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Vectors - how exotics get around
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Vectors
VECTORS - HOW EXOTICS GET AROUND
DAN MINCHIN1* & STEPHAN GOLLASCH2
Marine Organism Investigations, Ballina, Killaloe, Co Clare, Ireland
2
GoConsult, Hamburg, Germany
*
Corresponding author [email protected]
1
Abstract
The opportunities for the spread of exotics increase with the greater movement of goods and
people around the world. In early times human exploratory activities were responsible for some
selected species being moved as food or for cultural reasons but also inadvertently carried. As
trading links and colonisation of distant lands developed a more regular transport evolved. As a
result organisms spread to areas beyond their normal range, the ways in which this is done is the
subject of this chapter.
1 Introduction
Vectors responsible for exotic species spread arise from either primary or secondary
movements. International and national measures need to take account of these pathways
so that contingency plans for their management and control and prevention of their
spread are possible. However, the great majority of precautions follow at a time when
an invasion has been recognised often after it has had some economic impact. As a
result proactive measures in the prevention of primary inoculations are likely to be more
cost-effective in management plans. In order for this to be successful a good understanding of the vectors involved is needed. On the basis of current information a new
species is introduced to a new region worldwide every nine weeks. However, recent
indications are that this may be higher with approximately one new exotic species every
three weeks over the period 1998-2000 in European waters (ICES/IOC/IMO SGBOSV
2001).
1.2 PRIMARY AND SECONDARY INTRODUCTIONS
A species established in a locality in a different biological province for the first time
(usually between continents, from one side of an ocean to another or from one hemisphere to another) result from a primary introduction. More than one primary inoculation may occur at a similar time, however, this seldom occurs and should this happen, it
may be difficult to demonstrate, unless associated with deliberate movements. Most
primary inoculations have evolved as a result of trading in living organisms or as a
D. Minchin & S. Gollasch
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result of inadvertent carriage of species as fouling or in transported water. Shipping,
aquaculture activities and aircraft transmissions of living organisms are the main modes
of transmission for primary introductions today.
Secondary introductions result from the expansion of the exotic species from its first
location of establishment. This secondary spread will normally include a wider range of
vectors that may act either separately or together. Some vectors may be cryptic or just
not fully understood in the ways they operate. In order to manage the spread of a species
the likely modes for spread need to be clearly identified. With the secondary expansion
of a species new opportunities are created that enhance the further spread of the species
and so expansion can be accelerated. The expansion of both the Japanese brown alga
Sargassum muticum (Wallentinus 1999) and the Asian shore crab Hemigrapsus penicillatus (Noel et al. 1997; Gollasch 1999) are examples of species undergoing secondary
spread and have the potential to colonise most north European regions.
1.3 PRINCIPAL VECTORS OF EXOTIC SPECIES TRANSMISSION
There are a wide range of activities that either deliberately or inadvertently result in the
transmission of exotic species (Carlton 1994; Carlton et al. 1995; Gollasch 1996;
Minchin 2001). The majority of these relate to trade in some way but recreational activities can also be responsible for exotic species spread (Fig. 1, Table 1).
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Figure 1. Frequency of first records of nonindigenous species in European waters according to likely vector of introduction in 1998-2000. Ships include two vectors: ballast
water and hull fouling (white columns) (after ICES/IOC/IMO SGBOSV 2001).
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Table 1. Examples of exotic species spread in Europe other than by natural dispersal
mechanisms. Some of the acting vectors will also involve transport by ship, aircraft and
land vehicles.
