Size estimation of a giant jellyfish Nemopilema nomurai using quantitative echosounder in the East China Sea

Abstract

The maximum bell-diameter of giant jellyfish Nemopilema nomurai is over one meter. They are transported by the Tsushima current to the Sea of Japan and are sufficient to seriously damage coastal fisheries. It is necessary to monitor the population and the distribution of them for reduction of the damage. Therefore, acoustic-optical surveys of the giant jellyfish have been conducted around the Tsushima Strait by a visual observation, a quantitative echosounder (EK60, SIMRAD) and an underwater video camera from 2009 to 2014. The giant jellyfish’s size estimation method using an echotrace height was developed on the surveys. Using the method, the habitat and size distribution were obtained around the East China Sea in June and July 2015 to confirm the appearance of them earlier. The giant jellyfish were distributed from surface to about 40 m depth as results of the quantitative echosounder and the visual observation. The measured bell-diameter range was from 30 to 80 cm and giant jellyfish about 60 cm was most detected by the visual observation. The estimated bell-diameter range using the quantitative echosounder was from 10 to 80 cm. The value of highest frequency was 53 cm and agreed with the result of the visual observation.