Annex "Alien species"


National marine alien species monitoring programs

Within the Wadden Sea Area alien species are subject to national monitoring programs, which are presented below


The Netherlands

In the Netherlands, none of the ongoing monitoring programs for compliance with the Water Framework Directive and/or Natura 2000 regulations specifically focus on recording alien species in the Wadden Sea. When alien species are encountered during these programs, they are reported as such. In recent years, various studies and alien species-focused surveys were carried out (Gittenberger et al., 2009, 2010, 2012, 2015, 2019).



In accordance with a request by UNESCO, Germany initiated in 2009 a continuous specific monitoring of non-indigenous species (NIS) implementing the needs of the MSFD. The surveys focus on harbours, marinas, and aquaculture sites, which are assumed hotspots of species introductions. Annual, in which annual inspections were performed according to the Rapid Assessment (RAS) monitoring program by Buschbaum et al. (2012). Additionally, comprehensive investigations were conducted in the German Wadden Sea in 2014/2015 to provide baseline information on the presence of NIS (Rohde et al., 2015; Lieberum & Bock, 2016) considering also records of introduced species from other monitoring programs.

Since 2016, the RAS has been extended to a standardised Extended Rapid Assessment (eRAS) involving the deployment of settlement panels over summer at three different water depths at all monitoring sites. In autumn, after about 4 months of deployment, all species settled and associated to the panels are identified. This additional method is used to discover introduced hard-bottom species at an early stage of establishment and to detect species, which cannot be detected by the RAS alone. The investigations on alien species are conducted by several institutions and it is intended to compile all data and information in a joint marine Neobiota platform, which will be the link from science to authorities and policy makers.



In Denmark, the Danish National Aquatic Monitoring and Assessment Programme (NOVANA, also referred to as DNAMAP; Riemann et al. 2016) is implemented since 2004. The NOVANA programme replaced the Danish Aquatic Monitoring and Assessment Programme (NOVA-2003; Svendsen et al. 2005b; a), which had been running since 1998. NOVANA records, among other components, phytoplankton, zooplankton, angiosperms, algae and benthic invertebrates (Andersen et al., 2014). The Danish Nature Agency commissioned a study on the requirements for a monitoring program of alien species in Danish waters, including the Danish Wadden Sea (Andersen et al., 2014). The study proposed to supplement the conventional methods by molecular biological methods (DNA methods). Moreover, the monitoring programme should focus on hot spots and fully cover species groups, i.e., phyto- and zooplankton, macro-algae, benthic organisms, fish and marine mammals.

Stæhr et al. 2016 analysed the data from NOVONA and summarised the occurrence and trends of non-native species for the period 1989 to 2014, however, with the Danish Wadden Sea as a fraction of a greater area including the North Sea and Skagerrak. A report on the occurrence of alien species in Danish harbours is currently being prepared (Andersen et al., in prep.).


The trilateral alien species list update 2021

The following two tables list the marine alien species that have been detected in the Wadden Sea Area (Table 5) and in the margins of the Wadden Sea (Table 6).


Table 5: Trilateral aliens species list 2021 in the Wadden Sea Area (Gittenberger, 2021). The list considers species proofs up to 2020. The list is subject to continuous change and can only reflect an interim status, as knowledge about aliens is constantly expanding. New species, in comparison to the last QSR 2017, are highlighted by a red lettering.


Table 6: Alien species detected on the margins of the Wadden Sea Area (Gittenberger, 2021). The list considers species proofs up to 2020. The list is subject to continuous change and can only reflect an interim status, as knowledge about aliens is constantly expanding. Some of these alien species were included in the Trilateral alien species list of the QSR 2017 (see chapter 2.1).


A closer look at two marine species

Since that last QSR 2017, about 26 alien species were recorded in the Wadden Sea Area (see chapter 2.1, Figure 1). Two species, which have recently been found, are described in more detail by way of example.


Aoroides semicurvatus

Within the blue mussel monitoring in Schleswig-Holstein (Germany), the amphipod A. semicurvatus was found for the first time in the Wadden Sea Area in 2019 (Björn Nadarzinski, Labor für Meeresbiologie, Werkstätten Materialhof), on an intertidal oyster bed. The species originates from the Pacific Ocean off Japan and Korea, where it lives on oyster beds and among eulittoral algae. In Europe, A. semicurvatus occurred first on the French Atlantic coast in 2009 (Gouillieux et al., 2016) and in 2017 among red algae in the Dutch Oosterschelde (Faasse et al., 2018). Aquaculture and hull fouling are assumed introduction vectors. No invasive potential is known so far (Faasse et al., 2018). 