Principal activity
Shipping and floating
structures (drilling
platforms, dry docks)
Aquaculture
Main vectors or means of
transmission/establishment
Ballast water and sediment, hull
fouling, cargo, ships equipment
Imports for culture, transport
equipment, untreated shell, host
tissues
Fishing activities
Equipment transfer, bait fishes
Food processing
Untreated waste disposal of
imported produce, exports of
tissue
Escapes, releases, disposal of
tissue or contaminated water
Escapes, releases, disposal of
tissue or contaminated water
Releases, infested stock
Live food trade
Aquarium trade
Stock enhancement
Recreational activities
Opening of natural
barriers
Movements of sediment, aggregates
New trade agreements
Research studies
Political policy
Cultural preferences
New fisheries development
Contamination of boats, fishing
equipment
Opening of new water links,
canals
Attached to or living within substrate
Erosion of previous legal barriers
enabling free movement of
goods, new trading routes
Releases, disposal
Production of forage species
Imports of specific species,
festival releases
Releases to the wild and spread
Species
Mnemiopsis leidyi, Elminius modestus, Eriocheir sinensis
Anguillicola crassus, Bonamia
ostreae, Orconectes limosus,
Crepidula fornicata, Crassostrea
gigas
Rutilus rutilus, Carassius carassius
Fish diseases
Homarus americanus
Caulerpa taxifolia, Elodea canadensis, Carassius auratus
Oncorhynchus mykiss, Pacifastacus leniusculus
Dreissena polymorpha
Cordylophora caspia, D.
polymorpha
Mya arenaria
Mytilicola orientalis, Dreissena
polymorpha
Mastocarpus stellatus
Paramysis lacustris, Limnomysis
beneden, Hemimysis anomala
E. sinensis, Rapana venosa
Paralithodes camtschaticus, Acipenser spp., European and Pacific
salmonids
2 Shipping
Shipping is implicated in transmission of a great diversity of organisms. This is because
ships are capable of carrying a wide range of sessile species, their epibionts and parasites (hull fouling); planktonic species that will include those with even short free-living
stages (ballast water); species that bury or are otherwise associated with sediments
(ballast sediments). Ships are capable of transmitting large numbers of species and
given suitable conditions sufficient numbers may survive to create a viable inoculum in
a new region. Successful inoculations may have been relatively rare events in the past
but are likely to increase with:
(i) more berths available in ports where there are marine conditions and construction
of new ports (providing more suitable targets for primary inoculations to become
established; and enhancing opportunities for secondary spread).
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(ii) better management of water quality in port regions leading to better conditions for
imported organisms in ballast water to become established; and also increase the
opportunities of exporting larger numbers of organisms elsewhere (Carlton et al.
1995).
(iii) higher frequency of ship visits, rapid turn-around times in port and changes in
trading patterns as new opportunities arise that may not have been present before
(with a consequent increased volume of ballast water discharges).
For these reasons it is predicted that further exotic species will become established in
European waters.
2.1 SHIPS’ HULL FOULING
Ships while in dry-dock are supported on wooden blocks, the hull beneath these blocks
does not become painted and so here fouling may freely develop once the ship is reimmersed. Consequently vessels approaching their dry-docking time, despite the use of the
toxic anti-fouling ingredient tributyltin (TBT), can have a mature fouling on parts of
their hulls (Coutts 1999). Several invertebrates could spawn, once exposed to temperature fluctuations while entering ports, and release and leave behind a viable inoculum of
zygotes that could form a founder population, even following a visit of some hours.
Mature molluscs fouling hulls may have the potential to transmit diseases between ports
and this topic should be researched. Because of unwanted effects to native biota, aquaculture and fisheries in port regions, it is planned to ban the use of TBT in antifouling
coatings by 2008. The new generation of antifouling agents will need to be as effective,
or more effective than TBT, if it is to reduce fouling yet the new products should not
cause unwanted effects to the environment. Transport of slow moving or otherwise
stationary craft do not have the same requirement of effective antifoulants as do fast
craft where large savings in fuel consumption can be made. For this reason barges and
working platforms may accumulate large fouling communities and if transferred may
carry a high risk of spreading exotics.
