a collapse of the coastal fisheries in the Black Sea 25 years ago, an area already greatly weakened ecologically at that time due to overfishing and eutrophication. Specimens that were probably introduced with ballast water were first sighted there in 1982. The jellyfish spread rapidly and ravaged native species, especially fish, by feeding on their eggs and larvae. The commercial fishery harvest decreased by around 90 per cent. In 1989 counts of 240 specimens per cubic metre of water were made, the greatest concentration of M. leidyi in the world. Only the unintentional introduction of another comb jelly, Beroe ovata – a predator – was effective in repelling the population and allowing a comeback of the fish population. Invasive species are also causing problems on the eastern coast of North America. There the European common shore crab Carcinus maenas caused a decline in the clam fishery harvest. In some cases invasive marine organisms can even present a hazard to human health. One example of this is illustrated by microalgae of the genus Alexandrium, which produce a nerve toxin. Species of Alexandrium have recently been discovered in many coastal areas where they probably did not exist just a few decades ago. Such phenomena can obviously have extremely negative effects on tourism. Introduced species are not only transported unintentionally in the ballast water of ships. Entrepreneurs often import marine organisms from other countries to nonindigenous habitats for aquaculture breeding. This may provide short-term commercial profits, but also poses the risk of imported species displacing native organisms, leading to intermediate- or long-term economic or ecological damage. Studies have shown that at least 34 per cent of the 269 introduced marine organisms investigated were deliberately imported for aquaculture breeding. One example is the Japanese giant oyster Crassostrea gigas, which has taken up residence and established itself in at least 45 ecoregions. Between 1964 and 1980 in particular, large amounts of young oysters, called spat, were imported into Europe. In many cases the ecological impact was devastating. In North America and Australia the giant oyster forms dense colonies that displace native species. Furthermore, they frequently cause eutrophication of the coastal waters because they excrete indigestible particles agglutinated with mucus, which cause the additional organic contamination of the water. The presence of giant oysters in France has also led to pollution of the waters. In addition, a decline in the zooplankton as well as larger animals has been observed. In the Netherlands and Germany the giant oysters tend to settle on blue mussel banks. This is threatening an important species of traditional fishery. It is assumed that besides the giant oyster at least 32 additional species have been unintentionally introduced into the North Sea, including the common Atlantic slippersnail Crepidula fornicata and the alga Gracilaria vermiculophylla, both of which have proven to be ecologically problematic. In order to avoid this kind of hazard in the future, a standard assessment system would be helpful. This could be used to estimate the potential of a species displacing other organisms. In addition, it could be used to weigh up the advantages and disadvantages of introducing an external species to a certain habitat. By comparing problematic and harmless imported species, experts have been trying for some time to identify characteristics that indicate a high potential for displacement of the native species. For example, some algal species float while others sink. Whether the species drifts and can thus easily disperse depends essentially on this factor. But so far it has proven difficult to draw conclusions about the displacement potential of a species based on individual traits. Perhaps it will never be possible to make confident predictions about the behaviour of a species in a new location, because numerous critical factors are at play. This prediction is further complicated as a species establishes itself in a new habitat over an extended time period, living through numerous phases. After an initial expansion phase, during which a species thrives, there usually follows a decline before the species has completely adapted to the new habitat. Before the displacement potential of a species can be confidently estimated, it has to be known which phase the species is in at a given time. But that is very difficult to determine.
Can future introductions be avoided?Caution is necessary when dealing with foreign marine organisms because species introduction is largely irreversible. Any kind of mechanical removal of established species is virtually impossible. Many species go through microscopic dormant or larval stages during which they are free-floating. During such phases the organisms defy all efforts to control them. It may be possible to introduce natural enemies to the new habitat, but then these organisms could later become a threat themselves. Government policy and environmental management will therefore have to take a stronger stance to control the primary causes of species introduction. It is important that this includes uninterrupted monitoring of aquaculture and ballast water, for example. Unilateral efforts at the national or local levels, however, will hardly be effective. International strategies practiced by all states bordering an ecoregion have greater chances of success.