Target strength estimation of the large jellyfish Nemopilema   nomurai

The jellyfish Nemopilema nomurai, which reaches up to 2 m in diameter and 200 kg in weight, has occurred in large numbers in the Sea of Japan during the last several years and has had a negative effect on coastal fisheries in this region. Data on the abundance and distribution of jellyfish are needed to forecast when and where they will occur in coastal areas. Acoustic techniques are commonly used to study the distribution and abundance of fish and zooplankton because these techniques can survey large areas relatively quickly. However, before such surveys can be conducted, the acoustic characteristics of the target species must be known. In this study, the density of and speed-of-sound in live jellyfish were measured to clarify their acoustic characteristics using a theoretical scattering model. Jellyfish density was measured using the dual-density method, while the speed of sound was measured using the time-of-flight method. The acoustic characteristics were estimated with the distorted-wave Born approximation (DWBA) model using these material properties and the shape of free-swimming jellyfish. The results demonstrate the feasibility of investigating the abundance and distribution of jellyfish using acoustic methods. [Work supported by JSPS and KOSEF Core University Program on Fisheries Science (FiSCUP).]