For some hundreds of years hull fouling was the main means of species transmission
but more recently attention has been devoted to ballast water and the organisms carried
within it. Nevertheless, recent studies on exotics (North Sea, Australia and USA) indicate that hull fouling continues to be an important vector for invasions although it is
often difficult to determine whether these have evolved from hull fouling or from ballast
water. Historically the numbers of non-native species likely to have been introduced by
hull fouling is greater than for ballast water, and so we may expect a large proportion of
established exotics to have arrived in this way.
2.2 SHIPS‘ BALLAST WATER AND ITS SEDIMENTS
Most of the world trade depends on shipping and to travel safely ships must maintain a
correct immersion level by either carrying cargo, ballast water or both.
Early ballasting of ships was undertaken using solid ballast material (sand, gravel and
stones), the purpose being to submerge the propeller and rudder thereby providing better
control and increased stability. The use of solid ballast was very labour intensive and it
was in the late 1870s when ballast water became more regularly used. Ballast water is
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usually carried in segregated ballast water tanks or in emptied cargo holds and is taken
on board in ports, waterways and the open ocean. With the uptake of ballast water organisms, suspended solids (i.e. sediments) and chemicals, including industrial and human wastes are pumped onboard. Vessels almost always carry ballast water when no
cargo is carried and even when fully laden some ballast will remain in ballast tanks
because the pumps serving the ballast tanks are unable to remove all of the water. Ballast water tanks will have different configurations according to the ship design and will
have a complex network of pipes for ballasting and for adjusting water levels between
tanks to improve trim. As the tanks will be filled and drained in different sequences
either singly or collectively, the ballast water in one tank may be composed of water
from several ports (Gollasch 1996).
The amount of ballast water carried can be 30% of the overall cargo carrying capacity
of the ship (e.g. Gollasch 1996). Pumping such large volumes of water when undergoing ballast water exchanges at sea, as recommended by the International Maritime Organisation (1996), is time consuming, costly and not always safe. Since the tanks can
not be fully drained three ‘complete’ exchanges are recommended until pump suction is
lost. However, under certain swells and other sea-states it may be unsafe to undertake
these exchanges because the uneven distributions of ballast water in the tanks can compromise the structural integrity of the ship. In the absence of fully proven sterilisation
techniques the mid-ocean exchange (i.e. re-ballasting in mid-ocean) is the only current
‘preventive’ approach employed by existing ships. Exchanges at sea are probably most
effective when freshwater is exchanged for seawater. Up to 10 billion tonnes of ballast
water (Rigby & Taylor 1995) and several thousands of species are transported every day
(Carlton & Geller 1993; Gollasch 1996). Large numbers of organisms are in transit,
estimates of > 50,000 zooplankton and 110 million phytoplankton per m-3 have been
made by e.g. Lenz et al. (2000). The abundance of some organisms is difficult to quantify, as they may not be evenly distributed within ballast and tank sediments due to the
winnowing effects caused by water circulation within tanks. Nevertheless estimates of
150 to 22,500 cysts m-3 of sediment were made (Hallegraeff & Bolch 1992). These cysts
may remain viable for 10-20+ years. In 14 recent European ballast studies approximately 990 species were recorded from ballast tanks (water and sediment), ranging from
bacteria to 15 cm long fishes (Gollasch et al. in review).
To show a cause and effect that exotics transported in ballast water do become established and spread in this way is a difficult task. Nevertheless, the evidence is strong and
remains unchallenged. The main sites of introductions are in ports and waterways where
shipping is the principle activity and the great numbers of organisms that can be carried
in a viable state is strongly indicative that ships are vectors.
3 Aquaculture
Exotic species provide economic opportunities whilst others can impose serious financial loss and unemployment. In the marine environment a small number of exotics are in
cultivation or stocked in the wild. These and further exotics are likely to contribute to
future production. Species that are tolerant of wide ranges of temperature and salinity,
and easily manipulated to produce young and can be maintained at high densities, are
most likely to be favoured for production. However, unregulated movements of further
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exotics intended for culture could provide access for unwanted and harmful pests, parasites and diseases which could subsequently compromise future production. More than
100 species have been transported with living oysters either carried in the packing materials, attached to shells or as parasites and disease agents in the living oyster tissues
(Carlton 1992; Sindermann 1992; Minchin 1996). The International Council for the
Exploration of the Sea’s (1995) Code of Practice on Introductions and Transfers of
Marine Organisms provides a procedure whereby the risks of introducing species can be
considerably reduced. The introduction of a species by this means is costly because it
takes more than one generation of a species before its release from quarantine.
