In situ target strength measurements of the scyphomedusa Chrysaora melanaster

Abstract

Large medusae can dominate the pelagic biomass in marine environments, but their contribution to acoustic scattering remains poorly characterized. Here we report on in situ target strength (TS) measurements of Chrysaora melanaster, a dominant species of scyphomedusa in the North Pacific and Arctic, made with 38 and 120kHz split-beam echosounders in the Chukchi Sea. The average TS of a ∼25cm bell diameter C. melanaster is estimated to be −60.1dB re 1m2 at 38kHz and −60.5dB re 1m2 at 120kHz. There was no evident relationship between mean jellyfish size and TS, which is likely attributable to a combination of a small range in average jellyfish size at the sampling sites and geographic variability in TS corresponding to factors other than size and/or errors in determining the size of the jellyfish. The TS of C. melanaster is similar to that of a small swimbladdered fish, but on a mass-specific basis, the TS of these jellyfish is substantially lower. These measurements will aid in acoustic estimates of jellyfish abundance and in partitioning the relative contribution of jellyfish to acoustic backscatter in mixed species assemblages.