Assessment and Management of Risks from Biofouling
Vessel biofouling is a well recognised modern-day pathway for the transfer of nonindigenous species (NIS). However despite awareness of these risks, marine incursions as a result of vessel biofouling continue to occur at a growing rate. The objective of this thesis is to provide underpinning knowledge to improve pre- and post-border management strategies for vessel biofouling. Chapter 2 provides a baseline assessment of the biofouling extent and assemblage composition on slow-moving vessels arriving at New Zealand's border. Slow-movers were targeted because their operational profile is widely considered to favour the accumulation of extensive biofouling communities (i.e., potentially high risk vectors of NIS). Interestingly, this research revealed low fouling levels and a low incidence of NIS. Highest levels of fouling were observed in areas where antifouling paint condition was poor or absent (e.g., dry-docking support strips and niche areas), which is consistent with recent studies of biofouling on other vessel types. Despite these findings, there have been several documented examples where heavily fouled slow movers have had high risk NIS on them. As such, risk profiling of slow-moving vessels is recommended. This should be based on operational characteristics such as maintenance history, exposure to regions where pest species are known to be present and intended vessel movements in the recipient region, and should ideally be undertaken on a case-by-case basis prior to arrival from international or distant source-regions. There are limited biofouling risk mitigation options available upon the discovery of NIS at the border, particularly for large vessels (e.g., barges) or towed structures (e.g., oil rigs) where removal to land is often not feasible and in-water defouling may be the only option available. Chapter 3 provides a conceptual framework that identifies biosecurity benefits and risks posed by in-water defouling. Among the latter are the survivorship of defouled material, the release of viable propagules via spawning, and enhanced colonisation of recently defouled surfaces by high risk NIS. Chapter 4 then assesses the operational performance of two diver-operated defouling tools (rotating brush devices) that were designed to retain defouled material during operation (i.e., mitigating one of the main risks associated with in-water defouling identified in Chapter 3). These devices proved effective in removing low-to-moderate levels of fouling from flat and curved experimental surfaces. However, performance was generally poorer at removing more advanced levels of fouling. Furthermore, neither system was capable of retaining all material defouled; c. 4% was lost to the environment, of which around 20% was viable. A significant component of material lost comprised fragmented colonial organisms (e.g., the ascidian Diplosoma sp.), which are theoretically capable of forming new colonies from fragments. The study also concluded that the defouling brush devices were not suitable for treating niche areas of vessel hulls such as gratings and water cooling intakes, areas where earlier work in Chapter 2 identified fouling levels to be the greatest. Observations of fully intact and seemingly viable fragments being lost to the environment during in-water defouling trials led to a series of laboratory- and fieldbased experiments designed to elucidate factors influencing the survivorship of defouled material on the seabed (Chapter 5). This work showed that for some colonial organisms (e.g., ascidians), the size of fragments generated during removal affected reattachment success. Thus the defouling method is an important consideration for vessels fouled by colonial NIS. Manipulative field experiments demonstrated that exposure to sediments and benthic predation can play a major role in post-defouling survivorship. Sedimentinduced morality and susceptibility to predation was also taxon-specific. For example, soft-bodied organisms (e.g., sponges, colonial ascidians) were more affected by sedimentation and predation than calcareous taxa (e.g., tubeworms). Chapter 6 provides a "real world" example of in-water defouling. In December 2007, the defouling of an oil rig over soft-sediments in Tasman Bay, and the subsequent discovery of NIS amongst the defouled material on the seabed, led to a dredge-based incursion response whose goal was eradication of the NIS, in particular the brown mussel Perna perna. During the response, c. 35 tonnes of defouled material was removed from the seabed, and target pests were reduced to densities considered too low for successful reproduction (and therefore establishment in the region) to occur. This chapter evaluates the efficacy of the response method and demonstrates that where complete elimination of a pest (i.e., removal of all organisms) is not feasible, alternative eradication success criteria based on density thresholds can be developed to mitigate biosecurity risks posed by an incursion. The preceding technical chapters highlight the risks posed by biofouling and identify that there are presently limited post-border risk mitigation tools available. This reinforces the widely held belief that more effort should be put into pre-border management. In Chapter 7, I use two case studies of oil rig biofouling to highlight the many challenges associated with pre-border management, and identify the urgent need for the development of treatment tools and strategies to mitigate biosecurity risks posed by vessels and structures where removal to land (e.g., dry-docking) is not feasible.