Non-indigenous species threaten biodiversity in Europe’s seas and are targeted by EU policy. Since 1970, 640 species (excluding microalgae, pathogens and parasites) have invaded European waters, with an increasing average annual rate of new introductions. Between 2012 and 2017, on average 21.5 species per year were detected, up from seven in the 1970s. Introduction rates vary by species group, pathway and marine region, with the numbers of new species in the Mediterranean Sea being particularly notable. Transport – Stowaway/Shipping remains the main pathway of introduction, accounting for almost half of all new invasions, followed by unaided introductions from neighbouring non-EU waters (23%).

Figure 1. Number and cumulative number of marine non-indigenous species in Europe's seas, 1970-2020
Number and cumulative number of marine non-indigenous species in Europe's seas, 1970-2020

Non-indigenous species (NIS) can become invasive and spread rapidly, and this is widely recognised as a major threat to biodiversity worldwide. NIS can also threaten human and animal health and affect economic activities and livelihoods, and their impacts can be irreversible. Under ideal environmental conditions and ecological circumstances (e.g., in the absence of natural predators), NIS may spread and reproduce excessively, feeding on native species or out-competing them for space and other resources; sometimes also carrying parasites and diseases that can be lethal to native species or dangerous to human and animal health . Around 87 NIS are currently considered invasive in Europe's seas .

To address this, invasive alien species (IAS) are targeted by EU legislation and policy. Regulation (EU) No 1143/2014 establishes rules to prevent, minimise and mitigate the adverse impact on biodiversity of the introduction and spread of IAS within the EU. Moreover, the Marine Strategy Framework Directive (MSFD), aims to achieve good environmental status (GES) for all EU marine waters, such as by ensuring that NIS do not adversely alter ecosystems. It requires that the number of NIS newly introduced into the wild through human activity is minimised or reduced to zero and that Member States establish thresholds for NIS introductions.

Until 2020, at least 804 NIS (excluding microalgae and parasite/pathogen species) have been recorded in European marine waters, of which 640 have invaded since 1970. The vast majority are invertebrates (64.2%), followed by primary producers (18.4%) and vertebrates (17.3%). The proportion of each type of NIS and their annual rates of introduction vary among regional and subregional seas. The increase in the number of NIS at European level in recent years has been largely driven by introductions in the Mediterranean Sea and the North-East Atlantic, with an average of 14.0 and 11.7 new NIS per year between 2012-2017, respectively (Figure 1). Within the Mediterranean region, the Central Mediterranean is most prominent with an average of 10.5 new NIS introductions per year between 2012-2017. In the North-East Atlantic, the Bay of Biscay and Iberian Coast leads with seven new NIS per year during the same period (Figure 1 – extra).

The large increase, particularly of vertebrates and invertebrates, observed between 2006-2011 and 2012-2017 could indicate a general worsening of GES, but could also be the result of increased scientific effort and contribution of citizen science (Lehtiniemi et al., 2020). This increase could also be related to climate change, the role of which is further supported by the influx of tropical biota into European waters (e.g. Red Sea fish species in the Mediterranean). On the other hand, the decrease in the rate of introductions more recently, most notably of primary producers, does not necessarily indicate an improvement in GES but could be the result of the time elapsed between NIS being introduced and detected for the first time (Zenetos et al., 2019). In conclusion, the information available is currently insufficient to confidently assess GES based on the numbers of NIS introduced.

Figure 2. Pathways of introduction of non-indigenous species to Europe’s seas, 1970-2020
Pathways of introduction of non-indigenous species to Europe’s seas, 1970-2020

The transfer of NIS by seafaring vessels (transport-stowaway by hull fouling, ballast water and other) remains the main pathway of NIS dispersal, accounting for 46.6% of new introductions in European waters. This is followed by unaided introductions from neighbouring non-EU waters (22.8%), transport-contaminant, e.g. unintentional movement of live organisms (11.9%) and release in nature, i.e., intentional release (4.3%). Overall, the proportional increase in new introductions has risen over the last two six-year assessment periods for most of the pathway modes, most noticeably by hull fouling and escape from confinement, e.g., from aquaria, aquaculture and mariculture. These findings could be explained by increased scientific interest and effort to identify new NIS in hot spot areas such as ports and marinas (Azevedo, 2019; Tempesti et al., 2020). Nonetheless, they highlight that further action is needed to minimise future risks of new introductions and spread of invasive NIS and more knowledge on their impacts on biodiversity and ecosystems, in particular in a changing climate. For this it is crucial to have updated and validated NIS inventories, including baselines and threshold values so that policy implementation progress can be effectively monitored.