Summary 1.Biological invasions are characterized by occasional long-distance, human-assisted dispersal. Centres of human transportation that are connected by trade to a wide range of other locations ('transport hubs') may be important catalysts of the rate at which new populations of an invader are established. 2. We developed a spatially explicit stochastic model to simulate the spread of a hypothetical marine invader by hull fouling. The model was based on classic 'Susceptible-Infected-Resistant' models used in medical epidemiology. It was parameterized using empirical data on the colonization of vessel hulls by fouling organisms, and on maintenance and travel patterns of ~1300 domestic and international yachts around New Zealand. Thirty-six marinas were grouped into three categories that represented a gradient in the number of other transport nodes each marina was 'connected' to and the frequencies of yacht movements between them. Invasions were seeded in three locations from each category. Simulations were run over 10 years to determine differences in the trajectory of invasions originating from busy and less frequented transport nodes. 3. Busy 'hub' locations were 75% more likely to become infected by an invader than quieter locations. Infection of hub nodes occurred at an earlier average stage in the invasion sequence. This occurred irrespective of whether the initial source of the invasion was associated with low or high traffic volume and connectivity. 4. Biotic invasions originating from hub locations did not consistently result in faster spread, or a larger number of secondary infestations. However, the rate of spread from hubs was less variable than from quieter nodes and was less often preceded by a prolonged lag period. 5. Synthesis and applications. Rapid spread of invasive organisms can occur from busy and from seemingly unimportant transport nodes. Busy locations were consistently more likely to become infested by an invader and to accelerate spread to secondary locations faster. Busy transport hubs should be considered a priority for the allocation of preventative and management efforts, such as regular baseline or target surveys and the development of incursion response plans that minimize the risk of spread within the transport network.

Human transport hubs, such as shipping ports, airports and mail centers are important foci for the spread of non-indigenous species. High relative abundance in a transport hub has been proposed as a correlate of invasion success, since abundant species are thought more likely to colonize vectors and to be transported more frequently. We here evaluate the relative importance of vector characteristics and local source assemblages in determining the pool of species that is transported by hull fouling on recreational boats. We compared the resident fouling communities of three recreational boat harbors in Australia with the assemblages on the hulls of boats that travel between them. We used data on the recent travel and maintenance history of the boats to evaluate correlates of transport probability and the potential for intra-coastal spread of fouling organisms. Invertebrate assemblages on heavily fouled vessels reflected the composition of biotic assemblages within the marina in which they were moored, but by itself, relative abundance in the source port was not a reliable predictor of transport probability. More important was the age of the antifouling paint on the vessels' hulls, which acted selectively on some groups of organisms. Movements of vessels were characterized by 'fidelity' (vessels remaining close to homeport) interspersed with 'promiscuity' (vessels traveling to a diverse pool of destinations). In an infested harbor, measures taken to increase the resistance of vectors to colonization by the invader should be effective in slowing the rate of spread to other locations, by decreasing the overall frequency of transport.

Numerous studies have shown how interactions between nonindigenous species (NIS) can accelerate the rate at which they establish and spread in invaded habitats, leading to an “invasional meltdown.” We investigated facilitation at an earlier stage in the invasion process: during entrainment of propagules in a transport pathway. The introduced bryozoan Watersipora subtorquata is tolerant of several antifouling biocides and a common component of hull‐fouling assemblages, a major transport pathway for aquatic NIS. We predicted that colonies of W. subtorquata act as nontoxic refugia for other, less tolerant species to settle on. We compared rates of recruitment of W. subtorquata and other fouling organisms to surfaces coated with three antifouling paints and a nontoxic primer in coastal marinas in Queensland, Australia. Diversity and abundance of fouling taxa were compared between bryozoan colonies and adjacent toxic or nontoxic paint surfaces. After 16 weeks immersion, W. subtorquata covered up to 64% of the tile surfaces coated in antifouling paint. Twenty‐two taxa occurred exclusively on W. subtorquata and were not found on toxic surfaces. Other fouling taxa present on toxic surfaces were up to 248 times more abundant on W. subtorquata. Because biocides leach from the paint surface, we expected a positive relationship between the size of W. subtorquata colonies and the abundance and diversity of epibionts. To test this, we compared recruitment of fouling organisms to mimic W. subtorquata colonies of three different sizes that had the same total surface area. Secondary recruitment to mimic colonies was greater when the surrounding paint surface contained biocides. Contrary to our predictions, epibionts were most abundant on small mimic colonies with a large total perimeter. This pattern was observed in encrusting and erect bryozoans, tubiculous amphipods, and serpulid and sabellid polychaetes, but only in the presence of toxic paint. Our results show that W. subtorquata acts as a foundation species for fouling assemblages on ship hulls and facilitates the transport of other species at greater abundance and frequency than would otherwise be possible. Invasion success may be increased by positive interactions between NIS that enhance the delivery of propagules by human transport vectors.

