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Introduced Species

Marine organisms can move or be moved to a new sea area either by natural vectors such as water currents or as the result of human activities, in particular shipping (in ships' ballast water and as hull fouling) and aquaculture.

If such an alien (foreign, exotic, non-native, non-indigenous, introduced, neobiota, naturalized — names that are often used interchangeably) organism or species becomes established in the new sea area, then it is considered to be an introduced species.

Those introduced species that are harmful to biodiversity are often called invasive, aggressive, noxious, pest or harmful species.

Shipping and aquaculture are responsible for about 90% and 10%, respectively, of the introductions of marine alien species in Europe.

WWF-Germany 2004

The introduction of alien species [1] via marine aquaculture activities may be either intentional or accidental. Some species of finfish, shellfish (molluscs, crustaceans, and echinoderms) and aquatic plants (including seaweeds) are imported from other sea areas specifically for cultivation, either extensively for commercial reasons or otherwise for the aquaria trade or research. Some, such as mussels and oysters, are deliberately introduced to a location in the wild; others, such as farmed salmon, occasionally escape into the wild and establish themselves. In other cases, aquatic "hitchhikers", such as disease-causing and parasitic organisms or small larval forms of invertebrates, are unintentionally imported together with the species intended for cultivation.

Whether intentional or accidental, introduced species — both larval and adult stages — occupy similar niches in the marine environment to native species, with potentially serious impacts on the native species, for example by competing with them for food and/or space, or by interbreeding and so altering the gene pool. Introduced species may also potentially alter habitats and the balance of existing communities, resulting in changes to the structure and function of entire marine ecosystems [2]. Such ecological impacts and the consequent loss of biodiversity may not be detected during pilot studies. The consequent ecological and socio-economic cost may be profound. Therefore, key international agreements and instruments (e.g. the Convention on Biological Diversity) play a vital role in requiring international, regional and national level measures to prevent, reduce and control the introduction of alien species.

International conventions regulating introduced species

The United Nations Convention on the Law of the Sea came into force in 1994. Article 196 on the use of technologies or introduction of alien or new species requires states to take all measures necessary to prevent, reduce and control "the intentional or accidental introduction of species, alien or new, to a particular part of the marine environment, which may cause significant and harmful changes thereto".

The 1992 Convention on Biological Diversity (CBD): "Each Contracting Party shall, as far as possible and as appropriate - prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species".

The 1995 FAO Code of Conduct for Responsible Fisheries: "9.2.2 States should, with due respect to their neighbouring States, and in accordance with international law, ensure responsible choice of species, siting and management of aquaculture activities which could affect transboundary aquatic ecosystems. 9.2.3 States should consult with their neighbouring States, as appropriate, before introducing non-indigenous species into transboundary aquatic ecosystems." … "9.3.1 States should conserve genetic diversity and maintain integrity of aquatic communities and ecosystems by appropriate management. In particular, efforts should be undertaken to minimize the harmful effects of introducing non-native species or genetically altered stocks used for aquaculture including culture-based fisheries into waters, especially where there is a significant potential for the spread of such non-native species or genetically altered stocks into waters under the jurisdiction of other States as well as waters under the jurisdiction of the State of origin. States should, whenever possible, promote steps to minimize adverse genetic, disease and other effects of escaped farmed fish on wild stocks. 9.3.2 States should cooperate in the elaboration, adoption and implementation of international codes of practice and procedures for introductions and transfers of aquatic organisms. 9.3.3 States should, in order to minimize risks of disease transfer and other adverse effects on wild and cultured stocks, encourage adoption of appropriate practices in the genetic improvement of broodstocks, the introduction of non-native species, and in the production, sale and transport of eggs, larvae or fry, broodstock or other live materials. States should facilitate the preparation and implementation of appropriate national codes of practice and procedures to this effect."

The threat from introduced species was also emphasised in the Plan of Implementation of the World Summit on Sustainable Development, Johannesburg 2002, which called for actions at all levels to: "Strengthen national, regional and international efforts to control invasive alien species, which are one of the main causes of biodiversity loss, and encourage the development of effective work programme on invasive alien species at all levels".

In marine aquaculture, organisms are frequently moved (transplanted) from one location to another within their native distribution range. For example, shellfish spat (seed or spawn) may be dredged from subtidal beds and moved to areas where they will have a better chance of survival and produce a better crop. Apart from the physical impact of dredging on the natural habitat, there are concerns about genetic impacts and disease associated with organism transfer.