Vaucheria cf. velutina

The green alga Vaucheria velutina is a native species in the Wadden Sea (e.g. Krieg et al. 1988). The species colonizes poly- and euhaline sites, mostly close to the mean high tide line in the zone of Salicornia, Spartina and grassland (Puccinellia maritima salt-marsh community). Recently, however, extensive settlements of Vaucheria sp. have been detected on sandy tidal flats near the island of Sylt in the northern Wadden Sea, where the alga caused an increased sedimentation of fine particles , which leads to an accumulation of mud on the sandy sediment surface (Figure 1). Chloroplast DNA has been sequenced from native salt marsh V. velutina and V. cf. velutina found at the low tide level sandy flats at Sylt. The genetic difference justifies the recognition of a distinct species. Consequently, the name V. velutina refers to a species complex of cosmopolitan distribution and with a conservative morphology. The same applies to the congener Vaucheria longicaulis, also found at the low tide level near the island of Sylt. It is assumed that both have been introduced by transoceanic shipping or with shellfish translocations (Karsten Reise, pers. communication).


Spartina anglica – invasive alien and/or beneficial ecosystem engineer?

Common Cordgrass (Spartina anglica) is a widespread species in Wadden Sea salt marshes. Here, it is considered as a pioneer plant, establishing itself on bare tidal flats. In addition, it also occurs in depressions within the low and high marsh zones (see QSR chapter on salt marshes). The plant has a unique evolutionary origin: It developed from Spartina × townsendii, which is a naturally derived and sterile hybrid between the European native cordgrass Spartina maritima (Small Cordgrass) and the introduced American Spartina alterniflora (Smooth Cordgrass). Spartina × townsendii was recognized for the first time around 1870 in Southern England, to where Spartina alterniflora was unintentionally introduced, probably by ballast water some decades earlier. The allotetraploid Spartina anglica evolved by chromosome doubling out of Spartina × townsendii and was observed close to Southampton less than 25 years after the first appearance of this hybrid.

In the early 20th century, Spartina anglica has been planted at several sites along the Wadden Sea coast because it was considered to help build up new salt marshes. As the species was naturally not occurring in the Wadden Sea and as it was introduced here by humans, it has been considered an alien species. However, natural dispersal distances of salt marsh plants by ocean currents are comparatively high and it could thus be hypothesized that the species today would also have naturally reached the Wadden Sea from its place of evolutionary origin, which is only some 500 km southwest from the Wadden Sea. This is challenging the consideration of Spartina anglica being an alien plant in the Wadden Sea.

Spartina anglica was also introduced to coastlines in Asia, Australia, New Zealand and North America, areas far away from its place of evolutionary origin. Here, it is considered a serious invader to native coastal ecosystems. Consequently, the species is also listed by the World Conservation Union to be among the 100 ‘World’s Worst’ invaders (Lowe et al., 2000).

By oxygenizing its rhizosphere, by sediment trapping and increasing accretion rates and by contributing to the geomorphological development of newly establishing salt marshes in the Wadden Sea, Spartina anglica can also be considered as an important ecosystem engineer. For its establishment from seeds on bare tidal flats, it needs a certain ‘window of opportunity’, a period of time with comparatively low water levels allowing germination and establishment. Later on, the species develops into single tussocks, which mainly grow vegetative means and eventually merge and build dense Spartina swards. By lowering tidal currents within, but increasing them outside of tussocks, Spartina also determines the development of creeks in young pioneer marshes.

Spartina anglica is the dominant and characteristic species of the S. anglica vegetation type in the TMAP vegetation typology and of habitat H1320 (Spartina swards) of the European habitat directive (see also QSR chapter on salt marshes). It has not been proven and it is not expected that Spartina anglica is replacing native vegetation in Wadden Sea salt marshes in the sense that any native salt marsh plant might be prone to extinction due to the Spartina invasion. Although the species seems to speed up the development from tidal flats into salt marshes, this has to be considered also a natural process and in view of accelerating sea level rise in times of climate change the beneficial effects of Spartina seem to outweigh any anticipated possible negative impact.


