The Japanese Sea of Japan, a near-isolated body of water, is home to a fascinating ecosystem that has recently come under the spotlight due to the discovery of dolphins using its coastal waters. This region, with its unique geographical constraints, behaves like a small ocean with distinct currents and weather patterns. The presence of dolphins and porpoises in these waters has been a subject of interest, but the details about their behavior and habits have remained largely unknown. The story begins with local fishers reporting frequent sightings of dolphins near the shore, which caught the attention of researchers at Kyoto University. This sparked a study that aimed to uncover the mysteries of these coastal dolphins and their interactions with the environment.
The research team employed a novel approach called passive acoustic monitoring, which involves recording underwater sounds and allowing the animals to announce their presence. They chose two distinct locations: Wakasa Bay, a large open bay near the middle of the Japanese archipelago, and Aso Bay, a sheltered inlet on Tsushima Island. By deploying recorders on working fishing gear, they captured the high clicks used for hunting and the whistles and steady background hum of the water.
The findings revealed that dolphin sounds were not as frequent as expected, with detections occurring once every 10 days at each site. The species identified was the Indo-Pacific bottlenose dolphin, known for its preference for coastal waters. The study also utilized a click detector to differentiate between the narrow, high clicks of porpoises and the broader clicks of dolphins, with almost no instances of porpoise-like clicks in Wakasa Bay.
The timing of dolphin appearances played a crucial role. In Wakasa Bay, detections were most common during the early morning hours, while Aso Bay showed a more complex pattern with two daily peaks, one around 5 am and another near 7 pm, with higher activity from July to November. Water temperature and the phase of the moon also influenced dolphin activity, suggesting a connection to the availability of squid, their primary prey.
One of the most intriguing aspects of the study was the analysis of the whistles. Wakasa Bay's whistles had a higher proportion of rising tones, while Aso Bay's whistles were flatter and lower in pitch. The louder background sound in Aso Bay may have influenced the dolphins' behavior, as they are known to flatten and drop their whistles in noisy environments. However, the team could not yet determine whether the noise shaped the sound or if the dolphins themselves differed in their vocalizations.
The implications of this research are significant for conservation efforts. With limited long-term dolphin data for this region, the study provides a valuable baseline for understanding when and where small odontocetes use these bays and what they sound like. The passive acoustic monitoring method, which can be kept running for months without boat presence, offers a promising approach to tracking animal movements and understanding the dynamics of the marine environment.
Looking ahead, the researchers plan to continue using the recorders and collaborate with local fishing communities to improve species identification, individual tracking, and the connection to prey and currents. This ongoing work will contribute to a deeper understanding of the marine life in these coastal waters and inform conservation strategies. The study's findings, published in the journal Marine Mammal Science, highlight the importance of innovative research methods in uncovering the secrets of our oceans and the creatures that inhabit them.
