1Field Science Education and Research Center, Kyoto University, Maizuru, Kyoto 625-0086, 2National Fisheries University, Shimonoseki, Yamaguchi 759-6595, 3Marino-Forum 21, Taito, Tokyo 110-0016, 4Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, 5Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
Sargassum beds play important ecological roles as nursery grounds, feeding grounds and hiding grounds for coastal fauna in Kuruminose, Yamaguchi, Japan (12 km2). In this study, we tried to estimate the spatial distribution of a Sargassum bed using an acoustic method. For this purpose, binarization and geostatistical methods were used. The binarization provided the optimum thresholds to separate the bottom and head of the Sargassum bed. The bottom threshold and the head threshold were −22.4 dB, and −56.4 dB, respectively. The horizontal distribution was interpolated every 1 m2 using kriging. The area of the Sargassum bed was estimated to be 1.94 km2, with thickness of 0.50 to 2.24 m. The Sargassum bed in the south-southeast of Kuruminose was thicker and larger than that near Kuruminose, suggesting that the south-southeast Sargassum bed is important for the coastal ecosystem in Kuruminose. The estimated thickness and distribution corresponded with those by direct observation by diving and underwater TV camera, suggesting that acoustic measurement is adequate for estimating the spatial distribution of Sargassum beds.