Indicator Assessment

Trends in marine non-indigenous species

Indicator Assessment
Prod-ID: IND-344-en
  Also known as: MAR 002
Published 10 Dec 2019 Last modified 18 Nov 2021
28 min read
This page was archived on 18 Nov 2021 with reason: No more updates will be done

Available data show that  around 1 223 non-indigenous species (NIS) are present in the Europe’s seas, of which almost 81% (1 039) were recorded in the period 1949-2017. The species in question consist mostly of invertebrates (approx. 63 %).

The number of NIS is highest in the Mediterranean Sea, where almost 69 % (838) of all NIS have been recorded. A total of 21% (256) were recorded in the North-East Atlantic Ocean, 5 % (66) in the Baltic Sea and 3 % (32) in the Black Sea.

Mean numbers of new NIS recorded in the period of 2006-2011 (calculated for 6 yearly periods)  were 28 species per year. The rate of new NIS recording slowed down to 16 species per year during the 2012-2017 period.

More than 80 of NIS species were identified as 'invasive alien species' (IAS)  with a high potential to negative impact biodiversity and cause economic and social consequences.

Cumulative numbers of non-indigenous species in European Seas

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Cumulative number of non-indigenous species recorded in Europe’s regional seas, 1949-2017

NIS Introduction
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Temporal variability in numbers of new marine non-indigenous species per functional group, recorded in Europe's Seas

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Temporal variability in numbers of new marine non-indigenous species per functional group, recorded in each of Europe's regional seas

All seas
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North East Atlantic Sea
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Baltic Sea
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Mediterranean Sea
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Black Sea
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Icelandic Shelf
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Europes' sea

Of the 1 223 NIS recorded in Europe seas, 1 039 (81 %) were identified in the period 1949–2017 (Figure 1).  This is a current best estimate of new NIS introductions at the pan-European level, although many species have probably not been recorded yet. NIS consist primarily of invertebrates (63% — mostly crustaceans and molluscs), followed by primary producers (25% — marine plants and algae) and vertebrates (12% — mostly fish). 824 NIS were identified in the waters under jurisdiction of EU Member States, reported under Marine Strategy Framework Directive (MSFD; Tsiamis et al., 2018). 

This information was aggregated for regional seas (Figure 2). The number of NIS is highest in the Mediterranean, where almost 69 % (838) of all NIS have been recorded. A further 21% (256) of NIS was recorded in the North-East Atlantic Ocean, 5 % (66)  in the Baltic Sea and 3 % (32) in the Black Sea.

Between 2006-2011 a mean of 28 new NIS were recorded in European seas per year. Between 2012-2017 the mean number of new NIS recorded in European seas were reduced to 16 species per year (Figure 3). This decreasing trend is obvious for all EU regional seas (Figure 4) and for all high taxonomical groups. At the national level, the number of marine species introduced as a result of human activity has decreased in some countries (ICES, 2018).

The low number of NIS does not imply that there is less impact on the regional seas then on seas with a high number of NIS. The high number of such species in the Mediterranean Sea does not imply a higher overall impact because many of them are harmless. A number of NIS (fish, crabs, and shrimps) are commercially exploited.

More than 80 of NIS species were identified as 'invasive alien species' (IAS) during the work of experts in the period 2014 – 2019 (Katsanevakis et al., 2014; Katsanevakis et al., 2016; EEA, 2019).  IAS have a high potential to have negative impacts on biodiversity and cause economic and social consequences. IAS species establish themselves after introduction, spread rapidly and can threaten biological diversity in various ways — from reducing genetic variation and eroding gene pools, to the extinction of endemic species, and by altering habitat and ecosystem functioning (Katsanevakis et al., 2014).  While many NIS become part of the ecosystem into which they are introduced, only a few actually become IAS and cause the above mentioned negative impacts. IAS can hamper human activities and livelihoods that use or depend on the ecosystem, including shipping, fisheries, aquaculture production, recreation and tourism. They can also represent a threat to public health. Even when introduced locally, the impacts of marine invasions are generally widespread and irreversible. IAS can therefore have significant ecological, economic or social effects, threatening biodiversity and ecosystem services. The impacts of alien species on local ecosystems are still poorly assessed. 

