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Documenting temporal and spatial occurrence trends of Non-Indigenous Species (NIS) is essential to understand vectors and pathways of introduction, and for horizon scanning for future introductions. This study provides an overview of marine NIS found in the Republic of Ireland up to 2020. Taxonomic groups, species origin, and location of first reporting (counties) were compiled and analysed focusing on the last three decades. While the unambiguous characterisation of introduction events is challenging, analysis of 110 species corroborated the global weight of evidence that shipping activities to/from ports and marinas are the most likely vectors and pathways in Ireland. A comparable review study for the Netherlands revealed that most NIS were first introduced to mainland Europe and subsequently would take on average >15 years to reach Ireland. In the last two decades there has been an increase in NIS-focused surveys in Ireland. Incorporating data from these surveys in centralized national repositories such as the National Biodiversity Data Centre, will strongly aid the evaluation of potential NIS management responses. Furthermore, the availability of robust baseline data as well as predictions of future invaders and their associated vectors and pathways will facilitate the effective application of emerging monitoring technologies such as DNA-based approaches. [ABSTRACT FROM AUTHOR]

Information on temporal and spatial trends with regard to the introduction of non-indigenous species (NIS) is often sparsely available. These trends may potentially help improve the design and focus of monitoring programs, give insights into new pathways and hotspots, and facilitate horizon scanning. We provide an overview of 215 marine and brackish water NIS recorded in The Netherlands. Temporal trends over the most recent three decades for taxonomic groups, species origin, introduction vectors, and water systems were analysed. We attempt to explain the observed patterns and discuss factors that hamper their explanation. A shift in the region of origin from Pacific to W Atlantic can potentially be linked to legislation prohibiting Pacific oyster imports, whereas a subsequent shift backwards cannot. Case studies illustrate that NIS may not be first detected in the water systems where they were originally introduced. Additionally, it is shown that changes in allegedly native species' distribution or seasonal pattern should be linked to an introduced cryptic NIS instead. We also discuss the shortcomings of monitoring programs that were originally not focused on NIS, the importance of naturalists' observations, and the added value of a more recent network that is focused on NIS detection in the coastal waters of The Netherlands. [ABSTRACT FROM AUTHOR]