Development of a three-dimensional mapping system of biota and environment on the sea bottom with acoustics

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This study aimed to develop portable and small-size three-dimensional mapping system of marine organisms and environment on the sea bottom. The developed system can map precise coastal bottom topography acquiring accurate ship position, measuring bottom depths, substrate and distribution of benthic organisms such as seagrass with multibeam sonar, displaying bottom profile in real time. Since echosounders scan bottom depth with only short swath width, bottom topography must be interpolated between transects because of wide interval. It can sound three-dimensional form of bottom surface in aide of the multibeam sonar with wide swath width without interpolation classifying bottom substrate and measuring biomass of seagrass/seaweeds as a remote sensing system. The system also localizes not only the ship position but also bottom positions of ultrasound beams on the bottom by using accurate D-GPS, gyrocompass and motion sensor. We developed method mapping bottom topography and substrate by processing data with GIS. Field researches were conducted around Otsuchi Bay, Aburatsubo Bay and seamount in southern Tunisia. Since the research in Otsuchi Bay targeted seagrass beds, we made studies on ecology and biomass of seagrass as a sea truth survey. Before development of mapping software, we made researches on change in intertidal ecosystem polluted by the Nakhodka oil spill accident, and management and conservation of coastal areas as a research for GIS applying to studies on fisheries and oceanography. Conventional methods to measure bottom substrate and biomass of benthic organisms on the sea bottom are diving along a line transect or core sampling that are not efficient. It was verified that the developed system could map precisely bottom environment on the sea bottom surface with ecosystems varying in a horizontal scale of 1 to 100 m. The system contributes to not only scientific but also social domains for practice of coastal zone management.