Baltic Sea

Out of the total of 132 NIS and cryptogenic species recorded, 59 % are currently established in at least one country surrounding the Baltic Sea (Ojaveer et al., 2017).

According to our data, 66 alien species have been reported in the Baltic Sea since 1949. However, the number of new recordings is decreasing (Figure 4). In particular, no new marine species were observed in Denmark, Estonia, Finland, Germany, Lithuania, Poland, Sweden and Russia in 2017 (ICES, 2018).

On the other hand, NIS already introduced in many countries have significantly expanded their distribution range. The spread of already established species such as the round goby (Neogobius melanostomus) and the North American mud crab (Rhithropanopeus harrisii) are most significant. These two species have recently colonised multiple sub-basins of the Baltic Sea and because of their rapid range expansion, increasing densities and local novelty, they are expected to have strong separate or interactive effects on native communities. Laboratory experiments have demonstrated that the round goby and the mud crab have exerted a significant predation pressure on different benthic invertebrate species and that the effects of the predators studied were largely independent (Nurkse et al., 2018).

The economic effects of different species of Marenzelleria worm in the Baltic Sea were estimated to range from EUR 28 billion to EUR 105 billion, depending on the effect of sequestration of phosphorus by the worms. The average annual cost corresponds to 0.03–0.13 % of total gross domestic product of the nine coastal countries. However, the cost was unevenly distributed among the countries, with Poland bearing the largest share because of its large phosphorus loads and access to low-cost abatement options (Gren et al., 2018).

North-East Atlantic (Greater North Sea, Celtic Seas, Iberian Shelf and Bay of Bisca

As in other European seas, the rate of new recorded NIS is decreasing. From 49 new species in the period 2006-2011, the rate dropped to 21 species in the period 2012-2017 (Figure 4).

Rates of new NIS recording are a relatively constant in the three sub-regions assessed. Therefore, more effort to reduce the current rate of introduction should be considered. Differences in the rates of introduction and recording are relatively small between regions and not statistically significant over the reporting periods (OSPAR, 2017).

 Mediterranean Se

The trend in the introduction of NIS to the Mediterranean Sea peaked in the 2000-2005 period with approximately 21 new species per year (126 in total). The rate appears to have decreased over the last 6 years to 95 species (Figure 4). The relative decrease in the eastern Mediterranean (Zenetos, 2017), where the enlargement of the Suez Canal was of significant concern (Galil et al., 2015) is notable.

The alien species list includes the 821 multicellular species reported by December 2016 (Zenetos et al., 2017), and updated with new records and unicellular organisms. All analyses are based on 838 alien taxa observed for the first time after 1949. Figure 4c shows the contribution of marina taxa to NIS composition in the Mediterranean. Invertebrates dominate the list with >63 % (532) of species, represented mostly by molluscs and decapods (Zenetos et al., 2017; Galil et al., 2018). Primary producers follow with approximately 170 species, including macro-algae and, in particular, rhodophytes. Vertebrates (fish only) are dominated by Lessepsian migrants (Bariche & Fricke, 2018), but over the last decade the number of fish species intentionally released into the wild has increased (Zenetos et al., 2016; Marcelli et al., 2017; Deidun et al., 2018; etc).

Black Sea

110 NIS were recorded in the Black Sea as a whole (EU and non-EU). 52 of  NIS were invertebrates, 6 vertebrates and 52 primary producers, recorded after 1970;  64 of them were recorded after 2000. A decreasing trend is obvious with just 11 new NIS reported in the 2012-2017 period, against 21 in the 2006-2011 period (Figure 4). However, some past invaders seem to be established and spreading, such as the Arcuatula senhousia mussel. This mussel was originally found only in Constanza, from the Russian part of the Azov-Black Sea basin (Kovalev et al., 2017) and the Varna Gulf (Todorova in Chartosia et al., 2018).

