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Acoustic Distribution Maps of Phonating Species in Coral Reefs28Marine Research of the National Academy of Marine ResearchDOI: 10.29677/MR.202406_4(1).0003 Ocean was considered a silent world in the 1950s (a movie directed by Jacques-Yves Cousteau and Louis Malle's The Silent World). However, it is a noisy world due to many types of acoustical sources (Slabbekoom et al., 2010). In recent years, the effects of anthropogenic noise on marine creatures have gained international attention (Duarte et al., 2021). Commercial ships, oil surveys, offshore wind farms, and many underwater acoustic engineering works cause ''anthrophony.'' Waves, earthquakes, melting ice in polar regions, typhoons, and precipitations create intense underwater noise categorized as ''geophony.'' Whales, dolphins, dugongs, and many fish species produce unique vocalizations called ''biophony.'' All these sources contribute to theunderwater soundscape. Soundscape is defined as ''ambient sound in terms of its spatial, temporal, andfrequency attributes, and the types of sources contributing to the sound field,'' (ISO 18405:2017 - Underwater acoustics - Terminology, International, published by Organization for Standardization 2017) consisting of biophony, geophony, and anthrophony. Once the sound source characteristics are identified, remote acoustic survey in the water is possible. However, the application of PAM to fish and other species is significantly behind compared to marine mammal research. Many stationed PAM systems revealed periodical fish choruses during potential breeding season. Automatic separation of fish choruses is possible using unsupervised classification (Kim et al., 2023). Besides these efforts, insufficient descriptions of species-specific sound characteristics and small source levels of individual sounds prevent source identification in noisy coastal waters; see Parmentier et al. (2021) and Barroso et al. (2023). As a result, most soundscape research does not identify the source species of fish choruses.Fish produce low frequency sounds using their swim bladders, pectoral spines, teeth, bones, and tendons (Fine, 2023). The sound frequency of fish is much lower than that of echolocation sounds of odontocetes and similar to the ocean ambient noise in coastal waters. The source level of an individual animal is much smaller than that of cetaceans. In coral reefs, acoustic features were shown to differ between different reef morphologies, suchas barrier reefs, fringing reefs, and channels (Minier et al., 2023). As for PAM on mobile platforms, selfoperation noise such as flow noise and engine noise prevent wide range passive acoustical survey of fish.Therefore, the acoustically observed distribution of fish through moving platforms has not been triedextensively. Here we use a paddling boat as a silent platform to tow a stereo hydrophone system around a coral reef to observe the acoustic distribution of fish and crustaceans in a coastal water area. This method allows simultaneous visual observation from a boat or underwater using a box glass or scuba gears if necessary.However, we should note that direct visual observation is heavily affected by the turbidity of the water. Visual observation can be a supplemental tool in addition to PAM. Passive acoustic monitoring (PAM) methods are widely applied in cetacean research. Using a longhydrophone array cable, the distribution of several baleen whales could be visualized (Wang et al., 2016). Small odontocetes, such as dolphins and porpoises, frequently produce ultrasonic pulses for echolocation. Previous studies showed that the PAM can provide not only the presence of target animals, but also the number ofphonating odontocetes through a simple stereo monitoring device towed by a small boat (Kimura et al., 2014).As long as the relative bearing angles of multiple sound sources from the receiver are separated, the number of simultaneously phonating objects can be counted. Applying the density estimation model, the existing numberof animals within the observable area could be estimated by a fixed PAM assuming source level distributionand number of phonations per unit time (Marques et al., 2009). Long-term PAM data showed fin whales' clear distribution and migration patterns using a seismic submarine cable system (for example, Nakamura, 2020).1 INTRODUCTION