Aim Anticipated changes in the global ocean climate will affect the vulnerability of marine ecosystems to the negative effects of non‐indigenous species (NIS). In the Arctic, there is a need to better characterize present and future marine biological introduction patterns and processes. We use a vector‐based assessment to estimate changes in the vulnerability of a high‐Arctic archipelago to marine NIS introduction and establishment. Location Global, with a case study of Svalbard, Norway. Methods We base our assessment on the level of connectedness to global NIS pools through the regional shipping network and predicted changes in ocean climates. Environmental match of ports connected to Svalbard was evaluated under present and future environmental conditions (2050 and 2100 predicted under the RCP8.5 emissions scenario). Risk of NIS introduction was then estimated based on the potential for known NIS to be transported (in ballast water or as biofouling), environmental match, and a qualitative estimate of propagule pressure. Results We show that Svalbard will become increasingly vulnerable to marine NIS introduction and establishment. Over the coming century, sea surface warming at high latitudes is estimated to increase the level of environmental match to nearly one‐third of ports previously visited by vessels travelling to Svalbard in 2011 (n = 136). The shipping network will then likely connect Svalbard to a much greater pool of known NIS, under conditions more favourable for their establishment. Research and fishing vessels were estimated to pose the highest risk of NIS introduction through biofouling, while ballast water discharge is estimated to pose an increased risk by the end of the century. Main conclusions In the absence of focused preventative management, the risk of NIS introduction and establishment in Svalbard, and the wider Arctic, will increase over coming decades, prompting a need to respond in policy and action.

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In this experimental study the patterns in early marine biofouling communities and possible implications for surveillance and environmental management were explored using metabarcoding, viz. 18S ribosomal RNA gene barcoding in combination with high-throughput sequencing. The community structure of eukaryotic assemblages and the patterns of initial succession were assessed from settlement plates deployed in a busy port for one, five and 15 days. The metabarcoding results were verified with traditional morphological identification of taxa from selected experimental plates. Metabarcoding analysis identified > 400 taxa at a comparatively low taxonomic level and morphological analysis resulted in the detection of 25 taxa at varying levels of resolution. Despite the differences in resolution, data from both methods were consistent at high taxonomic levels and similar patterns in community shifts were observed. A high percentage of sequences belonging to genera known to contain non-indigenous species (NIS) were detected after exposure for only one day.

Greenshell TM mussel (Perna canaliculus) culture is the primary aquaculture industry in New Zealand. However, our knowledge of biofouling on Greenshell TM mussel farms, and its contribution to farm ecotrophic effects, is poor. We conducted a preliminary study of biofouling accumulation at two Greenshell TM mussel farms during Intermediate and Final seed on-growing stages (each of 6 months duration) with sampling of mussel ropes at 0, 3, 5 and 6 months during each on-growing stage. A diverse range of biofouling organisms (71 distinct taxa) accumulated on mussel ropes, with biofouling biomass dominated by suspension-feeding organisms (*88% of biofouling biomass) such as other bivalves, ascidians and, to a lesser degree, bryozoans. Biofouling biomass increased with culture time, varied between farms and was generally greater at 2 m than at 8 m depth. After 6 months, biofouling organisms on average comprised 54% of the total rope biomass. The reseeding of ropes between Intermediate and Final seed crops reduced the amount of non-Greenshell TM mussel biofouling. However, after 6 months, non-Greenshell TM mussel biofouling on average still comprised 15% of the total rope biomass. To evaluate potential ecotrophic effects of biofouling on Greenshell TM mussel farms, we compare the clearance rates of Greenshell TM mussel longlines based on Greenshell TM mussels alone and when combined with the two dominant biofouling species observed in our study (the mussel Mytilus galloprovincialis and the ascidian Ciona intestinalis). Our study shows that accumulated biofouling biomass on Greenshell TM mussel ropes can be significant and recommends further investigation as to actual ecotrophic effects of biofouling to ensure sustainable mussel farm practices.

Preventing the introduction of nonindigenous species (NIS) is the most efficient way to avoid the costs and impacts of biological invasions. The transport of fouling species on ship hulls is an important vector for the introduction of marine NIS. We use quantitative risk screening techniques to develop a predictive tool of the abundance and variety of organisms being transported by ocean-going yachts. We developed and calibrated an ordinal rank scale of the abundance of fouling assemblages on the hulls of international yacht hulls arriving in New Zealand. Fouling ranks were allocated to 783 international yachts that arrived in New Zealand between 2002 and 2004. Classification tree analysis was used to identify relationships between the fouling ranks and predictor variables that described the maintenance and travel history of the yachts. The fouling ranks provided reliable indications of the actual abundance and variety of fouling assemblages on the yachts and identified most (60%) yachts that had fouling on their hulls. However, classification tree models explained comparatively little of the variation in the distribution of fouling ranks (22.1%), had high misclassification rates (approximately 43%), and low predictive power. In agreement with other studies, the best model selected the age of the toxic antifouling paint on yacht hulls as the principal risk factor for hull fouling. Our study shows that the transport probability of fouling organisms is the result of a complex suite of interacting factors and that large sample sizes will be needed for calibration of robust risk models.