Environmental and ecological factors influencing the spillover of the non-native kelp, , from marinas into natural rocky reef communities.

The non-native kelp, , is considered one of the world's worst invasive species. The northeast Atlantic is a hotspot of invasion, yet there is limited knowledge on its invasion dynamics. In the UK its distribution is strongly associated with artificial structures, primarily marina and harbour pontoons, with relatively few records of on natural substrates. Here, the southwest UK is used as a case region, to explicitly link distribution-abundance patterns in artificial marina habitats with those in natural rocky reef habitats. Using a mixture of in situ recording and video survey techniques, was found at all thirteen marina sites surveyed; but in only 17 of 35 rocky reef sites, all of which were in 2 of the 5 larger systems surveyed (Plymouth Sound and Torbay). The distribution-abundance patterns of at reef sites were analysed using zero-inflated models. The probability of finding on rocky reef increased with increasing proximity to marinas with high abundances of . Total propagule pressure from marinas also increased the probability of occurrence, and was positively related to abundance and cover at reef sites. Increases in the cover of native kelps, spp., and wave exposure at reef sites were linked to a reduced probability of occurrence, and lower abundance and cover. Identifying high risk areas, natural boundaries and factors affecting the spread and abundance of non-native species in natural habitats is key to future management prioritisation. Where is confined to artificial substrates management may be deemed a low priority. However, the results of this study suggest that controlling the abundance and propagule pressure in artificial habitats may limit, to some extent, the spillover of into natural rocky reef habitats, where it has the potential to interact with and influence native communities.