Should an important industry suffer a decline following a serious disease or parasite
outbreak a separate exotic strain or species may be imported in large numbers to rapidly
replace this decline in production. The politics of such a situation may determine that a
precautionary approach is not adopted, because of the time involved using the approved
protocols. Direct imports of stock almost inevitably lead to an introduction of unwanted
species and some of these may reduce future production or have impacts on the cultivation of other species. For example, the introduction of the Pacific oyster Crassostrea
gigas to France by aircraft, following the decline in native oyster production, enabled
this species to become established but also enabled several associated organisms that
included pests to do so despite the use of brine dips as a preventative measure (Gruet et
al. 1976). Inevitably species successfully cultivated in one world region will be considered as a suitable species for culture elsewhere. This will include species for commercial and recreational benefit. The stocking of rainbow trout Oncorhynchus mykiss has
conferred many advantages for recreational angling throughout much of Europe (Lelek
1996; Löffler 1996).
Stocking of the spider crab Paralithodes camtschaticus in Russia during the 1960s has
resulted in its establishment and expansion to northern Norway (Kuzmin et al. 1996).
The species has spread with a combination of planktonic dispersal of its larval stage and
by walking. Adults could result in the spread of sessile organisms that may attach to its
carapace. Fishes are the most likely species to become widely distributed. Attempts at
cultivating the pink salmon Oncorhynchus gorbuscha in Russia have resulted in reports
of vagrant specimens in Britain and Ireland. Species in marine cage culture may escape
and spread in a similar way. Unless there is a sufficiently large inoculum to create a
founder population as well as a sufficient knowledge of its biology and interactions with
other species, the success of such a programme will have limitations. This is either
because of insufficient numbers present to create a reproducing progeny, because of a
poor knowledge of their physiological capabilities or because of unexpected interactions
with native species.
4 Trade
Agreements involving trade do not normally take into account those associated organisms that may extend their ranges as a result of the trading activity. This is because
should there be restrictions on the product the trade may not evolve. In an attempt to
overcome this difficulty veterinarians may classify a series of known lower impact
diseases that may be moved, and restrict trade for only the most serious diseases. Unfortunately some diseases, listed at a low level of priority, now can become transferred
Vectors - how exotics get around
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to areas where they did not previously exist. In addition diseases that have yet to be
described do not get sufficient attention until they have been spread. Dealing with pests
as well as diseases seems to be an additional set of criteria that could compromise the
trade activity itself and often appears to receive little or no attention.
The trade in live species, and in particular the trade of half-grown oysters which are relaid for further growth, continues to result in range expansions of molluscan pests and
diseases (Minchin 1996). Because oysters survive under cool damp conditions for several days, large consignments are easily transported long distances. Their shells provide
habitats for attaching, cryptic and boring species and harmful species can be carried in
this way or in molluscan tissues (Bower et al. 1992).
Aquarium species in ornamental trade are a further likely means for the spread of diseases. For example, fish prior to their departure are often held at high densities. Accidental or intentional releases of aquarium fishes that survive in the nature are frequent
events in freshwater but rare in the sea. The expansion of epizootic ulcerative syndrome
from the Indian Ocean, where it has been responsible for serious declines of fish production, is of concern and movements of aquarium species may aid its spread.