Kasapoglu et al. (2015) have summarised the effects of NIS in the Black Sea. They pointed out that some of the alien species i.e.Rapana venosa, Anadara inaequivalvis, Mnemiopsis leidyi, Beroe ovata, Mya arenaria, Balanus improvisus, Mugil soiuy, and Potamopyrgus jenkinsi caused significant impacts on the Black Sea ecosystem and fisheries. Many of these impacts, such as predation, food competition and major destruction in the food web, have led to reductions in total fish production in the Black Sea. The population status and trends of selected invasive species in the Varna Bay, Bulgaria, has been described by Ivanova et al. (2017), while a study of zoobenthic NIS along the Crimean Black Sea Coast revealed that despite an increasing trend in their abundance between 2010-2016, their impact on the diversity of the benthic communities was low (Shalovenkov, 2017).

Icelandic Shelf

The trend analysis for NIS in the Icelandic shelf is encouraging. No new species were reported for the 2012-2017 period (Figure 4).


References in assessment of trends

  •  EEA, 2019, Korpinen, S., Nurmi, M., Klančnik, K., Laamanen, L., Peterlin, M.,  Zupančič, G., Andersen, J.H., Murray, C., Harvey, T., Zenetos, A., Stein, U., Tunesi, L., Abhold, K., Bolman, B., E., Piet, G., Kallenbach, E., Agnesi, S., Vaughan, D., Royo Gelabert, E., Reker, J., 2019, Multiple pressures and their combined effects in Europe’s seas, ETC/ICM Technical Report X/2019, Magdeburg: European Topic Centre on inland, coastal and marine waters.
  • Katsanevakis, S., Wallentinus, I., Zenetos, A., Leppäkoski, E., Çinar, M. E., Oztürk, B., Cardoso, A. C., 2014, Impacts of invasive alien marine species on ecosystem services and biodiversity: a pan-European review', Aquatic Invasions 9(4), pp. 391–423.
  • Katsanevakis, S., Tempera, F., & Teixeira H. 2016. Mapping the impact of alien species on marine ecosystems: the Mediterranean Sea case study. Diversity and Distributions, 22: 694-707.
  • Tsiamis, K., Zenetos, A., Deriu, I., Gervasini, E., & Cardoso, A. C. (2018). The native distribution range of the European marine non-indigenous species. Aquatic Invasions, 13(2), 187-198.

Baltic Sea

  • Gren, M., Sandman, A. N., & Näslund, J. (2018). Aquatic invasive species and ecosystem services: Economic effects of the worm Marenzelleria spp. in the Baltic Sea. Water Resources and Economics.
  • ICES (2018). Report of the Working Group on Introduction and Transfers of Marine Organisms (WGITMO). 7-9 March 2018, Madeira, Portugal.
  • Nurkse, K., Kotta, J., Rätsep, M., Kotta, I., & Kreitsberg, R. (2018). Experimental evaluation of the effects of the novel predators, round goby and mud crab on benthic invertebrates in the Gulf of Riga, Baltic Sea. Journal of The Marine Biological Association of The United Kingdom, 98(1), 25-31.
  • Ojaveer, H., Olenin, S., Narščius, A., Florin, A. B., Ezhova, E., Gollasch, S., ... & Strāke, S. (2017). Dynamics of biological invasions and pathways over time: a case study of a temperate coastal sea. Biological invasions, 19(3), 799-813.

