The Sound-speed and density contrast of Nemopilema nomurai’s umbrella and oral arm

Abstract

The global warming increased the temperature on the sea surface and consequently, jellyfish numbers rapidly have been increasing. Especially toxic Nemopilema nomurai increased in our coast and gave a lot of damage. To reduce damage, we have to know of distribution the biomass of the jellyfish.<BR> In recent years, to identify biomass distribution of the jellyfish, acoustic technology has been extensively used. This acoustic assessment is required for precise estimate of target strength (TS) of a jellyfish. Among methods to estimate TS, theoretical scattering model method is generally dependent on the sound speed contrast (h) and density contrast (g) of jellyfish.<BR> So, we measured sound-speed contrast and density contrast of the large jellyfish Nemopilema nomurai. This experiment was carried out by separating umbrella and oralarm. The sound-speed contrast was measured by using the time-of-flight method and the density contrast was measured by using volume-mass method. The measured sound-speed contrast varied between 0.9977 and 1.0027 (mean 1.0005) in umbrella and between 0.9957 and 1.0014 (mean 0.9985) in oralarm. The measured density contrast varied between 0.9878 and 0.9981 (mean 0.9930) in umbrella and between 1.0029 and 1.0060 (mean 1.0044) in oralarm. There was a difference in sound speed contrast and density contrast of umbrella and oralarm of Nemopilema nomurai.