Assessment of Vessel Noise Impact on Finless Porpoises in the West Part of the Seto Inland Sea, Japan

Anthropogenic noise is a new type of marine pollution that impacts marine life. It is essential to accurately assess and mitigate its effects on each species. In coastal waters with heavy vessel traffic, noise impacts are a particular concern, and immediate impact assessments are needed for species living in these areas. The narrow-ridged finless porpoise Neophocaena asiaeorientalis sunameri, a small cetacean species, is found in the coastal waters of East Asia. Therefore, they are vulnerable to the effect of vessel noise. However, knowledge regarding these effects is limited. This study aimed to determine the time interval after vessel passage at which click trains of narrow-ridged finless porpoises, inhabiting the western part of the Seto Inland Sea, Japan, were first detected using fixed passive acoustic monitoring data. Furthermore, the effect of vessel presence or absence on the detection of click trains was investigated, along with the correlation between the number of vessels and the number of detected click trains. Fixed passive acoustic monitoring was conducted intermittently using an event recorder, A-tag, from July 2021 to January 2024. The click trains were most frequently detected within 0 to 5 minutes after the vessel passed, while detection rates decreased and remained relatively stable beyond 5 minutes. The detections were increased in the presence of vessel noise. In this study, no acoustically distinct avoidance behavior in response to vessels was observed. After the reduction of noise following vessel passage, the porpoises may increase their click train emission rates, possibly as a form of vigilance or a strategy to detect environmental disturbances caused by the vessels.