Intentionally introduced species

Transboundary issue

The introduction of marine organisms takes place across ecological boundaries, not state borders. However, the intentional introduction of an alien species by an individual state could impact an entire marine region.

One example of an intentionally introduced species in Ireland is the Pacific oyster (Crassostrea gigas), which has been imported from Japan because it is larger and faster growing than the native oyster (Ostrea edulis), otherwise known as the European flat oyster. Heffernan (1999) points to several potential problems associated with bringing the Pacific oyster into Ireland:

Genetic implications
In general, the genetic implications depend on whether introduced and/or transferred organisms are capable of breeding and establishing a self-sustaining population and, if so, whether they are likely to out-compete native species and populations.

The genetic effects of introductions on native populations may be defined as direct or indirect. Direct effects occur when the gene pool of the native population is open to the invasion of genes from the introduced population. Indirect effects occur when hybridisation between the native and the introduced population is not possible, but alterations in gene frequencies result from ecological interactions with the introduced organism. Only the indirect effects apply to Pacific oyster cultivation, as they cannot hybridise with the native oyster.

Heffernan (1999) states that the likely impact of Pacific oyster cultivation will be negligible as, although it has been observed spawning in Ireland in Donegal Bay in 1993, it has not been recorded as establishing wild populations: "This is presumably due to the limited occurrence of sufficiently high temperatures for successful reproduction." However, ecological interactions will drive genetic changes in both the native (Ostrea edulis) and the introduced species (Crassostrea gigas) and, therefore, climate change and ocean warming in particular "may enable the Pacific oyster to spawn successfully and so cause other impacts, genetic and well as ecological" (Heffernan 1999).

Transfers of non-target species
There are numerous examples of the introduction of parasites and pathogens and other unwanted (non-target or "hitchhiker") organisms with shellfish in general.

Minchin et al. (1993) discovered that consignments of Pacific oyster certified as being free of Bonamia, Marteilia and other species actually harboured Mytilicola orientalis, Myicola ostrea, Crepidula fornicata [slipper limpet], Ostrea edulis and Mytilus edulis. The biomass of the importations and the frequency of Mytilicola orientalis and Myicola ostrea in the consignments suggest that they may become established in Irish waters. Furthermore, recent research has confirmed that the copepod Mytilicola orientalis is now established in Ireland.

Minchin et al. (1993) make the point that the discovery of Ostrea edulis and Mytilus edulis in Pacific oyster consignments is worrying as they are both vectors of Martelia refringens and in the case of the protozoan Bonamia ostreae, Ostrea edulis is a vector. The presence of Crepidula fornicata also has serious implications as should it become established there may be significant changes in (a) trophic competition, (b) changes in the texture of the seabed and (c) modification of the benthos. Another species which was not detected by Minchin et al. (1993), but which is also high risk is Sargassum muticum. Being a monoecious species a single plant can result in the development of a whole population and come mature within a year (Minchin et al. 1993).

Phytoplankton species have also been imported with live consignments of oysters. In fact, sixty-seven species of phytoplankton (43 diatoms, 22 dinoflagellates and 2 silicoflagellates) were recorded in addition to other microspecies such as foraminiferans and tintinnids. Fifteen types of dinoflagellate cysts were recorded. There is concern that potentially harmful species of phytoplankton may be imported accidentally into Ireland with shellfish transfers (O’Mahony 1993). It is possible that a phytoplankton species, capable of causing a red tide, could be imported to Ireland in Pacific oysters from France. One "red tide", in Dungarvan in 1994, virtually wiped out all the cockles and lugworms in the area (P. Cullen, pers. comm.). This could have serious consequences for the birds that feed on these species.

Heffernan 1999

Heffernan (1999) states: "To date, Pacific oysters have not adversely affected the indigenous fauna. This may be due primarily to its contained status."

Manila clam
Another intentionally introduced species is the Manila clam (Tapes semidecussatus or Ruditapes phillipinarum).

In the long-term, it is possible that Manila clam spawning could lead to the production of a self-sustaining population due to:

1. Acclimatisation.
2. Elevated temperatures in sea lochs.
3. Movements in water currents and isotherm patterns (Meikle and Spencer 1989).

Concerns have been expressed about the likelihood of escapes from intertidal Manila Clam culture plots where only a top containment net is used (Meikle and Spencer 1989). The Manila clam was recorded as spawning in Sligo in 1989 (Burnell and Cross 1989).

