Personal tools


Skip to content. | Skip to navigation

Sound and independent information
on the environment

You are here: Home / Data and maps / Indicators / Trends in Marine Alien Species pathways/vectors

Trends in Marine Alien Species pathways/vectors

Topics: ,

Assessment versions

Published (reviewed and quality assured)
  • No published assessments

Justification for indicator selection

Marine alien species (MAS) are being introduced into Europe at unprecedented rates (Bax et al., 2003; Streftaris et al, 2005; Hulme et al, 2008.  The introduction of species into habitats outside their native ranges is closely linked to the increasing globalization of trade and travelling (Mooney & Hobbs 2000).

MAS may arrive and enter a new region through three broad mechanisms: importation of a commodity (aquaculture), arrival of a transport vector (mostly shipping), and/or natural spread from a neighbouring region where the species is itself alien. Speed of MAS transfer has changed.

Historically, aquaculture and stock transfers of aquatic species resulted in a significant amount of taxa being transported worldwide. The speed and access of different transport modes in the spread of cultured species, their pests, parasites, diseases and associates, has changed over this time. At one time shipping journeys would have taken some months but now aircraft can spread biota within a day or less over the same distances. Elton (1958) stated: "the greatest agency of all that spreads marine animals to new quarters of the world must be the business of oyster culture". He was unaware of the impact that shipping will already have had by that time and of the trade that was to follow.

Knowledge of the invasion process is essential in designing management plans to cope with the potential detrimental effects of invasive species, and to attempt to prevent their large-scale spread.

Analyses of management actions vs pathways and number of species over time may help prioritize pathways. This can be done for all Alien species or for a selected list of species and for different environments.

The Marine Strategy Framework Directive (MSFD; EU, 2008), which is the environmental pillar of EU Integrated Maritime Policy, sets as an overall objective to reach or maintain "Good Environmental Status" (GES) in European marine waters by 2020. Trends in abundance, temporal occurrence and spatial distribution in the wild of non-indigenous species, particularly invasive non-indigenous species, notably in risk areas, in relation to the main vectors and pathways of spreading of such species is proposed as an indicator for assessing progress towards good environmental status relevant to the MSFD Descriptor 2 "Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystem".

Scientific references:

Indicator definition

This indicator shows trends in pathways of marine alien species. It shows the number of species introduced via 5 main categories of primary pathways of introduction (i.e. aquaculture, shipping, corridors, aquarium trade and others) per decade since the 1950´s. Results are shown for each EU regional sea, covering alien species in both in the marine and estuarine environment.

Pathways describe the processes that result in the introduction of alien species from one location to another. The classification of primary pathways of introduction was an adaptation of the frameworks proposed by Hulme et al. (2008) and Molnar et al. (2008) and analysed by Katsanevakis et al (2013). Alien species may arrive and enter a new region through three broad mechanisms: importation of a commodity, arrival of a transport vector, and/or natural spread from a neighbouring region where the species is itself alien.

Five pathways are associated to these broad mechanisms: commodities intentionally released or escaped (aquaculture, aquarium & live food/bait trade), contaminants of commodities (aquaculture), stowaways on modes of transport (shipping), and exploitation of corridors that is resulting from transport infrastructures (Suez canal, inland canals). The sixth pathway proposed by Hulme et al. (2008) is a secondary pathway and is not considered herein; it refers to alien species that may arrive unaided in a region as a result of natural spread following a primary human-mediated introduction in a neighbouring region.

In this analysis for simplicity and to be more specific to the marine environment, we used five broad categories defined on a human activity basis, as also proposed by Molnar et al. (2008): ´aquaculture´(subdivided to  ´commodity´ and ´contaminant´), ´shipping’ (subdivided to ´ballasts´ and ´fouling´), ´corridors’ (subdivided to ´Suez´ and ´inland canals´), ´aquarium trade´, and ´other´ (including live food / Angling bait trade i.e. Fucus spiralis;  Freshwater barrels i.e. Potamopyrgus antipodarum ; introduced with packing material i.e. Spartina versicolor; Floating objects; imported for military purposes). In other are also assigned species introduced for research and education: with equipment, intentional release, waste discharges (Ojaveer et al. 2013).


Number of new alien species arriving in European Seas per decade per pathway/vector.

Policy context and targets

Context description

The EU Biodiversity Strategy (EU, 2011) specifically stresses the need to assess pathways of biological invasions through its Target 5: "By 2020, invasive alien species and their pathways are identified and prioritized, priority species are controlled or eradicated, and pathways are managed to prevent the introduction and establishment of new invasive alien species".

Article 11 of the European Commission (2013) Regulation on the prevention and management of the introduction and spread of invasive alien addresses Action plans on the pathways of invasive alien species and specifies:
Member States shall, by [18 months from the entry into force of this Regulation– date to be inserted] at the latest carry out a comprehensive analysis of the pathways of unintentional introduction and spread of invasive alien species in their territory and identify the pathways which require priority action ('priority pathways'), because of the volume of species or of the damage caused by the species entering the Union through them. In doing so, Member States shall in particular focus on an analysis of the pathways of introduction of invasive alien species of Union concern.

