Measurement of swimming speed of giant jellyfish Nemopilema nomurai using acoustics and visualization analysis

Abstract

A species of giant jellyfish, Nemopilema nomurai, which has appeared only recently in the East China Sea, is an emerging nuisance in the northeastern region of Asia because of its extensive damage to fisheries. Until now, the biomass estimates of these jellyfish have mainly been obtained using trawl sampling and sighting survey methods. However, it is also necessary to determine the origin and diurnal migration patterns of these jellyfish. Drawbacks of the trawl sampling method are that it is effective only in estimating the density of jellyfish population distributed throughout the entire water column and requires considerable time. Another common analysis technique is the sighting method, which is effective only in the estimation of he density of jellyfish distributed in surface areas. The sighting method can determine distributions over wide areas in a short time. This method has limitations in investigating the vertical distribution and swimming behavior of jellyfishes. In our study, we utilized an echo sounding method extensively and effectively to overcome these limitations. Our method involved the use of a scientific echo sounder, acoustic camera, and conductivity-temperature-depth instrument during the drifting of a research vessel at various stations in the Yellow Sea. The acoustical method of particle tracking velocimetry (PTV) was used to analyze the swimming speed according to the vertical distribution of N. nomurai jellyfish. Results of the scientific echo sounder indicated that the jellyfish were mainly present in the water column from the surface up to a depth of 40 m. The mean swimming speed of the jellyfish was estimated as being 0.6 times the bell size (BS), with a tendency to maintain a certain speed. Further, results of a Monte Carlo simulation showed that the swimming speed was in the range of 0.46–0.89 BS. These results might be used as an index in a migration model, which may be useful to forecast the behavior and origin of the giant jellyfish entering inshore areas on a massive scale in northeastern Asia.