1Yasuda Fisheries Office, Tsuruga, Fukui 914-0821, 2National Research Institute of Fisheries Science, Fisheries Research Agency, Yokohama, Kanagawa 236-8648, Japan
We developed a tool for measuring the diameter of the Nomura's jellyfish Nemopilema nomurai, whose weight often exceeds 60 kg, without lifting them out of the seawater. Use of the newly developed tool made it possible to collect sufficient bell diameter data to enable analysis of the population growth. Change of diameter of jellyfish at set-nets of Mihama, Fukui Prefecture was monitored and compared with on-deck measurements during seven surveys from 4 September to 1 November, 2006; each survey measured 23-70 individuals by the tool and 5-31 individuals on deck. Comparison of the measurements found a significant difference in only a single survey out of the seven. This difference was probably because of bias in selecting the specimens to measure rather than any specific difference between the methods. According to the analysis of data measured by the tool, mean diameter was largest on the first sampling day, then decreased gradually to the end of September, and was rather stable in October with a slight increase at the end of the month.