Passive acoustic monitoring is a powerful tool for understanding the behavior patterns of underwater phonating animals, which contributes to conservation and management. The Ogasawara Islands, located approximately 1,000 km south of Tokyo, are breeding areas for humpback whales (Megaptera novaeangliae), where males produce vocalizations known as “songs” between December and May. From 2016 to 2018, we recorded the occurrence of singers using long-baseline hydrophone array in the coastal waters off Chichijima Island. Results showed the acoustic occurrence patterns are related not only to seasonal cycles but also to diel and tidal cycles. The number of singers peaked before sunrise and in the evening and decreased during the daytime. It also increased during flood tide and decreased during ebb tide. Furthermore, since the influence of lunar phase on singing activity has been reported in other regions, we conducted statistical modeling that incorporated time of day, tide, and lunar phase to improve our understanding of the environmental factors influencing the periodicity of singer occurrence. Our model revealed that a variable used as an indicator of tidal rise and fall and tidal speed influenced singer occurrence, supporting the possibility that the periodicity of singer occurrence is explained by distribution changes caused by tidal fluctuations.
Tsujii et al. (2025) studied this question.