The majority of fishes imported to Europe in ornamental trade are tropical species from
freshwater and marine ecosystems. Their management needs to ensure that they are not
exposed to prolonged cool periods or sudden temperature changes. Such species are
unlikely to thrive in the wild in Northern Europe except possibly in thermal discharges
and warm water springs. In tropical and semi-tropical regions exotic fishs have commonly established themselves following release - known from the 1930s (Myers 1940).
In temperate waters fish from similar climates are more likely to become established.
Some such as the pumpkinseed Lepomis gibbosus were deliberately released in central
and southern Europe but this species is also an aquarium species and could well become
established further to the north.
One of the main concerns is that movements of exotic aquarium species inevitably result in the movement of other species that include diseases associated with the fishes
themselves. Robertson & Austin (1994) noted several pathogens associated with introductions of exotic cyprinids, some considered to be harmful to salmon and rainbow
trout. Shotts et al. (1976) examined the bacteria associated with imported exotic fishes
and the water in which they were carried, coming from Taiwan, Singapore, Hong Kong
and Bangkok. Eighteen genera were found associated with the fish and 14 with the
associated water demonstrating that transmissions of disease causing bacteria may easily be spread. Tropical aquaria can develop cultures of Mycobacterium marinum. Fish
normally become moribund and those subsequently cleaning the tanks can develop a
sporotrichosis-type condition as a result of infections of this bacterium (Adams et al.
1970).
The ulvophycean alga Caulerpa taxofolia may have been accidentally released from an
aquarium as it was first discovered in the Mediterranean Sea off the coast of Monaco in
near the Oceanographic Museum where it was cultivated for display in the aquaria. The
species began to spread rapidly from the northern part of the western Mediterranean to
regions further south. This invasive Mediterranean strain of C. taxifolia differs from
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tropical strains of C. taxifolia by its great resistance to lower temperatures (Meinesz &
Boudouresque 1996).
The aquarium trade needs to consider certification of its products and have regular
health inspections. Undue mortalities of stock need to be recorded and explained. Those
involved in rearing or collections should be made aware of a protocol of expected standards so as to reduce stresses on collected organisms. They should also be made aware
of those species not permitted for trade. This requires a close co-operation between
producers and regulators. In many regions of south-east Asia there is a need to treat
aquarium fishes as a fishery, conservation of many species will only be achieved with
public education (Ng & Tan 1997). According to Davenport (1996) the majority of
ornamental fish imported to Europe are exported from Singapore, with less from Israel,
USA, Czech Republic, Indonesia, Japan, Brazil, Thailand and Hong Kong. Other countries export smaller amounts. Most of these are imported to Germany, Britain and
France.
5 Natural dispersal
Once established within a new locality a species may remain confined to a small region,
as do many tunicates (e.g. Styela clava) because of their short larval period or because
of a lack of a larval stage (e.g. the gastropod Urosalpinx cinerea). However the majority
of organisms have a pelagic phase that will result in an incremental spread of its range
locally. For some species nearest neighbour distances following their planktonic dispersal will be important, barnacles (Elminius modestus) practice internal fertilisation and so
normally need to be close to each other. Those species likely to be widely distributed
include algae with air bladders (e.g. the brown seaweed Colpomenia peregrina); thereby
allowing water and wind currents to rapidly disperse them. Because marine macrophytes are close to neutral buoyancy these may be easily carried with water currents.
Planktonic species may become distributed along coastal fronts and blooms such as
those of the naked dinoflagellate Gyrodinium aureolum, now known as Karenia mikomotoi, have swiftly spread throughout northern Europe since their first appearance in
Norway (Tangen 1977).
6 Other vectors
Additional vectors may be associated with trading, for example the use of marine algae
used as a packing material with movements of living lobsters or oysters. The algae,
epibionts and associated organisms could become established elsewhere, should this
material be disposed to the wild. Bait organisms may be exported beyond their normal
range and may become discarded alive to the wild. Movements of infested fishing gear
may also allow species to colonise new regions (Wallentinus 1999).
