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Acoustic Distribution Maps of Phonating Species in Coral Reefs30Marine Research of the National Academy of Marine ResearchDOI: 10.29677/MR.202406_4(1).0003 Ground truth sound data was collected in advance by placing a hand-held underwater recorder (AUSOMSmini, Aquasound, Kobe, Japan) and water-proof camera at specific places of fish aggregation in the coral reef. Sounds of damselfish species such as scissortailed sergeant (Abudefduf sexfasciatus), sapphire devil(Chrysiptera cyanea), and threespot dascyllus (Dascyllus trimaculatus) were recorded with camera images. The damselfish family is considered to produce sounds by stridulation of the jaw apparatus during courtshipbehavior (Maruska et al., 2007). Recorded waveforms and spectrogram of damselfish and possible snapping shrimp are shown in Figure 2. The typical sound pattern of damselfish is a pulse train, consisting of several successive pulses.2.2 Sound analysis The locations, where the extracted sound was detected, were calculated by comparing the time of the sound segment and the GPS record. The trajectories of the observation platform were calculated by the GPS positions, which were interpolated between the positions recorded every several ten seconds. The time resolution of the sound segment was very precise. The exact location of the sound was estimated as the linear interpolation between two GPS locations. 2.3 Location and mapping Dominant frequency and inter-pulse interval were used for the identification of damselfish sounds.Applying the rule-based filter described in the following section, candidates of damselfish sounds wereextracted (horizontal arrows in figure 2a). A rule-based detector for damselfish sounds was designed as a custom-made code on MATLAB R2016b (MathWorks, USA) by the first author that selected the dominant frequency of the pulse train, average inter-pulse interval, and the number of pulses in a train. The rules were based on the separatory observed acoustic characteristics of damselfish sounds described above. Speciesdifferences in sound characteristics of damselfish were reported, but acoustic separation seemed insufficient due to the overlapping of many acoustic parameters among species (Parmentier et al., 2009). The designed detector could select the sound events matching the rules, but did not discriminate species of damselfish, rather it extracted sounds similar to the damselfish family sounds. We must also note that some other biological sound sources producing pulse trains could be contaminated. To detect damselfish sounds, the rule-based filter calculated the integrated energy level between 200-700Hz of each 370 ms segment of the entire recording data. If the energy level exceeded the background noiselevel, the algorithm continued the analysis procedure. An envelope detector of MATLAB function was appliedto extract the pulse train's envelope shape to identify the height and the time of each pulse. The time of thehighest peak was selected as the center of the new segment file. The re-calculated envelope of the new segment file was used to count the number of peaks within the segment, which should be 4 or more. Average inter-pulse intervals should be within 10 %u2013 70 ms and the standard deviation of the inter-pulse interval should be within 17 ms. All new segments satisfying the rules above were identified as damselfish sounds. These rules were tailored to detect potential damselfish sounds in the monitoring area. They may not apply to other sites, even for thesame family/species. To detect snapping shrimp sounds, a band level between 6,350-10,079 Hz was used to identify highfrequency intense pulse sounds. As shown in Figure 2 (b), intense pulse sounds had a high-frequency componentof around 8,000 Hz. The band level within +/- 1/3 octave from 8,000 Hz was used as an indicator of the intense pulse sounds for the current study. Since many crustaceans produce pulse sounds, the current rule-based detectordid not discriminate against them. The snapping shrimp family also includes many species. Hereafter, the sourceof the pulse sounds is referred to as ''snapping shrimp'' just for labeling, but it is not limited to any specific species.