Heffernan 1999 (incl. citations)

Intentionally transferred species

Bottom cultivation of native blue mussels (Mytilus edulis) involves the location, collection and transplantation of wild mussel spat into richer, shallower waters using a dredger (Heffernan 1999). As Mytilus edulis is a native species any impacts relating to transfers would be on a national rather than an international level. Heffernan (1999) points to three basic concerns associated with such organism transfer:

Recruitment — The dredging of several thousands of hectares of subtidal beds for mussel spat must have some impact on the natural system, including reducing the chance that beds will develop into mature mussel beds, and that mature beds will regenerate following disturbance from human or natural causes.

Genetic impacts — Any genetic dilution could theoretically lead to a weakening of the species and ultimately a drop in survival. "If hatchery production of seed for commercial mussel farming were to become a major source of supply for the various mussel industries there would be a requirement for further research into its genetic implications. In addition, care should be taken, insofar as possible, to avoid moving mussels further than is absolutely necessary as this may cause dilution of any unique genetic traits in the local population."

Disease — Disease is not documented as a problem in mussel cultivation. "However, care should taken to avoid introducing mussels carrying disease or parasites into an uninfected area. Care must also be taken that the mussel does not inadvertently act as a carrier of disease of some other organisms."

The genetic and disease implications, above, of moving shellfish from one area to another also apply to suspended mussel cultivation.

Cultivation of the native scallop (Pecten maximus) in Ireland involves collecting spat, which may then be transferred to another area.

Studies of the reproductive ecology of different populations of Pecten maximus indicate that there are genetically isolated stocks in different areas. This has important implications for stock assessment, for restocking and aquaculture programmes.

Differences between populations reflect not only differing responses to differing local environmental cycles but also genetic adaptation on the part of local self-recruiting stocks, and hence a degree of genetic isolation between stocks (Orensanz et al. 1991). This uniqueness must be protected from introductions of other native species for mariculture or restocking purposes, even from within Ireland, as they could dilute the gene pool. If this genetic variation became diluted it may have the effect of weakening the whole population and reducing the probability of survival.

In this situation, where the endemic population is small and locally adapted transfers may destroy the unique phenotype of the local population, even if overall fitness is not compromised. The homogenising effect is popularly labelled genetic pollution and results in the loss of interpopulation diversity and distinct local phenotypes (Gaffney and Allen 1992). In contrast, if the local population is not highly adapted to a changing environment then it is possible that the introduction will bring genes which may result in immediate benefits (Gaffney and Allen 1992).

Heffernan 1999 (incl. citations)

Native oyster
The usual cultivation method employed in Ireland for the native/flat oyster (Ostrea edulis) is extensive culture, which involves collection of wild spat by dredging and re-laying in a more productive area (Heffernan 1999).

When considering the ecological impact of transfers of native oysters, the main concern is the possible accidental introduction and transfer of disease-causing non-target species, such as the oyster parasite (Bonamia ostreae), see below, as a result of imports.

Accidental introductions and transfers


Probably one of the best documented disasters resulting from the transfer of organisms is the case of Bonamia. Bonamiasis is a disease of Ostrea edulis which was first described in Brittany, France, in 1979 (Meikle and Spencer 1989) where it caused serious mortalities in flat oyster stocks. It has since been recorded in the Netherlands, Spain and Ireland. Losses due to the disease are usually high, up to 80% or even higher. The organism responsible is a simple protistan, Bonamia ostreae. The organism has been recorded in Cork Harbour, Galway Bay and Clew Bay. In Cork mortalities of 90% were recorded in the 4-year old classes in 1986 and by the spring of 1987 significant mortalities were evident in all classes except one year olds. The Galway Bay outbreak was confined to a small inner outlet. Mortalities up to 70% were reported. Oysters from Clew Bay tested positive, but although no obvious mortality occurred, their condition was very poor. Interestingly the two areas where Bonamia outbreaks were most severe the numbers of oysters were high (McArdle et al. 1991). It is believed that the disease was originally introduced through an illegal consignment of oysters (Ostrea edulis) from France into the south-west of Ireland in the early 1980s (McArdle et al. 1991; D. Hugh-Jones pers. comm). When the disease was recognised, French imports of live Ostrea edulis were banned.

Heffernan 1999 (incl. citations)

Other examples from around the world of accidental introductions and transfers include:

  • The American whelk tingle (Urosalpinx cinera) introduced into England along with American oyster (Crassostrea virginica). This unwanted predator gastropod became established in some areas of Essex and Kent and caused a great deal of damage to the juvenile stages of the native oyster (Ostrea edulis).

