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Harbours are highly modified habitats that differ from natural areas. They are hotspots of non-indigenous species (NIS) and act as stepping-stones in invasive processes. However, local communities can exert biotic resistance against biological invasions through trophic interactions and competition. This study assesses the biotic effects of predation on the recruitment of fouling assemblages in three marinas of NE Atlantic Portugal (Cascais, Setúbal and Sines), with particular emphasis on NIS, using predator exclusion experiments. Predation increased the relative abundance of NIS, mainly Watersipora subatra, in the estuarine marinas of Cascais and Setúbal, while no predation effects were registered in the coastal marina of Sines. Therefore, predation can increase the risk of NIS invasion (biotic facilitation). Furthermore, local ecosystems may have different effects and differ in vulnerability against NIS invasions. Finally, a better understanding of coastal invasive ecology and biotic effects in coastal artificial habitats will improve our capacity for NIS management. Agência Regional para o Desenvolvimento da Investigação, Tecnologia e Inovação M1420-09-5369-FSE-000002 Ministerio de Ciencia, Innovación y Universidades PRE2018-086266, CGL 2017-82739-P Fundação para a Ciência e a Tecnologia UIDB/04292/2020, LA/P/0069/2020, CEECINST/00098/2018, 2020.01797.CEECIND