Automatic detection and bell-diameter estimation of giant jellyfish Nemopilema nomurai using echotrace shape

¹National Research Institute of Fisheries Engineering, Kamisu, Ibaraki 314-0408, ²Marine Fisheries Research and Development Center, Yokohama, Kanagawa 220-6115, ³Japan Sea National Fisheries Research Institute, Chuou-ku, Niigata 951-2121, Japan

Since 2009, distribution surveys of giant jellyfish Nemopilema nomurai have been conducted around the Tsushima Strait every July. During the surveys, expert researchers visually counted echotraces of giant jellyfish that appeared on a high resolution echogram of a quantitative echosounder (EK60, SIMRAD). Bell-diameter information was obtained from catch data of midwater trawls. In order to improve the efficiency of the survey, methods to automatically detect echoes and to estimate the bell-diameter of giant jellyfish from acoustic data were developed. Firstly, echotraces similar to the body size of giant jellyfish in July were pre-selected. Secondly, three morphological parameters—perimeter, image compactness, and rectangularity—of selected echotraces were examined to identify giant jellyfish. Distribution trends obtained from our automatic detection method were spatiotemporally consistent with those determined by expert researchers. To estimate the bell-diameter distribution of giant jellyfish, jellyfish silhouettes were captured by underwater video camera. The relationship between the bell-diameter measured from jellyfish silhouettes and the maximum height of the echotrace for one ping was obtained. The bell-diameter distribution was estimated using the relationship as well as the maximum height of detected echotrace. The mode of bell-diameter estimated from acoustic data agreed well with that obtained from catch samples.