This study assessed the influence of summer seawater temperature and shipping on the introduction, establishment, and spread of nonindigenous fouling species on both local and regional scales in coastal regions of the USA. Using photographic surveys of 80 marinas on the east and west coasts of the USA, we defined thermal niches and ranges of summer sea surface temperature (SSST) for 27 abundant fouling species. We calculated percent cover of all abundant tunicates and bryozoans across sites and correlated species richness with water temperature and cargo shipping volume in each region. We quantified the relative importance of cargo shipping, seawater temperature, and distance between sites using Jaccard similarity between paired sites. Native species richness was positively correlated with SSST, while nonindigenous species (NIS) richness displayed a parabolic relationship with a peak at 20 °C. Temperature and cargo shipping traffic explained 53 % of variability in NIS richness, and only temperature was correlated with similarity between sites. We also found no link between similarity and distance between sites, and site–site comparisons showed no effect of NIS on native species richness on the scale of this study. It appears that cargo shipping may play a regional role in introduction of new species, but on local scales NIS distributions are more haphazard, possibly driven by local recreational boat traffic and associated larval dispersal or by other vectors affecting the local spread of these species. Our study demonstrates the importance of seawater temperature in allowing spread of NIS and influencing similarity between sites and regions.

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