Life history stage and vessel voyage profile can influence shipping-mediated propagule pressure of non-indigenous biofouling species

Abstract

To control the spread of non-indigenous species it is necessary to understand how early stages of the invasion process, such as age of propagule at time of entrainment and transport, influence the quality and quantity of propagules delivered to recipient environments (i.e., the propagule pressure). Using ship biofouling as a model pathway scenario and the bryozoan Bugula neritina, the effect of two early-stage selective filters—the age of recruits and the pattern of the voyage—on reproductive output post-arrival were examined. Voyage scenarios were created by manipulating food levels in a series of field experiments. Recruit Age had three levels (1-day, 1-week and 1-month), and Voyage Pattern (four levels) represented vessels that undertake frequent short voyages or infrequent long voyages, plus control treatments. Spawning success, number of larvae and larval size were measured over multiple spawning events after each scenario. One-day and 1-week old colonies that had been exposed to short-voyage scenarios had higher reproductive output than those that had been exposed to longer and less frequent voyages. In contrast, 1-month old colonies did not release larvae under any voyage scenario. The survivorship of bryozoans in all treatments was relatively high, but reproductive output was influenced by colony age and voyage type. Our results suggest that vessels undergoing frequent, short intra-coastal or domestic voyages are able to deliver viable propagules to subsequent destinations even if they have prior port residencies as short as 1 day. At least for bryozoans, juvenile recruits may even pose a higher risk of spread compared to reproductively mature recruits.