Policy, management, and the ‘Level of Fouling’ scale to transform marine invasion risk reduction from recreational boats

Categorical scales have a rich history in environmental management, providing pragmatic means to assess ecological phenomena when full quantification is not feasible or necessary. The Level of Fouling (LOF) scale was developed 20 years ago to facilitate biosecurity policies that reduce the spread of marine invasive species by recreational boats. Applications of the six-category scale have since varied, and divergent implementation practices may undermine its value for marine biosecurity. To support researchers and practitioners to maintain accuracy and precision when applying the scale, we provide a review of LOF-based research and management and updated guidance on its implementation. Focussing on underwater observations and percentage cover metrics (rather than above water observation and species richness) can reduce uncertainty and error. To support users when applying the scale we provide an online application - LOFeR - which features: (i) thousands of underwater vessel images for training and self-assessment, and (ii) an LOF Calculator that can be used in the field to automatically calculate whole-vessel LOF ranks from multiple ranks applied to different underwater surfaces. The LOF scale has been used most extensively in New Zealand to establish biological thresholds and underpin regulations and monitoring for biosecure boating. New Zealand's three-tiered approach to managing boat biofouling addresses boats arriving at the border from overseas, travelling within and between different regions, and visiting marine protected areas. Over 47,000 LOF-based vessel surveys in recent years support New Zealand's approach, which provides a roadmap for other jurisdictions toward managing this large and unwieldy transfer mechanism.