Assessment of enforcement strategy highlights opportunities for more effective deterrence in a large-scale no-take MPA

Abstract Illegal, unreported, and unregulated (IUU) fishing poses a significant threat to fisheries sustainability, global food security, and biodiversity conservation. Many regions rely on patrol-based enforcement to promote compliance, yet the effectiveness of such strategies in deterring IUU activity remains underexplored. In this study, we employed a mixed-methods approach to assess the impact of management and policy decisions on illegal fishing within the Chagos Archipelago marine protected area (MPA). This MPA has maintained near-continuous patrols since its implementation in 2010, providing a unique opportunity to evaluate the influence of patrolling on illegal fishing deterrence. First, we analyzed semi-structured interviews with fishers in Sri Lanka, the origin of most intercepted illegal fishing vessels, to contextualize perceived deterrence within the broader socio-economic environment of the MPA. We then examined patrol vessel movements from onboard automatic identification systems to compare the effectiveness of different enforcement strategies. Finally, we evaluated the relationship between patrol effort and illegal fishing interception rate as an indicator of deterrence. Interviews indicated that strengthened port monitoring and regulatory improvements in Sri Lanka contributed to perceived deterrence. Although variations in patrol strategy significantly affected interception rates through differences in vessel activity and spatial coverage, no deterrence effect of any patrolling strategy was detected. Our findings highlight the value of multidisciplinary approaches in evaluating enforcement effectiveness and emphasize the need for diverse, context-aware policies to address illegal fishing within MPAs that incorporate patrolling, robust national regulations, and community-focused strategies.