Biomass distribution and zooplankton composition of the sound-scattering layer in the waters of southern East China Sea

Abstract

Fuzzy classification was used to analyse hydroacoustic survey data (Sv, an acoustic volume backscattering strength index of biomass) and oceanographic data (temperature, salinity, water density, and chlorophyll a fluorescence) collected from the waters of the southern East China Sea off northeastern Taiwan. Four types of water masses were identified. Type A water mass corresponded to warm continental-shelf mixed water and had a high acoustic volume backscatter (Sv>−59.37 dB). Type D corresponded to cold upwelling Kuroshio subsurface water with Sv<−65.24 dB, i.e. about 6 dB lower than that of Type A. Type B and C waters were intermediate between Type A and D waters. Twenty-two IKMT (net) samples were taken, and their estimated biomasses showed a significant relationship to corresponding average Sv values. Copepoda were most dominant in the zooplankton community. Decapoda, appendicularia, and chaetognatha were abundant in Type A waters, and proportionally rarer in the other three types of water masses. Conversely, the percentage contribution made by ostracoda and gastropoda was greater in Type D waters than in Type A waters. The combination of IKMT net sampling and hydroacoustic measurements was found to be useful for assessing the biomass distribution and species composition of zooplankton. We also conclude that the fuzzy classification method used here can usefully be applied to hydroacoustic data for the classification and identification of water masses and zooplankton composition in the ocean, particularly in an upwelling area.