National implementation of the European Union policy framework for monitoring introduced non-indigenous species in marine and brackish waters: a Belgian case study based on an updated species inventory

Based on a thorough literature review and expert consultation, this study provides an inventory of all introduced non-indigenous species (iNIS) reported for Belgian marine and brackish waters. The data indicate a strong increase in iNIS in the study area from the 1990s onward, averaging 2.2 newly detected species per year, with a cumulative total of 108 iNIS between 1800 and 2024. The majority of these iNIS have the Northwestern Pacific or Northwestern Atlantic as their native region and are primarily introduced in Western Europe via shipping or aquaculture. In addition to compiling the inventory, the context in which the iNIS are detected is examined, distinguishing between official monitoring programs, project-based data collection efforts, and citizen science initiatives. Our findings indicate that while the EU aims to promote coordination between its Marine Strategy Framework Directive (MSFD) and Water Framework Directive (WFD), a misalignment occurs in the practical implementation of iNIS monitoring at the Belgian level. For example, a coherent and integrated monitoring framework across marine, brackish, and freshwater systems is still lacking. Furthermore, despite the EU’s ambition to ensure comprehensive iNIS monitoring, no legal framework currently mandates targeted monitoring in coastal ports, despite their well-documented role as hotspots for new marine introductions. After all, iNIS monitoring is only mandatory under the MSFD, which in essence applies only seaward from the coastal baseline and therefore does not cover waters within these ports. In addition, while the EU’s IAS Regulation has recently incorporated a few marine species on the Union list, its monitoring requirements remain primarily focused on terrestrial and freshwater species. As a result, observations published by citizens with significant expertise in the field represent the primary source of marine iNIS data in coastal port areas in recent decades in Belgium. The fragmentary nature of iNIS data complicates the efficient flow of information to international or European iNIS reference databases that support policy and decision-making. Yet, even species officially reported by Member States under MSFD Descriptor 2 are not always included in these reference databases. Nonetheless, accurate data on iNIS presence and distribution are essential for effectively targeting and managing iNIS.