Dolphins dropping in to the Sea of Japan
Sound detection in two coastal communities reveals visits from small odontocetes
Kyoto University
image:
Passive acoustic monitoring revealed the occurrence patterns and whistle characteristics of small odontocetes in Wakasa Bay and Aso Bay, two coastal regions on the Sea of Japan.
view moreCredit: Chihiro Kinoshita
Kyoto, Japan -- When locals of two coastal communities along the Sea of Japan reported observations of dolphins in nearby waters, it caught the attention of some scientists. The marine environment has been changing rapidly in the Sea of Japan, which is partially enclosed by the Japanese archipelago and the Eurasian continent and connected to the ocean through shallow straits. Though sightings and bycatch records of dolphins have been reported along the coast, basic ecological information on small odontocetes, such as dolphins and porpoises, has remained limited.
Once they heard about dolphin sightings in both Obama City on Wakasa Bay and Tsushima City on Aso Bay, a team of researchers at Kyoto University headed there to investigate. They wanted to know how these upper-level predators use areas like Wakasa Bay and Aso Bay, and what kinds of sounds they produce.
"This study began through local connections: we heard from acquaintances and local fishers that dolphins might be visiting nearby waters," says first author Satoko S Kimura.
The team conducted passive acoustic monitoring from January 2022 until November 2024 in Wakasa Bay, and from March 2023 through October 2024 in Aso Bay, recording echolocation clicks, whistles, and ambient sounds produced by small odontocetes, as well as sighting information from local fishers. Locals helped the scientists deploy and retrieve recording devices and took photographs when possible.
The recordings revealed dolphin sounds coming from both Wakasa Bay and Aso Bay. However, the detection rate was low in both, occurring only about once every ten days. The whistle characteristics in Aso Bay, where background noise levels were higher, differed from those in the quieter Wakasa Bay. The team could not determine whether these differences came from sound environments or the animals themselves, but based on the acoustic characteristics, sightings, and photographic evidence, the visiting odontocetes are most likely Indo-Pacific bottlenose dolphins.
These findings provide baseline information on the spatiotemporal occurrence and acoustic characteristics of small odontocetes along the Sea of Japan coast. This study also demonstrates that passive acoustic monitoring is an effective method for studying animals in coastal areas where infrequent sightings make them difficult to observe directly.
These methods may also help detect future changes in marine environments and identify the species visiting these coastal waters more accurately. By further developing long-term monitoring, the researchers aim to provide scientific evidence that helps support both marine conservation and the sustainable use of coastal waters by local communities.
"With the help of local fishing communities, we were able to listen to the sea over long periods and begin revealing the lives of dolphins in familiar coastal waters," says Kimura. "We hope to continue working with local communities to better understand and conserve the marine life that shares the sea with us."
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The paper "Occurrence and Whistle Characteristics of Small Odontocetes in Two Coastal Regions of the Sea of Japan" appeared on 25 May 2026 in Marine Mammal Science, with doi: 10.1111/mms.70200
About Kyoto University
Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en
Method of Research
Observational study
Subject of Research
Animals
Article Title
Occurrence and Whistle Characteristics of Small Odontocetes in Two Coastal Regions of the Sea of Japan
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