  • In British Columbia, Canada, the Japanese oyster drill (Ceratostoma inoratum) was introduced along with the Pacific oyster (Crassostrea gigas).

  • In France, introduction of the Pacific oyster led to other Japanese species living on the French coast in shellfish farming areas. The source was probably species imported with the spat. The species found included the annelid Hydroides ezoensis, coelenterate Aiplasia pulchella, mollusc Anomia chinesis, cirripeds Balanus amphitrite amphitrite and Balanus albicostatus.

A literature review conducted by the UK JNCC (Eno 1996) concluded that some 50 species now known to be present in UK waters ought to be regarded as non-indigenous. Most were introduced accidentally either via shipping or through movements of shellfish for cultivation purposes. The Pacific oyster was a deliberate introduction for aquaculture development purposes and the hard shelled clam (Mya arenaria) may have been deliberately introduced. Of the 50 species classed as non-indigenous, seventeen are found in waters off the west of Scotland and of these only seven are animal species (the review excluded species < 20µm in size). Some of the introduced species are now common, for example Spartina anglica or common cord grass.

The introduction of other non-indigenous marine molluscs (e.g. abalone and Manila clam (Ruditapes semidecussata)), as well as the transfer of molluscs from one area to another, for mariculture purposes includes a risk of transporting competitors, predators, parasites, pests and diseases. Of the total of 126 species imported into the North-east Atlantic region, thirty have been recorded in Irish waters. Whilst some of these species were intentionally introduced for mariculture, the vector of introduction for many is unknown but may have been shipping. Others are known to have been incidental species associated with importations of bivalve molluscs for mariculture. Until about 1920, American oysters (Crassostrea virginica) were regularly imported into Region III. One species introduced in this way is the slipper limpet, Crepidula fornicata. Any populations that became temporarily established in Ireland have not survived but it still persists in some UK waters. In the 1920s to 1950s, native oysters from France were imported to restock certain Irish bays and may have been the source of Chinese hat shell (Calyptraea chinensis) on the west coast of Ireland.

Following the implementation of EC Directive 91/67/EEC in January 1993, the movement of shellfish species between member countries is, in principle, free of restrictions. The trade in half-grown Pacific oysters from France has resulted in the oyster-gut parasite Mytilicola orientalis being introduced to Ireland. In 1993, samples taken in Carlingford Lough on the North Channel, and Dungarvan, Cork Harbour and Oysterhaven on the Celtic Sea, revealed the presence of this organism. As far as is known, it has become established only in Dungarvan harbour. M. orientalis may harm Pacific oysters and other molluscan species in areas where it becomes abundant. Finally, a number of phytoplankton species have been recorded in importations of Pacific oyster, including cysts of toxin-producing dinoflagellates.

OSPAR QSR 2000 for Region III Celtic Seas



1. The term "alien species", or similar, is used for all species outside their native distribution range, and also refers to exotic sub-species, races and other exotic organisms within the same species.     [Back]

2. The ecosystems and habitats where introduced species arrive are already greatly altered by human activities and are more vulnerable to impact from alien species.     [Back]


Minchin D., Duggan C.B., Holmes J.M.C. and Neiland S. 1993. Introductions of exotic species associated with Pacific oyster transfers from France to Ireland. International Council for the Exploration of the Sea, Copenhagen (Denmark). Mariculture Comm. Counc. Dublin, Ireland, 23 Sep-1 Oct 1993, 11, CM 1993/F:27.

O’Mahony J.H.T. 1993. Phytoplankton species associated with imports of the Pacific oyster Crassostrea gigas, from France to Ireland. International Council for the Exploration of the Sea, Copenhagen (Denmark). Mariculture Committee. C.M. 1993/F:26. ref: K=L.

WWF-Germany. 2004. Biodiversity assessment and threats analysis for the WWF Global 200 Ecoregion "North-East Atlantic Shelf". WWF-Germany, Frankfurt am Main.

WWF-Norway. 2005. On the run – Escaped farmed fish in Norwegian waters. Report 2/2005. WWF-Norway, Oslo. (pdf 2.1Mb)


Impacts of Aquaculture
Impacts of Finfish Farming
Impacts of Shellfish Farming
Introduced Species
Intentionally introduced
Intentionally transferred
Accidental introductions
ICES Code of Practice
EU policy on introductions
Genetic Modification
Welfare of Farmed Fish

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