Biosecurity risk associated with bilge water from small vessels: an evaluation of systems and operator behaviours.
Vessel movements are considered the main anthropogenic pathway for secondary spread of marine non-indigenous species. Recent studies have highlighted the potential for viable organisms to be transported within bilge water of small vessels. In this study, a survey was conducted to (1) describe the bilge water operational profiles of a range of domestic vessels operating in New Zealand and (2) identify factors driving the level of biosecurity risk. The proportion of vessels with pre-discharge treatments was low (13.7%), confirming the potential for organisms to be viable at discharge. Significant differences in risk scores among vessel types were shown; risk scores were highest for yachts, followed by launches, trailer boats and commercial vessels. Yachts were characterised by a high frequency of use, visits to multiple regions and a high likelihood of bilge water on board when leaving port (i.e. high-risk source regions). Almost all yachts operated automatic bilge systems, suggesting that discharge occurs primarily when the volume is sufficient to trigger the pump mechanism and that associated bilge water could be carried significant distances. Validation of vessel risk profiles through targeted sampling is recommended, along with research to identify potential practical and cost-effective mitigation measures such as public awareness campaigns.