Indicator Assessment

Pathways of introduction of marine non-indigenous species

Indicator Assessment
Prod-ID: IND-365-en
  Also known as: MAR 003
Published 03 Mar 2015 Last modified 11 May 2021
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The main pathways for marine non-indigenous species (NIS) introduction in Europe´s seas are shipping (51%) and the Suez Canal (37%). These are followed by aquaculture related activities (17%) and, to a much lesser extent, aquarium trade (3%) and inland canals (2%). This is a pattern observed in all regional seas, except for the Eastern Mediterranean where introductions via the Suez Canal exceed those by shipping.

Trends in pathways show an increasing rate of introductions by shipping and corridors (in particular the Suez canal) since the 1990s, while aquaculture mediated introductions have been decreasing since the 2000s. This can be attributed to the adoption of effective EU regulation. Aquarium trade emerges as a lower but increasing pathway since the 2000s.

Main pathways of introduction of marine non-indigenous species

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Rate of introduction of marine non-indigenous species per pathway

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Aquaculture introductions of marine non-indigenous species in Europe's seas

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Main pathways of introduction of marine non-indigenous species in regional seas of Europe

Note: This figure shows the relative importance (%) of the main pathways of introduction of non-indigenous species (NIS) across regional seas of Europe in 2014.

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Of the 1 416 marine non-indigenous species (NIS) in European Seas, 7% were introduced intentionally (via aquaculture and aquarium trade), while 93% invaded unintentionally. The main pathway for NIS introduction in European Seas is shipping (51%) followed by unintentional introductions via the marine Suez canal (37%), aquaculture related activities (17%) and, to a much lesser extent, aquarium trade (3%) and inland canals (2%) (Figure 1). However, it should be underlined that these percentages are only a best estimate because the introduction pathway is often uncertain.

Previous studies have also shown that the most prominent invasion pathways are shipping and aquaculture activities (Streftaris et al., 2005). An exception to this general trend is the Mediterranean Sea, where the dominant invasion pathway (or vector) is the Suez Canal, which enables Red Sea species to migrate into the southeastern Mediterranean Sea and vice-versa. This phenomenon is also known as Lessepsian migration. Hull fouling is thought to have been the vector of introduction for many algal species (Mineur et al., 2007). A further analysis on the available Pan-European data shows that species introduced with hull fouling seem to dominate those associated with ballast water by 58% and 42% respectively of ship mediated introductions. This is also in agreement with other analysis (Gollasch, 2007). 

Trends in pathways of introduction (Figure 2) show that the rate of ship mediated NIS and those introduced via corridors have been increasing since the 1990s, while the rate of species introduction via aquaculture activities (imported and accidentally introduced with contaminants) is decreasing since the 2000s (Figure 3). However, to a lesser extent, aquarium trade emerges as a growing pathway in the past decade.

Following the issue of biological invasion awareness, which was raised in the 1970s, a significant reduction in the main pathways of introduction is observed in the 1980s, except for shipping. Species introduced via the Suez Canal clearly reduced in the 1980s, but increased abruptly in the 1990s, following the deepening/widening of the Suez Canal. The decrease in aquaculture introductions can be attributed to the adoption of a specific EU regulation in 2007, showing management measures can be effective if properly implemented.

This shows measures to prevent or control NIS introductions need to be stepped up. In addition, given that it is impossible to eradicate widely spread species, interception or closure of new pathways, following an integrated approach, seems to be the only effective strategy for reducing the spread of alien species and their future impacts.

Supporting information

Indicator definition

This indicator shows trends of marine non-indigenous species per pathway of introduction recorded in European seas since 1950. Analysis is shown at both European and regional sea level.


Number of non-indigenous species 


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 prioritised, priority species are controlled or eradicated, and pathways are managed to prevent the introduction and establishment of new invasive alien species".

The recently adopted European Commission Regulation on the prevention and management of the introduction and spread of invasive alien species determines that Member States carry out a comprehensive analysis of the pathways of unintentional introduction and spread of invasive alien species of EU concern. This should be carried out, at least, in their territory as well as in their marine waters as defined in Article 3(1) of Directive 2008/56/EC. Member States should also identify the pathways that require priority action ('priority pathways'), because of the volume of species or of the potential damage caused by the species entering the EU through them. They will also need to establish and implement action plans to address the priority pathways identified within three years of the initial assessment.

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

1. It has been recognised 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 the introduction of target and non-target species into the wild. In 2007, the first EC regulation on alien species was approved: No. 708 on 11 June 2007 (implemented by 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, minimise 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).


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 prioritised, priority species are controlled or eradicated, and pathways are managed to prevent the introduction and establishment of new invasive alien species”.

The MSFD aims to reach Good Environmental Status (GES) of the marine environment by 2020. Descriptor 2 addresses marine alien species and is defined as “Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystem".

Related policy documents



Methodology for indicator calculation

The list of species used is the same as that considered for the trends in introduction analysis (MAR002) and stored in the HCMR/EEA data base. It includes both alien and cryptogenic species. Information on the country, year of first introduction of each species and pathway(s) is publicly available through the species search widgets of EASIN updated as of October 2013 ( and is also provided in the supplementary file of Nunes et al. (2014) The HCMR/EEA data base is in full agreement with the classification given in EASIN, yet updated to June 2014.

Trends in introduction per pathway were calculated on a decadal basis, considering the very first sighting/collection of the species in European Waters. In the absence of an exact collection date, the date of publication was used as the best available information. All species, both established and casual, were taken into account.

The primary pathway/vector was filled in to the degree possible. Marine and estuarine species are those aquatic species that do not complete their entire life cycle in freshwater (modified after ICES, 2005). Vagrant species, mostly fish and crustaceans of tropical Atlantic origin, whose mode of introduction is unknown, were not considered in the calculations. Thus, the pathway was calculated for 1 416 species.

To categorise 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.

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 those imported species that have escaped from aquaculture and spread in the wild under the category ‘aquaculture escape’, even if they are released freshwater species if found in estuarine areas of the Baltic Sea. The so called aquaculture contaminants are hereby assigned as ‘aquaculture accidentally introduced’ species.

Shipping: With regard to NIS, 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 and cargo. Estimates 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 tonnes of ballast water are transported annually. In ballast tanks and other ship vectors including hulls, anchor chains and sea chests, ships may carry 4 000 to 7 000 taxa each day (Gollasch, 1996). Under shipping we have also included species transferred by recreational boats, mostly encountered in marinas, and species transferred by fishing boats.

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, also called Lessepsian immigrants) and those Ponto-Caspian species spreading in the Baltic via inland canals.

A final category is aquarium trade. The recent focus on the aquarium trade as a possible mechanism for environmentally sustainable development poses an especially dangerous threat (Papavlassopoulou et al, 2014), although this has so far escaped the attention of most environmentalists, conservationists, ecologists, and policy makers.

When investigating gateways of introduction in European Seas, the country and year of initial introduction of marine alien species in Europe were identified (for 1 380 species in Nunes et al 2014). The country through which a species was first introduced in Europe will hereafter be called ‘recipient country’. For 31 species, more than one recipient country was associated with their introduction into European Seas. This may happen when a species has been collected independently in the same year from different countries, such as, for example Desdemona ornata (Banse, 1957) from Italy (Lardicci and Castelli 1986) and Greece (Panagopoulos and Nicolaidou 1989-90), Fibrocapsa japonica (Toriumi and Takano, 1973) from France (Billard 1992) and Germany (Elbrächter 1994) or Hemigrapsus sanguineus (De Haan, 1835) from France and the Netherlands (Breton et al. 2002). 

Given these potential multiple pathway of introduction at Pan-European level, the percentages add to more than 100% (i.e. 116.3%) as some species are linked to more than one pathway. Temporal trends in the numbers of newly recorded marine alien species in Europe are given in relation to the pathways of introduction. Some species that were linked to more than one pathway were given a value of 1/k, for each of the k associated pathways, so that the overall contribution of each species to the total number of new aliens per decade was always one (Katsanevakis et al, 2013).


Methodology for gap filling

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

Information on vectors is 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 demand 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 characterised 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 microorganisms 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 100 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 ballast water has been deemed 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. Human activities near to the site of the first records are generally assumed to be responsible for an introduction event. However, such deductions are not always secure and, for this reason, more than one vector has been calculated where the likely vector remains unclear. Forty species of tropical Atlantic origin have recently entered the Mediterranean via Gibraltar. These are classified as ‘vagrant’ and ‘range expansion’ and are not included in the pathway analysis.

Where more than one pathway of introduction is suspected/documented, the analysis has considered both modes of introduction. Thus, the resulting percentage of contribution per pathway amounts to more than 100%. 

Data sets uncertainty

See rationale uncertainty

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) Almost 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 species reported elsewhere.

(4) Unknown: Where there is doubt as to any specific pathway explaining an arrival. Herein, the pathway of 100 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.

Data sources

Other info

DPSIR: Pressure
Typology: Policy-effectiveness indicator (Type D)
Indicator codes
  • MAR 003
Frequency of updates
Updates are scheduled every 3 years
EEA Contact Info


Geographic coverage

Temporal coverage