Sighting records of small cetaceans of the family Delphinidae in Omura Bay, Nagasaki, Japan

Omura Bay is a highly enclosed bay located in central Nagasaki Prefecture, Japan.Finless porpoises (Neophocaena asiaeorientalis) inhabit the bay, and their abundance is estimated at 200-300 individuals.Besides finless porpoises, there are records of past observations of Delphinus sp. in the bay.We conducted irregular observational surveys on small cetaceans since 2009.In addition, we carried out land-based surveys almost every month since 2010.In this study, we report the sighting records of small cetaceans of the family Delphinidae in Omura Bay.From December 2009 to May 2025, we conducted 182 days of land-based surveys and 30 days of irregular observational surveys, during which 157 sightings were recorded, including 150 sightings of common bottlenose dolphins (Tursiops truncatus), 6 sightings of Indo-Pacific bottlenose dolphins (Tursiops aduncus), and 1 sighting of long-beaked common dolphins (Delphinus delphis).For common bottlenose dolphins, a single individual was observed almost every month from March 2015 to September 2024.Additionally, a small group consisting of two individuals (increased to three individuals after May 2020) was observed almost every month from February 2020 to June 2024.A group of approximately 30 to 50 Indo-Pacific bottlenose dolphins was observed several times from September to December 2016.A group of 30 long-beaked common dolphins was observed in December 2009.From these results, we found that small cetaceans of the family Delphinidae that usually inhabit areas outside Omura Bay occasionally enter the bay.It is suggested that probably the same single individual and the same small group of common bottlenose dolphins showed long-term residency for approximately nine and four years, respectively.