Scientists have spotted dolphins in two bays along the coast of the Sea of Japan. (representational image)
People living along Japan’s Sea of Japan coast have been saying for years that they occasionally see dolphins near shore. Local fishermen have also reported seeing them from time to time.
These sightings were interesting, but scientists did not have enough information to understand how often the animals actually visited these coastal waters or what they were doing there.A new study by researchers from Kyoto University has provided some answers. Instead of waiting to see the dolphins on the surface, the team listened to them underwater using special recording devices. Their findings showed that dolphins visit two bays along the coast of the Sea of Japan only occasionally, with their calls being detected about once every 10 days, Science X reported.The research, published in the journal Marine Mammal Science, focused on Wakasa Bay near the city of Obama and Aso Bay near the city of Tsushima. These are the coastal areas where locals and fishermen have reported seeing dolphins.The Sea of Japan has changed rapidly in recent years. It is partly surrounded by the Japanese Islands and the Eurasian continent and is connected to the open ocean via shallow straits. Although dolphins and porpoises have been reported in these waters before, scientists say there is very little basic information about these small-toothed whales, known as odontocetes.
Local reports have begun the study
The idea for the research came from conversations with people living near the coast. “This study began through local connections: we heard from our acquaintances and local fishermen that dolphins might be visiting nearby waters,” said first author Satoko S. Kimura.The researchers wanted to know how often dolphins visited these bays and what types of sounds they made while using these coastal waters.This is why scientists used passive acoustic monitoring.
This method involves placing underwater microphones, called hydrophones, in the sea to record sounds continuously without disturbing marine animals.The equipment recorded different types of sounds made by dolphins and other small odontocetes. These included echolocation clicks, which dolphins use like natural sonar to find their way around and locate prey, as well as whistles, which are mainly used for communication.Monitoring continued for nearly three years in Wakasa Bay, from January 2022 to November 2024. In Aso Bay, recordings were collected from March 2023 to October 2024.The project also relies heavily on local communities. Residents helped scientists place the recording devices and retrieve them from the water. The fishermen also exchanged information about dolphin sightings, and took photos of the animals whenever possible.
Findings from passive acoustic monitoring in two coastal communities.
What dolphins revealed
Recordings confirmed that dolphins were visiting both bays. However, visits were much less frequent than some had expected.Researchers have detected dolphin sounds only once every 10 days in Wakasa Bay and Aso Bay. This suggests that dolphins use these coastal areas from time to time rather than staying there continuously.The researchers also noticed differences in the whistles recorded in the two Gulfs.
In Aso Bay, where underwater background noise was higher, whistle characteristics differed from those recorded in the quieter Wakasa Bay.The team was unable to determine whether these differences were caused by the noisier environment or whether different groups of dolphins naturally produced different whistles.Based on recorded sounds, sightings of local fishermen and photographs, researchers believe the visiting animals were most likely Indo-Pacific bottlenose dolphins.Although the evidence strongly points to this species, the study was unable to identify each individual animal with complete certainty.
Why are results important?
Scientists say the study provides important background information about when baby dolphins visit these coastal waters and what sounds they make.This information was limited for the Sea of Japan coast, making it difficult to understand how these animals use the area or whether their behavior changes over time.The research also highlights the value of passive audio monitoring. Because dolphins don’t always appear where people can see them, underwater listening allows scientists to spot the animals even when they remain out of sight.Long-term monitoring can also help researchers notice changes in the marine environment. If dolphin visits become more or less frequent in the future, or if different species begin to appear, underwater recordings may help detect those changes.The researchers hope that continued monitoring will improve understanding of marine life along the Sea of Japan coast and provide evidence to support conservation efforts and sustainable use of coastal waters by local communities.