The opening up of new waterways and canals, as happened between the Red and Mediterranean seas has resulted in a flow of species (Lessepsian migration) mainly from the
Red Sea. Such corridors allow for spread by natural dispersal. Similarly connections
between the North Sea and The Baltic (Kiel Canal) have enabled the spread of species
(e.g. Chinese mitten crab Eriocheir sinensis). In eastern Europe, the building of canals
has enabled a transmission and spread of species between the Baltic, Black and Caspian
Vectors - how exotics get around
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Seas. There has been a policy to add exotic species to rivers in former USSR under
stock enhancement programmes. Mysid and gammarid species were introduced as a
food source for commercial fishes.
Scientists and public institutions also have an obligation to act responsibly when disposing wastes. There are many exotic organisms held captive in aquaria in European
research institutions. The numbers of species are not known. Unfortunately secondary
spread of such organisms to other aquaria is a common feature and some species such as
a modified form of the alga Caulerpa taxifolia is believed to have spread to the Mediterranean Sea from an aquarium. Likewise, the spread of the golden snail Pomacea sp., a
serious pest in Southeast Asia and also used in aquaria, are frequently on sale in hobbyist shops. Although unlikely to become established in the wild in Northern Europe they
may do so in warmer regions to the south.
The macroalga Mastocarpus stellatus, native in Europe, was not occurring in all habitats likely for colonisation. Researchers assessed the reasoning for the limited distribution by planting it into the wild. Today it is well established where it was formerly absent and it is suspected to outcompete some co-existing native species (Wallentinus
pers. comm.).
Some species may be imported as live food because they have a cultural value. The
recent appearance of the whelk Rapana venosa to France may have been a release following imports for the Asian community.
7 Overlapping vectors and risk
Many shipping ports in harbours are situated close to aquaculture activities for reasons
of shelter and a nearby market. This proximity of shipping to aquaculture activities
poses the unquantifiable threat that some imported organism carried by ships may in
some way impair survival, compromise growth, or cause the cultivated product to be
unmarketable and this may render the benefits of previous quarantine regulations useless (Rosenthal 1980). Ballasting of water by ships in ports, for example, may result in
loading untreated discharges of human sewage and bacteria, such as Vibrio cholerae,
which on release might enter the food chain in distant ports through cultured filter
feeding molluscs (McCarthy & Khambaty 1994).
Small vessels such as yachts and motor boats may develop a fouling compliment on
their hulls that may include established exotics acquired in marinas in a shipping port
and these may spread to small inlets and lagoons, where ships do not trade.
In ports vectors are likely to overlap because many people normally live in these regions
and engage in a wide range of activities. Managing the overlap of vectors in such regions may lead to some hard decisions where some activities may need to be restricted
in some way so as to reduce risk. Ports will almost certainly benefit from studies of the
exotic species present and when there is a risk of that port acting as a donor to other
regions. All relevant activities within the port region should be evaluated where the port
may act either as a donor or recipient for unwanted invasives, as was demonstrated for
five north-west European port regions by Gollasch & Leppäkoski (1999). Small
changes in practice could result in reduced risk, for example, it may be sufficient to
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reduce the probability of an inoculum becoming established by extending the ballast
water discharge trails of ships on entry to a port.
8 Conclusions
This chapter demonstrates how vectors act in the movement of exotic species. Examples
are drawn from different taxonomic groups with varying impacts on humans and on
ecosystems and will indicate the value of proactive management measures. Some vectors are elusive, and through a better understanding as to how species are spread, together with the knowledge of the critical numbers needed to form new populations and
when and where this is most likely to happen, will greatly aid understanding. Today
movements throughout the world are continuous; the short transit times by aircraft over
large distances are of particular concern because they provide a whole suite of opportunities with reduced challenges. Shipping will continue to be an important vector and
those port regions with a large compliment of exotic species may expect to receive
more. In the coming century, should predicted changes in climate evolve (global
warming), natural ranges of organisms native to northern Europe are likely to change
and this will provide new opportunities for exotic species to expand their ranges.