A visual-acoustic combined observation method to monitor seagrass bed utilization by dugongs (Dugong dugon) in a tidal flat

Abstract

Dugongs (Dugong dugon) are herbivorous marine mammals distributed in the tropical and subtropical shallow waters in the Indian and the Pacific Oceans. Okinawa Island in Japan is the northern limit of the range of dugongs. Although the isolation of this extremely small population has not been confirmed, a package of measures to prevent their extinction is needed. Unfortunately, the lack of basic research especially the use of seagrass patches by dugongs in the Okinawa waters prevents us from taking any effective countermeasures for the conservation. In this paper, we propose an acoustic- visual combined observation method using AUSOMS-Ds (automatic underwater sound monitoring systems for dugong) and digital cameras on aerial and land based platforms. Attendance of dugongs and birds-eye view of seagrass patches with grazing trails can be obtained. This method has several advantages. First, a map of seagrass species composition, seagrasses coverage and bathymetries in the focal tidal flat can be obtained. Second, the attendance of vocalizing dugongs and the shape and length of dugongs' grazing trails can be observed daily. This new observation method will be tested in Thailand water for validation.