In the 1956 film "The Silent World" by Jacques Cousteau and Louis Malle, the ocean was portrayed as a realm of quietude. However, subsequent research into underwater acoustics has revealed that the ocean is anything but silent. It teems with sounds produced by marine organisms, geophysical events, and human activities. This composition of biophony, geophony, and anthropophony creates what we now understand as a soundscape. Since the first study of marine soundscapes in 1978, the number of studies has increased rapidly, but most studies still focus on depths shallower than 200 m, leaving a substantial gap in our understanding of the deep-sea soundscape.
In 2020, an Edokko Mark I Type 365 and an icListen HF hydrophone were deployed for one year in the abyssal plain (5552 m depth) south of Minamitorishima Island. This audio dataset revealed a distinct soundscape pattern of the abyssal plain from other deep-sea ecosystems. Based on long-term spectrogram visualization, we observed a clear phenological pattern in the year-long soundscape data, a discovery that challenges the longstanding notion of the deep sea as an environment without discernible rhythms. The application of source separation analysis to the soundscape phenology uncovered the presence of two distinct biological choruses occurring between 20:00 and 00:00, each in different seasons. These evening choruses, similar to those detected across various depths (277 to 5500 meters) in the Indo-Pacific, are thought to be associated with the diel vertical migrations of Myctophidae fishes, although evidence for their sound production remains lacking. Standing out against the background noise, these fish choruses may serve as circadian cues for marine organisms capable of auditory perception in aphotic zones, thereby playing a critical role in the pelagic coupling essential for deep-sea biodiversity.
Despite the potential significance of underwater soundscapes in deep-sea ecosystems, previous studies have highlighted their vulnerability to anthropogenic noise. To date, the response of deep-sea organisms to noise remains largely unexplored, rendering the prediction of noise impacts a formidable challenge. Given the growing interest in mining operations and the global increase in underwater noise, there is an urgent need to develop a collaborative deep-sea soundscape monitoring network. Such a network would establish baseline soundscapes across habitats, providing critical information for the stewardship and conservation of deep-sea environments.