バラスト水によるプランクトンの導入(総説)

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Introduction of marine plankton via ship ballast water and cultured fish and shellfish has hitherto caused serious social problems, and has greatly influenced native ecosystems. The present paper briefly reviews previous data concerning newly introduced plankton. At least 25 marine benthic species have been introduced to Japan since the early 1900's, whereas alien plankters have never been confirmed in Japan. This may be partly due to the relatively a small volume of ballast water discharged into Japanese ports. In contrast, the eastern Pacific coasts of North and South America are one of the main receiver areas for brackish and coastal zooplankters such as copepods from East Asia. Some East Asian copepods are so invasive in the USA that they appear to have highly changed the ecosystems into alien-dependent ones, and eliminated native copepods. These copepods could have been introduced either as (resting) eggs or as post-embryonic stages. When many benthic and planktonic invertebrates are simultaneously introduced, more complex ecological changes have occurred than were expected.Since introduction is mainly caused by economic activities, distinct patterns can be recognized, considering the intensity of international trade leading to intensive introduction. Fourteen major routes of transoceanic invasion are identified in the Pacific, in which the main receiver areas are Hawaii, the Pacific coasts of North and South America, and Australia, while donor areas are East Asia. In particular the San Francisco estuary has been already dominated by East Asian copepods, the Origin and evolutionary history of alien species also seems to affect their establishment. The major donor area in East Asia is a special area with a remarkably high diversity of brackish organisms that have evolved as the phylogenetically young "East Asian Initial Element" during the Miocene to Pleistocene. Hence East Asian species may potentially become strong competitors against American indigenous ones.Abundance and taxon numbers of both zoo- and phytoplankters generally declined sharply in ballast tanks within a few days after departure. However some detritus-feeding, semiplanktonic copepods propagated in the tanks to become approximately 100 times as high as the initial density at the end of the cruise. Ballast tanks can be incubators for some invertebrates rather than coffins. Some phytoplankton groups, such as dinoflagellates, diatoms and raphidophytes, are known to produce resting cysts. They are resistant and can survive during the transportation within the ship's ballast water tanks.Three main, complementary hypotheses have been proposed concerning introduction: "Propagule Supply Hypothesis", "Invasion Resistance Hypothesis", and "Enemy Release Hypothesis". Introduction is determined by density, frequency and duration of inoculation, by the condition of propagules, and by different donor areas. Establishment of introduced species is facilitated or prevented by physico-chemical and biological factors in the receiver area. Loss of predators and parasites in a newly introduced area is considered to facilitate establishment of the alien species greatly. Recently the release from parasite pressure has arthered in creasing attention to explain why alien species flourish in an introduced area. In fact zooplankters harbor many parasites and parasitoids, some of which greatly influence the host population dynamics.Many physical and chemical treatments of ballast water are applied to remove plankton in the ballast tanks, all of which are still incomplete in consideration of cost and environmental contamination. Since the rate of invasion is enhanced by increases in size and speed of modern vessels, improvement of effective sterilization methods and the issue of relevant laws is urgent.In addition the ecology of potentially invasive brackish and coastal invertebrates and algae should be studied prior to introduction in order to minimize their invasion. Long-term monitoring and assessment of ecological niches of introduced species are needed to detect changes in the ecosystem.
日本プランクトン学会の論文

バラスト水によるプランクトンの導入(総説)

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