Other international agreements cover different groups of MAS or pathways of their introduction and start to address MAS as a threat to biodiversity:

1. It has been recognized that aquaculture and related activities (e.g. sport fishing, fishery stock enhancement, ornamental trade) have been important drivers of alien species in Europe in the past and that the trade in alien species needs specific rules in order to prevent target and non-target species introduction into the wild. In 2007 the first EC regulation on alien species was approved: No 708 on 11 June 2007 (implemented rules: No. 535 on 13 June 2008) concerning the use of alien and locally absent species in aquaculture.

2. The International Maritime Organisation (IMO) adopted the "International Convention for the Control and Management of Ships' Ballast Water and Sediments". The aim of the Convention is to prevent, minimize and ultimately eliminate the transfer of harmful aquatic organisms and pathogens through the control and management of ships' ballast water and sediments.

3. The Convention for the Control and Management of Ship’s Ballast Water and Sediments under the International Maritime Organisation addresses ballast water as the main pathway for MAS.

4. The Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES) adopted a resolution on IAS during COP13 (and reviewed at COP14) (resolution 13.10, trade in alien invasive species).

Once introduced alien species, have a high probability of establishment and spread. While there is a consensus that shipping along with aquaculture are the primary conduits for MAS introductions, and a key locus for preventive regulation, few policies have been evaluated by scientists for effectiveness.


  • Reduce the number of MAS intentionally imported and  accidentally introduced with aquaculture imports: regulations (708/2007)
  • Reduce the number of MAS transferred in ship ballasts and as fouling -IMO BW Convention
  • Reduce the number of MAS spreading via man-made inland and marine corridors
  • Prevent spreading of MAS imported for aquarium trade – private/public
  • Prevent spreading of MAS from one regional Sea to another – secondary introduction

Related policy documents


Methodology for indicator calculation

The primary pathway/vector was filled in for about 1394 species. Marine and estuarine species are those aquatic species which do not complete their entire life cycle in freshwater (modified after ICES, 2005). Vagrant species, mostly fish and crustaceans of tropical Atlantic origin occurring in the Mediterranean but exhibiting a distinct distribution, whose mode of introduction is unknown were not considered in our calculations. Thus, the pathway was calculated for  1305 species.
To categorize pathways of primary introduction of alien species into a new region, we have followed the framework proposed by Katsanevakis et al (2013).

Five pathways are associated with human activity either as commodities (release and escape), contaminants of commodities, stowaways on modes of transport and opportunists exploiting corridors resulting from transport infrastructures. The sixth category highlights alien species that may arrive unaided in a region as a result of natural spread (rather than human transport) following a primary human-mediated introduction in a neighbouring region.

The main categories and subcategories of pathways of primary introduction of marine alien species used in this work:

    • Aquaculture: Historically, aquaculture and stock transfers of aquatic species resulted in a significant amount of taxa being transported worldwide. Of the species imported for mariculture, it is not known how many have been intentionally released for stocking. This is a practice for introduced species in inland waters not considered in this analysis. So, this category is not analysed here. However, we have included under the category ´aquaculture escape´those imported species that have escaped from aquaculture and spread in the wild, even if they are released freshwater species if found in estuarine areas of the Baltic Sea. The so called aquaculture contaminants is hereby  assigned as ´aquaculture accidentally introduced´ species.
    • Shipping: With regard to MAS, stowaways include organisms that foul the hulls of ships, are transported as seeds or resting stages in ballast water, as well as in shipping containers, cargo.  Estimations reveal that more than 480,000 annual ship movements occur worldwide with the potential for transporting organisms. Various calculations have been made on the amount of ballast water carried with the world’s fleet of merchant ships – it has been estimated that 2–12 billion t of ballast water are transported annually. In ballast tanks and also other ship vectors including hulls, anchor chains and sea chests, ships may carry 4,000 to 7,000 taxa each day (Gollasch, 1996).
      Given the IMO Policies on Ballasts after 2004 (BWC), it was deemed important to examine the trends in the two different modes of introduction (ballasts and fouling) separately.
    • Under Corridors, we have separated those species progressively introduced via the Suez Canal (mostly Indo-Pacific species occurring in the eastern Mediterranean, called also Lessepsian immigrants) and those Ponto-Caspian species spreading in the Baltic via inland canals.
    •  The unaided pathway describes situations where natural spread results in alien species arriving into a new region from a donor region where it is also alien. Since we are addressing only the primary introduction pathway this category is not analysed here.
    •  Finally we have added the category other to assign species introductions via aquaria trade. The recent focus on the aquarium trade as a possible mechanism for environmentally sustainable development poses an especially dangerous threat, although this has so far escaped the attention of most environmentalists, conservationists, ecologists, and policy makers.


Methodology for gap filling

Unless found when deliberately moved, evidence of actual transmission is seldom known.

