Passive acoustic monitoring of baleen whale detections during winter off Northeastern Japan

Between November 2024 and February 2025, passive acoustic monitoring was conducted off northeastern Japan using an autonomous recorder (AUSOMS-mini Plus, model AQM-0007) deployed at a depth of 29.2 m on the continental shelf off Ōhata, Aomori Prefecture. The recorder operated in continuous mono mode at a 48-kHz sampling rate. Analysis of 2245 h of recordings using automated detection and manual verification revealed vocalizations of blue whales (Balaenoptera musculus), fin whales (Balaenoptera physalus), sei whales (Balaenoptera borealis), and common minke whales (Balaenoptera acutorostrata). Blue whale and common minke whale vocalizations potentially represent the first acoustic records from waters around Japan. Blue whale calls were detected in 3.1% of recordings and exhibited distinct frequency characteristics, acoustic features, and sequential patterns not previously described. Fin whale 20-Hz pulses were present in 8.8% of recordings, while sei whale downsweep calls were rare (0.3%). Common minke whale calls accounted for 12.4% of the data, with detection rates increasing from late December. These findings align with previous visual surveys and contribute critical data in regions and seasons where visual monitoring is limited, particularly during winter in coastal Japan, while facilitating observation of elusive species such as the common minke whale.