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
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.
The introduction of alien species  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 .
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
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
"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
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
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
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:
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
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) states: "To date, Pacific oysters have not
adversely affected the indigenous fauna. This may be due primarily
to its contained status."
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:
2. Elevated temperatures in sea lochs.
3. Movements in water currents and isotherm patterns (Meikle and
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)
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
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
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)
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
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
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
- 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
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
OSPAR QSR 2000 for Region III Celtic Seas
|What does it mean?
Try the pop-up Glossary