Estimates of acoustic target strength for giant jellyfish Nemopilema nomurai Kishinouye in the coastal Northwest Pacific

Abstract

Acoustic target strength (TS) measurements were made of ex situ giant jellyfish Nemopilema nomurai Kishinouye at 38 and 120 kHz. These TS data may be useful for developing acoustic scattering models, and surveying giant jellyfish distributions and biomasses. Each jellyfish was tethered in seawater using a monofilament line that vertically penetrated its bells centre. During the acoustic measurements, an underwater video camera was used to continuously monitor the jellyfishs behaviour. Acoustic measurements were made using split-beam transducers. TS measurements were made of 27 individual jellyfish, but data were analysed for 23 specimens (bell diameter in air, D-air 2165 cm) at 38 kHz, and 19 specimens (D-air 2146 cm) at 120 kHz, respectively. Least-squares regression fits of TS vs. log(D-air) were TS38kHz 20log(10)D(air)82.7 (r 0.76) and TS120kHz 20log(10)D(air)86.7 (r 0.79). The mean TS values at 38 and 120 kHz, using the average D-air 40.3 cm and 35.5 cm, respectively, were 50.6 and 55.7 dB. The reduced TS, a function of the ratio of D-air to wavelength (), was RTS(D-air/) 6.1log(10)(D-air/) 36.1 (r 0.51). These RTS values decreased with increasing D-air/. Symbiotic medusa shrimp (Latreutes anoplonyx Kemp) contributed negligible bias to our TS measurements of giant jellyfish. These ex situ TS measurements may be used in acoustic surveys to estimate the distributions and biomasses of N. nomurai.