Policies and directives

Current national policies addressing alien species management in the Wadden Sea are based upon relevant regional, international and European policies (Schuchardt & Sevilgen, 2015; van der Have et al., 2015). International policies include the IMO Ballast Water Management Convention, CITES trade in endangered species, and IPPC (plant health). Several global policies, regional conventions, articles and guidelines including the UN Biodiversity Convention as well as nature conservation guidelines are already integrated within the national policies for alien species management.

 The EU Regulation on invasive alien species addresses the problems on alien species directly and several European Directives address them indirectly (Table 7). Relevant EU directives include the Habitat and Bird Directives, the Marine Strategy Framework Directive, the Water Framework Directive and the Council Regulations No. 1143/2014/EC (IAS Prevention and management), No.304/2011/EU (IAS in aquaculture) and No. 338/97/EC (IAS in trade). In the Marine Strategy Framework Directive (MSFD) the descriptor (2) examines alien species and aims “Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystems”. Within the COMMISSION DECISION (EU) 2017/848 criteria and methodological standards on good environmental status of marine waters for Descriptor (2) are defined.

 According to the EU regulation 1143/2014/EC, countries are obliged to establish appropriate management measures for widespread invasive species, and to set up an action plan of measures to prevent the non-intentional introduction and spread of invasive species. The central element of the EU Regulation is the list of invasive alien species of Union importance (Union list). In 2016, the European Union published a first list of Invasive Alien Species of Union concern, which was updated in 2017 and 2019 (EU 2016, 2017, 2019). The invasive species, occurring on the Union, list must be monitored. Furthermore, pathways of non-intentional introduction and spread are to be analysed and prioritised (Art. 13 (1)). For the identified priority pathways, so-called action plans are to be set up and implemented (Art. 13 (2)). The Regulation explicitly states that Member States may compile a list of invasive alien species of importance to Member States (Art. 12). 

The development of the trilateral "Management and Action Plan for Alien Species" (MAPAS) is based on the relevant EU- and other international policies mentioned above and the three Wadden sea countries currently focus on different elements of the MAPAS as priority management options for alien species.

In the Netherlands, the elements of protection and early warning system in the MAPAS are currently being elaborated for implementation in the Wadden Sea. These elements of the MAPAS, and the others being elaborated by Germany and Denmark, will inform the trilateral  management measures for alien species in the Wadden Sea.

NEHRING & SKOWRONEK (2020) delivered information on the occurrence of the species of the Union list in Germany, on possibilities of confusion with native species as well as general advice on possible removal and control measures. The action plan on priority pathways of invasive species with public participation has been completed and the Action Plan is in further preparation . In Denmark, the “Action plan against invasive species was published in 2017.


Table 7: European Directives and Regulations, which address alien species directly or indirectly.


The Trilateral Strategic Framework for Alien Species and objectives of the Expert Group Alien Species

At the 11th Trilateral Governmental Conference on Sylt (2010), it was agreed to develop a common strategy for the Wadden Sea to deal with the introduction and the impacts of alien species (CWSS, 2010a) within the framework of the proposal from the UNESCO World Heritage Committee in 2009 : to “implement a strict monitoring program to control invasive species associated with ballast water and aquaculture in the property”. Following this decision, a Trilateral Strategic Framework for Alien Species management (TSFAS) was elaborated. 

At the 12th Trilateral Wadden Sea Conference in Tønder in 2014 (CWSS, 2014), it was agreed to further develop the TSFAS and to coordinate the further development of an alien species management action plan. The trilateral Task Group Management and Monitoring (TG-MM) was mandated to coordinate these initiatives with input from an Expert Group Alien Species (EG-AS).
The Trilateral Wadden Sea Management and Action Plan for Alien Species (MAPAS) was adopted at the 28th meeting of the Wadden Sea Board on 14 March 2019. It compiles the measures, which are already established as obligations in international and EU law regarding the Wadden Sea Area and proposals for trilateral actions. Main elements of the MAPAS are prevention, monitoring, risk assessment, decision making, management and/or control and an evaluation of effectiveness of applied measures (WG-AS & Gittenberger, 2019).

