The following tables are included: APPENDIX 1. Overview of the iNIS in marine and brackish waters in Belgium, including the year of first observation in Belgium, the status of establishment in Belgium, the year of first observation in Europe and first observation in Western Europe, the native region(s), and the primary introduction pathways and vectors responsible for introductions in Western Europe. Abbreviations used to indicate introduction pathways and vectors are explained below. APPENDIX 2. Overview of the iNIS in marine and brackish waters in Belgium, as recorded in WRiMS and Verleye et al. (2024) prior to the present study, with an explanation of the actions taken to optimize (in terms of completeness) and align both inventories. APPENDIX 3. Inventory of marine and brackish iNIS first observed in Belgium after the initial MSFD benchmarking period (2011), including the data collection context and publication level of the first observation for each species, their habitat type, observed distribution and level of establishment in Belgium. Further, the table indicates whether the species occur within the legal implementation area of the MSFD, if they were included in formal Belgian MSFD reportings (Belgian State 2018a, Kerkhove and Kerckhof 2024) and if the species’ occurrences in Belgium were documented in reference databases prior to this study.

The dataset on introduced non-indigenous species in the Belgian marine and brackish waters can be considered complete for the period 1801–2024. However, as new species are introduced every year, the dataset represents a snapshot of the situation in 2024. Consequently, the status of the data collection was marked as 'completed' in the context of the present dataset and accompanying publication. In reality, however, monitoring continues and additional species will still be identified in the future.