 North-East Atlantic

  • OSPAR, 2017, Trends in New Records of Non-Indigenous Species Introduced by Human Activities,

Mediterranean Sea

  •  Bariche, M., & Fricke, R. (2018). Dipterygonotus balteatus (Valenciennes, 1830)(Teleostei: Caesionidae), a new alien fish in the Mediterranean Sea. BioInvasions Records, 7(1), 79-82.
  • Deidun, A., De Castro, D., & Bariche, M. (2018). First Record of The Azure Demoiselle, Chrysiptera Hemicyanea (Actinopterygii: Perciformes: Pomacentridae), In The Mediterranean Sea. Acta Ichthyologica et Piscatoria, 48(1).
  • Galil, B. S., Boero, F., Campbell, M. L., Carlton, J. T., Cook, E., Fraschetti, S., ... & Marchini, A. (2015). ‘Double trouble’: the expansion of the Suez Canal and marine bioinvasions in the Mediterranean Sea. Biological Invasions, 17(4), 973-976.
  • Galil, B. S., Marchini, A., & Occhipinti-Ambrogi, A. (2018). East is east and West is west? Management of marine bioinvasions in the Mediterranean Sea. Estuarine, Coastal and Shelf Science.
  • Marcelli, M., Dayan, A. R., & Langeneck, J. (2017). Finding Dory: first record of Paracanthurus hepatus (Perciformes: Acanthuridae) in the Mediterranean Sea. Marine Biodiversity, 47(2), 599-602.
  • Zenetos, A. (2017). Progress in Mediterranean bioinvasions two years after the Suez Canal enlargement. Acta Adriatica, 58(2).
  • Zenetos, A., Apostolopoulos, G., & Crocetta, F. (2016). Aquaria kept marine fish species possibly released in the Mediterranean Sea: first confirmation of intentional release in the wild. Acta Ichthyologica et Piscatoria, 46(3).
  • Zenetos, A., Çinar, M. E., Crocetta, F., Golani, D., Rosso, A., Servello, G., ... & Verlaque, M. (2017). Uncertainties and validation of alien species catalogues: The Mediterranean as an example. Estuarine, Coastal and Shelf Science, 191, 171-187

Black Sea

  • AquaNIS. Editorial Board, 2015. Information system on Aquatic Non-Indigenous and Cryptogenic Species. World Wide Web electronic publication. Version 2.36+. Accessed 2018-07-25.
  • Alexandrov, B. (2014). Non-indigenous species in the Black Sea and the Sea of Azov. Communication presented at WGITMO, Palanga 2014.
  • Boltachev, A. R., & Karpova, E. P. (2014). Faunistic revision of alien fish species in the Black Sea. Russian journal of biological invasions, 5(4), 225-241.
  • Çinar, M. E. (2016). The alien ascidian Styela clava now invading the Sea of Marmara (Tunicata: Ascidiacea). ZooKeys, (563), 1.
  • Ivanova, P. P., Trayanova, A. T., Stefanova, K. B., Stefanova, E., Raykov, V. S., & Doncheva, V. G. (2017). Population Status of Some Alien Species in Varna Bay, Bulgarian Black Sea Coast (2015-2016). Acta Zoologica Bulgarica, Supplement, 9, 73-82.
  • Kasapoglu, N., Duzgunes, E., Saglam, N. E., & Saglam, H. (2015). Alien Species And Their Impacts In The Black Sea.
  • Kovalev, E. A., Zhivoglyadova, L. A., Revkov, N. K., Frolenko, L. N., & Afanasyev, D. F. (2017). First record of the bivalve Arcuatula senhousia (Benson, 1842) in the Russian part of the the Azov-Black Sea basin. Russian Journal of Biological Invasions, 8(4), 316-320.
  • Shalovenkov, N. (2017). Non-native zoobenthic species at the Crimean Black Sea Coast. Mediterranean Marine Science, 18(2), 260-270.
  • Turan, C., Gürlek, M., Özeren, A., & Doğdu, S. A. (2017). First Indo-Pacific fish species from the Black Sea coast of Turkey: Shrimp scad Alepes djedaba (Forsskål, 1775)(Carangidae). Natural and Engineering Sciences, 2(3), 149-157.




Supporting information

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



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:

Data sources

Other info

DPSIR: Pressure
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • MAR 002
Frequency of updates
Updates are scheduled every 3 years
EEA Contact Info


Geographic coverage

Temporal coverage


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