Long-term assessment of the impact of vessel noise on finless porpoise presence in Mikawa Bay, Japan

Marine noise pollution is an increasingly recognized environmental concern. Among anthropogenic underwater sound sources, vessel traffic has drawn attention due to its global prevalence and continuous acoustic footprint. Small cetaceans, which rely on sound for echolocation and communication, are considered vulnerable to such noise. However, most previous studies have focused on short-term responses to nearby vessels. To assess long-term effects, we conducted a 10-year passive acoustic monitoring study in Mikawa Bay, a semi-enclosed coastal area in central Japan. This bay supports one of the highest densities of narrow-ridged finless porpoises (Neophocaena asiaeorientalis) and experiences heavy vessel traffic, providing an ideal setting to examine chronic noise exposure. We collected data on 1872 monitoring days, detecting porpoises on 99.4% of days. Logistic regression models using 1-min resolution data and generalized additive mixed models using 1-hour resolution data were applied to evaluate the relationship between vessel noise and porpoise presence. Both models revealed no significant negative effect. These findings suggest long-term coexistence between porpoises and regular vessel activity. Overall, vessel noise is unlikely to have caused habitat abandonment in this population.