[M-IS17] Ocean Plastics, an earth science perspective

[MIS17-02] Detection of Plastic Debris in Coastal Areas Using Underwater Drones and its application to Osaka Bay

*Toshiki Muraoka¹, Yosuke Alexandre Yamashiki¹, Katsuaki Koike², Kei Saito¹ (1.Graduate School of Advanced Integrated Studies in Human Survivability, Kyoto University, 2.Department of Urban Management, Graduate School of Engineering, Kyoto University)

Keywords:Marine Plastic, Macro Plastic, Underwater Drone

Due to global economic development and population growth, the amount of unprocessed plastic that is released into the ocean continues to increase. Although large-scale surveys have been conducted in the Pacific Ocean using the Shinkai 6500 to study macroplastics, which are plastic debris that has not yet degraded in the ocean, it is not possible to survey the entire ocean due to the enormous cost involved. In addition, the degradation of plastic debris into microplastics, which have recently been recognized as having a particularly problematic impact on ecosystems, occurs more frequently in coastal areas than offshore areas, but the distribution of macroplastics in coastal areas has not been investigated. In this study, we conducted a research using an underwater drone in the coastal area of Osaka Bay to clarify the distribution of macroplastics on the seafloor and their controlling factors, and classified the litter observed from underwater images. Fourteen sandy beaches facing Osaka Bay, including the east coast of Awaji Island, were selected as survey sites. The reasons for choosing sandy beaches as survey sites were twofold: to match the topographical conditions, and because mud on the seafloor would roll up with the drone's screw and make it impossible to capture clear images. At each location, at least three underwater images were taken using a drone, and the number of pieces of plastic debris on the seafloor was counted. The data for each location were compared by counting the number of pieces of litter found visually in the images and dividing by the area photographed. The total area photographed was calculated from the camera's mouth angle and distance from the seafloor to determine the area visible to the camera, which was then combined with the drone's speed listed in the catalog to determine the area photographed in the video. The area of the area captured by the camera was verified for accuracy by placing the drone indoors in the same position under the same conditions and measuring the area that was reflected in the image. The survey was conducted at seven locations on the east side of Osaka Bay, from Suma Beach in Hyogo Prefecture to Kata Beach in Wakayama Prefecture. The results suggest that the distribution of plastic litter in the interior of Osaka Bay is influenced by the accumulation of plastic litter in the eastern part of Osaka Bay due to the East Coast Evergreen Current, in the central part of Osaka Bay due to the Okinose Reflux, and in the western part of Osaka Bay due to the Tomogashima Reflux.