Information on vectors are mostly derived from expert judgement on an extensive review of the referred databases, since specific research projects aimed at identifying vectors and occurrences are complicated and demanding large resources. The only exception is published reports issued on maritime traffic worldwide (BWM, 2005). In a large number of cases, likely pathways are merely inferred, for example taking into account the most common activity occurring in a specific location (shipping, aquaculture), but no scientific evidence is provided.

Vertebrate pathways tend to be characterized as deliberate releases, exempting the Lessepsian immigrants that arrived unintentionally via the Suez Canal, invertebrates were introduced mostly  as contaminants and plants as escapees. Pathogenic micro-organisms and fungi are generally introduced as contaminants of their hosts.

Methodology references


Methodology uncertainty

In many cases, it is impossible to identify the introduction vector. Thus the pathway of 89 species is assigned as ´unknown´. In bivalves, for example, introductions may be attributed to larval transport in ballast water releases, adults in hull fouling of ships or imports of stock for aquaculture purposes, or for direct human consumption but released into the environment.

For species that are most frequently associated with hull fouling, this form of transport was assumed to be the responsible vector. For planktonic taxa and microscopic resting stages we have deemed ballast water to be the most likely vector since such species that are associated with hull fouling might be expected to become flushed away during ship journeys at sea. The human activities near to the site of the first records generally are assumed to be responsible for an introduction event. However, such deductions are not always secure and for this reason we have calculated more than one vectors where the likely vector remains unclear. Forty species of tropical  Atlantic origin have entered recently the Mediterranean via Gibraltar. These are classified as ´vagrant´ and ´range expansion´ and are not included in the 1394 for the pathway/vectors analysis.


Data sets uncertainty

No uncertainty has been specified

Rationale uncertainty

Different levels of certainty are associated with alien species that become introduced. A scheme proposed by Dan Minchin (2007), provides a basis for an improved quality of information for pathways and vectors.

(1) There is direct information of a pathway/vector: The species was clearly associated to a specific vector(s) of a pathway at the time of introduction to a particular locality. This is the case in intentional introductions (i.e. aquaculture/commodity) and in many cases of Lessepsian immigrants (when there was direct evidence of a gradual expansion along the Suez Canal and then in the localities around the exit of the Canal in the Mediterranean).

(2) Amost likely pathway/vector can be inferred: The species appears for the first time in a locality where a single pathway/vector(s) is known to operate and there is no other rational explanation for its presence except by this pathway/vector(s). This applies to many species introduced by shipping or aquarium trade or as aquaculture contaminants. In some cases a specific vector could not be inferred, e.g. some species probably introduced by shipping could not be further linked to ballasts or hull fouling and were classified as ‘shipping/unknown’. In many cases inference is based on known examples of introductions elsewhere for the same or similar species,the biology and ecology of the species, the habitats and locales it occupies in both the native and introduced range, and its pattern of dispersal (if known), e.g. for a fouling species frequently recorded in ports, shipping has been assumed to be the most probable vector.

(3) One or more possible pathways/vectors can be inferred: The species cannot be convincingly ascribed to a single pathway/vector. Inference is based on the activities in the locality where the species was found and may include evidence on similarly behaving pecies reported elsewhere.

(4) Unknown: Where there is doubt as to any specific pathway explaining an arrival. Herein, the pathway of 89 species has been assigned as ´unknown´.


Gruet, Y., Heral, M., Robert, J.-M., 1976. Premie`res observations sur l’introduction de la faune associe΄e au naissain d’huı tres Japonaises Crassostrea gigas (Thunberg), importe΄ sur la coˆ te Atlantique FrancΈaise. Cahiers de Biologie Marine 17, 173–184.

Hare, J.A., Whitfield, P.E., 2003. An integrated assessment of the introduction of lionfish (Pterois volitans/miles complex) to the Western Atlantic Ocean. NOAA Technical memorandum NOS NCCOS 2, 21

McMahon, R.F., 1983. Ecology of an invasive pest bivalve, Corbicula. In: The Mollusca, Vol. 6. Academic Press, pp. 505–553.

Minchin D., 2007. Aquaculture and transport in a changing environment: Overlap and links in the spread of alien biota. Marine Pollution Bulletin 55 (2007) 302–313

Lilly, E.L., Kulis, D.M., Gentien, P. & Anderson, D.M. (2002) Paralytic shellfish poisoning toxins in France linked to human-induced strain of Alexandrium catanella from the western Pacific: evidence from DNA and toxin analysis. Journal of Plankton Research, 24, 443–452.

Ruiz, G.M. & Carlton, J.T. (2003) Invasive Species: Vectors and Management Strategies. Island Press, Washington, DC.

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.

General metadata

Responsibility and ownership

EEA Contact Info

Constança De Carvalho Belchior


European Environment Agency (EEA)


Indicator code
MAR 003
Version id: 1
Primary theme: Water Water


Permalink to this version
Permalink to latest version


DPSIR: Pressure
Typology: Policy-effectiveness indicator (Type D)

Related content

Data references used

Relevant policy documents

Geographical coverage

[+] Show Map

Document Actions


European Environment Agency (EEA)
Kongens Nytorv 6
1050 Copenhagen K
Phone: +45 3336 7100