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Passively listening animal sounds is useful to confirm presence of vocalizing species. However, abundance estimation have been difficult since the number of sounds is not the proxy of the number of existing individuals. Essentially, silent animals are not detectable acoustically although estimating number of living animals is needed for the ecological studies. Sound source separation technique combined with a virtual mark recapture method gave the answer to apply acoustical observatories for the ecological studies. Including undetected animals, especially in the water, can be counted using acoustic technique. With an array of hydrophones, direction or position of the phonating animal can be measured by triangulation. Once the sound sources are separated, number of independent sound sources can be counted. This would be the minimum number of phonating animals within the acoustically observable range. In case to detect same animal by independent detectors such as two hydrophones separated several tens meters away each other, some animals detected by both detectors but other animals may or may not be detected by one hydrophone. This is the recapture situation. High recapture rate means the most of the animals could be detected by the sensor. Low recapture rate means big portion of non-vocalizing animals exist, which were not detected by the sensor. Virtual acoustical mark recapture provides the number of existing animals including silent individuals. We need to note several biases such as the phonating rate per unit time could different among individuals, especially the animal was isolated or in a group of conspecifics. Demand of communication or sensing may be different according to the behavioral context. We tested the group effect on the acoustic detection performance and compare the detection ability of different species distributed scattered or condensed. In conclusion, passive acoustic monitoring can be used for the conventional ecological observation to count the number of individual with identifying species. It is a kind of remote sensing of animals in a short range.
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