Acoustic target strength of single Jellyfish, Cyanea nozakii, and its symbiotic fish

摘要

Acoustic surveys are used to evaluate the numerical abundance and biomass of many pelagic fish species and some crustacean zooplankton. As jellyfish are important consumers of zooplankton and ichthyoplankton, and may detrimentally affect fish populations, knowledge of jellyfish distribution and abundance is therefore of economic as well as ecological importance. In recently, the acoustic detection has been used to monitor distribution and biomass of jellyfish, and simultaneously target strength (TS) measurements are conducted for the several jellyfish species. In this study, acoustic measurements of ex situ TS were made on live jellyfish, Cyanea nozakii, and its symbiotic fish, using 38, 120, 200, and 420 kHz split-beam transducers. The jellyfish were tethered using very thin line penetrated to their umbrella, and an underwater video camera was used to continuously monitor behavior. Data for 19 individual Jellyfish ranging from young to adult sizes (bell diameter, D=19 -46 cm) and data for symbiotic fishes (total length=3.3 -8.0 cm) were analyzed. In C. nozakii, the mean TS values were -50.45 dB at 38 kHz, -54.94 dB at 120 kHz, -54.12 dB at 200 kHz, and -54.74 dB at 420 kHz. Overall, the mean TS with bell diameter in air was higher at 38 kHz than at 120, 200, and 420 kHz. Least-squares regression fits of TS vs. log(D) were: TS38kHz=19.98.log10(D) -80.57 (r=0.70), TS120kHz=21.41.log10(D) -87.22 (r=0.74), TS200kHz=19.84.log10(D) -84.03 (r=0.69), and TS420kHz=20.13.log10(D) -85.09 (r=0.76). In the case of symbiotic fish, the mean TS values were -50.45 dB, at 38 kHz, -54.94 dB at 120 kHz, -54.12 dB at 200 kHz, and -54.74 dB at 420 kHz. The difference between 38 kHz and higher frequencies was between 3.7 dB and 4.5 dB. These ex situ measurements of TS can be applied to improve acoustic detection and estimates of the jellyfish, Cyanea nozakii.