The acoustic characteristics of three jellyfish species: Nemopilema nomurai, Cyanea nozakii, and Aurelia aurita

摘要

Reports about jellyfish damaging nets and reducing fish catches are increasing, and data on the abundance and distribution of various jellyfish species are needed to forecast where and when their blooms will happen. Acoustic techniques can be used to obtain this information if the acoustic characteristics of the targets are known. This is a study of acoustic scattering from three jellyfish species: Nemopilema nomurai (order Rhizostomeae), Cyanea nozakii (Semaeostomeae), and Aurelia aurita (Semaeostomeae). Target strength (TS) at 38, 120, and 200 kHz, specific density, and sound speed were measured with live specimens. Specific densities were measured using the displacement-volume-weight and the dual-density methods. The sound speeds were estimated using the time-of-flight method. The reduced TS (RTS), i.e. the TS normalized by bell area, was smaller for N. nomurai (bell diameter range 19-38 cm) than for C. nozakii (bell diameter range 30-40 cm), and the differences in RTS between the species were 17.8, 12.6, and 5.8 dB at 38, 120, and 200 kHz, respectively. The respective sound-speed contrast h and density contrast g were 1.0008±0.009 (mean±s.d.) and 1.004 ±0.015 for N. nomurai; 1.038 and 1.073 for one C. nozakii; and 1.0001 ± 0.025 and 0.989 ±0.019 for A. aurita.