Current alien species management measures in the Wadden Sea are based on the draft TSFAS, which consists of five main elements: (1) prevention; (2) early warning and rapid response; (3) eradication and control; (4) raising awareness and (5) implementation (see chapter 8.7). Since 2014 TG-MM and WG-AS have elaborated an alien species management and action plan to contain the five main elements, which is presented schematically in Figure 8. This scheme is divided into seven elements, (2) Early warning and rapid response is covered by monitoring and risk assessment to create a better understanding of the model. The work done so far is briefly described in the paragraph below. 


Project 1. Development of an inventory of alien species related policies for the Wadden Sea: 

This project was completed in 2015. Existing national, European, and international policies related to alien species management were evaluated for their relevance for the Wadden Sea. Furthermore, priorities for the trilateral alien species management and action plan were defined (Schuchardt & Sevilgen, 2015; van der Have et al., 2015). 


Project 2. Development of a trilateral alien species list: 

This work has been completed for the aquatic part of the Wadden Sea (GITTENBERGER 2016; see Annex 1). Work on the terrestrial part of the Wadden Sea in the three countries was done simultaneously. Alien species inventory in the Netherlands was completed in 2015 (Lensink et al., 2015) and in Denmark and Germany the list of terrestrial alien species was prepared (Büttger et al., 2017).


Project 3. Development of a proposal for a harmonised monitoring program for alien species in the Wadden Sea (TMAP-AS): 

This project is ongoing. The project objective is to harmonise existing national monitoring and assessment activities with the desired TSFAS-based monitoring requirements for alien species in the Wadden Sea (Van der Have & Lensink, 2017).

The focus of EG AS activities are the following:

  • extending the 2017 list of aquatic alien species to cover terrestrial alien species in the Wadden Sea Area
  • developing a common methodology for alien species risk assessment.
  • developing a scheme for awareness raising


Figure 8: A schematic presentation of the management action plan for alien species in the Wadden Sea (WG-AS & Gittenberger, 2019).


Measures of the Trilateral Strategic Framework for Alien Species management in the Wadden Sea 

The relevant national, regional and international policies, guidelines and directives (see chapter 8.5) inform the TSFAS which is used as the bases to develop the trilateral Management and Action Plan for aliens (MAPAS). The MAPAS describes management measures including prevention, early warning and rapid response, eradication and control, and raising awareness and education in the relevant communities. The current mode of application of these measures is shown below: 


(1) Prevention: All three Wadden Sea countries have ratified the IMO-Ballast Water Management Convention . Specific measures against introduction of alien species to the Wadden Sea through the import of seed mussels for aquaculture have been developed in the Netherlands. Measures against mussel transportation are irrelevant for Germany and for Denmark. In Germany the import of seed mussels into the Wadden Sea is forbidden and there is no mussel farming activity in Denmark. 


(2) Early warning and rapid response to new alien species introductions: Monitoring and surveillance teams in the Netherlands carry out risk assessment procedures to inform decision making at the Ministry of Economic Affairs. In Germany, a benthic monitoring network and a neobiota knowledge platform is set up to record new introductions to the Wadden Sea. In Denmark, various monitoring activities are in place to enhance the recording of new introductions. A common approach to early warning and rapid assessment with relevant procedures and protocols need to be developed with a website to facilitate the process of information sharing.


(3) Eradication and control: Current measures to control alien species transportation are a necessity. In the Netherlands, the management and control systems for mussel transport are enforced by Dutch law specifically for importing mussels into the Wadden Sea. This management and control systems based on the Shellfish Import Monitoring Protocol (Gittenberger et al., 2010) is applied during mussel transportation from the Scheldt estuaries to the Wadden Sea. More effort is needed to develop possible eradication and control measures for trilateral application in future. These may include exchange of experiences and best practices in the field and in the management sector.

(4) Raising awareness and education: More effort is needed to develop the elements of the TSFAS at national and trilateral levels. Informing stakeholders about alien species introductions is part of the Management action plan for alien species (see Figure 8). 


Collection of internet links

International, European, trilateral and national websites with information on alien species are listed below (without claiming completeness).


Global and European webpages and information:


Trilateral webpages and information:


Alien species information:


The Netherlands: