Indicator Specification

Trends in marine non-indigenous species

Indicator Specification
  Indicator codes: MAR 002
Published 10 Dec 2019 Last modified 03 Dec 2020
15 min read
This indicator shows the cumulative number and trends in the introduction and recording of marine NIS in the regional seas of Europe since 1949.

Assessment versions

Published (reviewed and quality assured)
  • No published assessments


Justification for indicator selection

Biological invasions are widely recognised as one of the main threats to biodiversity, alongside habitat destruction and pollution. Non-indigenous species (NIS), also known as alien species, are species introduced to areas outside their native range. NIS are defined as any live specimen of a species, subspecies or lower taxon of animals, plants, fungi or micro-organisms introduced outside their natural range. These include any part, gamete, seed, egg or propagule of such species, as well as any hybrids, varieties or breeds that might survive and subsequently reproduce (EU, 2014).

The introduction of alien species is closely linked to the increasing globalisation of trade and travel. The development of maritime activities has also provided new and enhanced pathways for the spread of alien invasive species. The vast majority of European marine NIS have their native distribution in the Western and Central Indo-Pacific, being mostly associated with introductions into the Mediterranean Sea through the Suez Canal. However, this overall pattern is heavily influenced by the fact that 76 % of all NIS primary introductions to Europe were first recorded in the Mediterranean Sea (Tsiamis et al. 2018). Member states are in the process of implementing legislation that will reduce new introductions of NIS.

Scientific references

Indicator definition

This indicator shows the cumulative number and trends in the introduction and recording of marine NIS in the regional seas of Europe since 1949.


The unit of measurement is the number of NIS per taxonomic group (primary producers, invertebrates and vertebrates) at pan-European and regional sea levels, expressed as the number of new NIS every 6 years.


Policy context and targets

Context description

Several policies for the marine environment address IAS, in particular through the animal health regime (various regulations and directives) and the Regulation on the use of alien and locally absent species in aquaculture (EC 708/2007). More broadly, the Birds Directive (2009/147/EC, BD), the Habitats Directive (92/43/EEC,HD), the Water Framework Directive (2000/60/EC, WFD), the Marine Strategy Framework Directive (2008/56/EC, MSFD) and the Regulation on aquaculture (EC 708/2007) require the restoration of ecological conditions and refer to the need to take NIS into consideration. Nevertheless, it was considered that this existing union action left most IAS unaddressed.

The European Commission formally recognised the urgent need to tackle invasions in Europe in its Communication 'Towards an EU Strategy on Invasive Species' (COM 789/2008), in 2008. The EU Biodiversity Strategy Regulation (COM/2011/0244 ) — which translates the international commitments adopted by the parties to the Convention for Biological Diversity in 2010 in Nagoya, Japan — further identified combating IAS as key to safeguarding European biodiversity, and sets a dedicated target and actions. In particular, it identified the need for a specific EU legislative instrument that could tackle outstanding challenges relating, inter alia, to IAS pathways, early detection and response, and containment and management of IAS. As a result, in 2014, the European Commission adopted a Regulation on the prevention and management of invasive alien species in Europe (EU 1143/2014). It aims to bring a more comprehensive approach to deal with IAS in Europe, across all environments. The Regulation establishes rules to prevent, minimise and mitigate the adverse impact on biodiversity of their intentional and unintentional introduction and spread within the EU. It indicates three types of intervention: prevention, early warning and rapid response; and management to tackle the problem. A list of marine invasive NIS of EU concern was updated in 2019, so as to guide implementation of the Regulation. This regulation should therefore be able to integrate and ensure consistency of existing EU, global, regional and national initiatives in order to increase their effectiveness in combating invasive alien species.

Other international agreements cover different groups of NIS and begin to address NIS as a threat to biodiversity:

  • 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 rules: No. 535 on 13 June 2008) concerning the use of alien and locally absent species in aquaculture.
  • The International Maritime Organisation (IMO) adopted the 'International Convention for the Control and Management of Ships' Ballast Water and Sediments'. The IMO Ballast Water Management Convention (BWMC, 2004) requires all ships to implement a ballast water management plan. All ships will have to carry a ballast water record book and are required to carry out ballast water management procedures to a given standard. Parties to the convention are given the option to take additional measures, which are subject to criteria set out in the convention and to IMO guidelines.
  • The Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES) adopted a resolution on trade of alien invasive species.

The four Regional Sea Conventions (OSPAR, HELCOM, Black Sea Commission, UNEP/MAP) have also been active in developing regional action to address NIS and are increasingly streamlining their efforts with relevant EU policy implementation. Synergies in the work to implement the ecosystem approach — taking into account the HELCOM Roadmap — are discussed between HELCOM and OSPAR, Bonn Agreement, Black Sea Commission and International Council for the Exploration of the Seas (ICES) Joint Assessment and Monitoring Programme (JAMP) and set the basis on which the OSPAR Contracting Parties will work together in fulfilling these obligations over the period 2010-2014. UNEP/MAP (2014) set a monitoring protocol to be implemented by contracting parties for producing the data necessary to calculate the NIS trend indicator. The Black Sea Commission is also working on MSFD guiding improvements in the Black Sea Integrated Monitoring System (MISIS).

OSPAR continues its close cooperation with the Helsinki Commission in the Baltic Sea on the development and adoption of Joint Guidelines on the granting of exemptions from the IMO BWMC, allowing a consistent approach across Northern Europe to minimise the risk of the introduction of NIS.


Target 5 of the EU Biodiversity Strategy to 2020 on combating invasive alien species (IAS) determines that '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 IAS'.

The MSFD's main objective is to reach Good Environmental Status (GES) of the marine environment, by 2020. It has 11 environmental quality descriptors to determine GES. Descriptor 2 addresses NIS, stating 'Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystems'. The initial reporting of Member States in 2012 did not provide regionally harmonised information. A baseline for the number of NIS in EU Member States has been recently established. Number of introductions compatible with good environmental status (so called threshold values) are currently being discussed.

Related policy documents

Key policy question

Are the numbers of marine non-indigenous species increasing in European seas?



Methodology for indicator calculation

Methodology for indicator calculation

A simple information system — the HCMR/EEA database — has been in development at HCMR since 2002. It serves as a resource in developing a trend indicator and for reporting to  the EEA. The Mediterranean component of it has been transferred to EASIN, the European Alien Species Information Network (, and the official EU repository of alien species. EASIN is a dynamic inventory that is continuously updated to follow the latest scientific findings about new alien species in Europe and their status.

Geographical aggregation and respective country data availability

Geographical coverage: Europe’s seas. Marine and estuarine species data were extracted from the working database and grouped by country at pan-European level (i.e. EU and non-EU). Country data were further aggregated at regional sea level, following the geographical delineation of the regional seas surrounding Europe.

Sub-regions of the regional seas:

  • Baltic Sea (no subdivision)
  • North-East Atlantic Ocean (Greater North Sea, Celtic Sea, Bay of Biscay and Iberian Coast)
  • Mediterranean Sea (Western Mediterranean Sea, Ionian Sea, Central Mediterranean Sea, Adriatic Sea, Aegean-Levantine Sea)
  • Black Sea
  • Iceland Sea

Species filtering and taxonomic aggregation

For each regional sea, marine and estuarine species were grouped by selected taxonomic group: vertebrates, invertebrates and primary producers (i.e. Chromista, vascular plants, algae and fungi). Estuarine species are those aquatic species that do not complete their entire life cycle in freshwater. Birds have been excluded from calculations. Established, non-established and cryptogenic species are included. Species currently reported as 'extinct' in the literature, and those considered to be observed due to natural or climate-driven expansion from one regional sea to the neighbouring one, are excluded from calculations.

Data analysi

Individual species records were analysed to determine the year of first reporting from the environment, done separately for each regional sea.  Graphic presentation illustrates the number of alien species which were recorded within the 6 year periods.

Data are then used to calculate to the cumulative number of NIS and the rate of new recordings as a proxy of new introductions, presented per decade since 1950. The total number of NIS recorded up until 1949 is also presented. These analyses are made both at pan-European level and per regional sea. NIS that have been registered in more than one regional sea have been recorded in each of them. This means regional assessments do not add up to the pan-European assessment since the latter only considers species once. Moreover, NIS that have been recorded, but for which the year of introduction is unknown were, nevertheless, considered when providing the total number of NIS, both at the pan-European and regional sea levels.

Care has been taken to ensure that the nomenclature problems encountered have not resulted in multiple separate recordings (e.g. the same species recorded in different regions or species lists, databases with different synonyms for the same species). Recent scientific literature was consulted to solve taxonomic problems and revise nomenclature in some cases (i.e. Marenzelleria, Mnemiopsis). Thus, some species reported as alien in national databases have been excluded and considered native. Synonyms of species reported differently in existing European systems have been sorted using the nomenclature of WoRMS (World Registry of Marine Species) — WoRMS Editorial Board (2014), available at

Methodology for gap filling

When the exact date of the first sampling of a NIS is not known, the year of the relative publication has been used instead.

Personal communication with national and taxonomic experts was carried out when relevant to verify the data. 58 experts from 21 countries were involved in the process. Number of experts from each country is written in brackets: Belgium (3), Croatia (1), Denmark (2), Estonia (2), Finland (2), France (4), Germany (3), Italy (6), Ireland (1), Israel (2), Lithuania (1), Malta (2), Netherlands (5), Norway (2), Poland (2), Portugal (7), Russia (4), Spain (2), Sweden (1), Turkey (2), UK (4).



Methodology references

  • ICES, 2007,  Status of introductions of non-indigenous marine species to the North Atlantic and adjacent waters 1992–2002. ICES Cooperative Research Report No. 284. 149 pp.
  • ICES, 2018,  Interim Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO), 7–9 March 2018, Madeira, Portugal. ICES CM 2018/HAPISG:11. 179 pp.


Methodology uncertainty

Uncertainty in alien status

  • The diversity of marine microalgae is scarcely known in European seas. This makes it difficult to determine whether a suspected microalgal invader was already present as part of the rare, hidden and un-sampled phytoplankton.
  • Miscategorising alien species as native is not rare. Many pseudo-indigenous species occur in the Mediterranean, precisely because many old taxonomic works originated in the Mediterranean.
  • There are many records of cryptogenic species and phylogenetic studies have revealed that some of them are true aliens.

Uncertainty in year of introduction

  • The year of introduction is based on reported first collection dates but does not necessarily imply the true year of introduction, which may be years earlier. For 2016-17, the rate of introduction is underestimated; due to a time lapse between observation and publication, species collected in the 2016-17 period are expected to be published in the next 2 years.



Zenetos, A. et al, 2017, Uncertainties and validation of alien species catalogues: The Mediterranean as an example, Estuarine, Coastal and Shelf Science 191 (2017) 171e187

Data sets uncertainty

Data sets on NIS in official sources, such as NOBANIS, HELCOM and AquaNiS, are often contradictory. This is mainly due to the fact that these sources include freshwater species, which are encountered in the upper estuarine reaches (oligohaline waters with salinities < 5 psu), in their inventories.

Geographical discrepancies between regional or national sources of data and those presented here may occur, namely in the North Sea. These arise from the definition of borders between regional seas. Such discrepancies may not concur with the map of regional seas around Europe used in this indicator. Individual experts were also contacted in many cases.

Data quality from Black is still behind that of other Seas. For example, in AquaNIS (AquaNIS. Editorial Board, 2015), 294 alien and cryptogenic species are reported compared with approximately 180 in the Black Sea Commission report (TDA, 2007) and 261 in Alexandrov (2014). This discrepancy can be attributed to the different understanding of the definition of ‘alien’ by the Black Sea countries. Most Black Sea scientists include range expanding and cosmopolitan species among aliens, without any evidence of their anthropogenic transfer. Such is the case for many monocellular algae and also Atlanto-Mediterranean fish. For example, Boltachev & Karpova (2014) reported 25 alien marine species of fish in the Black Sea for the period 1998-2013, while only 5 species were identified between 1994 and 2017. Overall, the number of NIS per country is decreasing, with the exception of Turkey, because of the Sea of Marmara. Among the most recent newcomers to the Black Sea are species reported to date only in the Sea of Marmara, such as the ascidian Styela clava (Çinar, 2016) and the Alepes djedaba fish (Turan et al., 2017). In addition, in the Black Sea, divergence between the regional experts used for reference makes the compilation of an agreed list impossible at the moment. As a result, the number of marine invertebrates (mostly copepods) and fish are considered to be overestimated and need further revision. Moreover, six species, which were intentionally imported for aquaculture, are not included as they are mostly freshwater species (see Yankova et al., 2014). Cryptogenic species are included by most scientists but considered native by others and excluded. In many cases, historical introductions, such as the ship worm Teredo navalis, are not perceived as aliens and not included in lists. Species reported from drift material washed ashore are included as casual records.

The list for Mediterranean alien species is still an underestimate as it does not include monocellular algae. The diversity of marine microalgae is scarcely known in wide areas of the Mediterranean Sea. This makes it difficult to determine if a suspected microalgal invader was already present as part of the rare, hidden and unsampled phytoplankton. Therefore, phytoplankton has not been included in the analysis of the indicator. 

The uncertainties behind data sets on NIS reflect the challenge of adequately discovering and reporting new species or mapping their distribution, but also of compiling data at a national level (Zenetos et al., 2017). 


Zenetos, A. et al, 2017, Uncertainties and validation of alien species catalogues: The Mediterranean as an example, Estuarine, Coastal and Shelf Science 191 (2017) 171e187. Available:

Yankova, M. et al, (2014). Marine fishes in the Black Sea: recent conservation status. Mediterranean Marine Science,15/2, 366-379. Available at:

Rationale uncertainty

Climate change is proved to enhance the establishment of alien species (see Raitsos et al, 2010). Consequently, the presented analysis skews the real magnitude of the phenomenon since it focuses on NIS directly introduced by human activities.


Raitsos, D. E., Beaugrand, G., Georgopoulos, D., Zenetos, A., Pancucci-Papadopoulou, A. M., Theocharis, A., & Papathanassiou, E. (2010). Global climate change amplifies the entry of tropical species into the Eastern Mediterranean Sea. Limnology and Oceanography,55(4), 1478-1484. Available at:

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

Monika Peterlin


European Environment Agency (EEA)


Indicator code
MAR 002
Version id: 2

Frequency of updates

Updates are scheduled every 3 years


DPSIR: Pressure
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)


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