Seasonal synchrony between vessel arrivals and larval production may influence the likelihood of biofouling introductions

Abstract Biofouling of ships and boats is a dominant global transport pathway for marine invasive species. Many biofouling non-indigenous species (NIS) can only be entrained by vessels and subsequently ‘discharged’ during the short-lived but highly mobile larval stage. This means their introduction to new locations is reliant on vessels arriving in suitable habitat while biofouling adults on the hull are spawning. Vessel arrivals and spawning are, however, both transient events. The influence of synchrony between shipping traffic and spawning on introductions of marine NIS is not well understood. Using New Zealand as a case study, we show that seasonality of domestic vessel arrivals is much more prevalent than for international vessel arrivals. Seasonality was identified in 35 of 58 nationwide locations receiving commercial maritime traffic. Seasonal locations most commonly experienced their shipping peaks during spring and summer, while troughs were most common during winter and autumn. This seasonal pattern produces more opportunities for introductions of spring–summer spawners, while potentially limiting introductions of autumn and winter spawners around New Zealand. Our findings indicate the importance of incorporating seasonality into models and risk-assessments of vessel-mediated incursions, and the need for more empirical data on the timing of introduction and entrainment events. There may be an opportunity to optimise surveillance and vector management efforts during periods of peak synchronicity, where the likelihood of incursions is higher.