Vertical signature in acoustic estimates of zooplankton around the Yellow Sea Bottom Cold Water, Korea

Abstract

The Yellow Sea Bottom Cold Water (● 10▲C ), one of the several water masses in the Yellow Sea, is formed during the winter, occupies the lower layer of the Yellow Sea’s water body, and remains throughout the year until fall. This cold water mass directly affects the Yellow Sea ecosystem and marine living resources. Particularly,the population of Euphausia pacifica, known as one of the cold water euphausiid species and a key species in the Yellow Sea, could be significantly related with the dynamics of this cold water mass. In this study, vertical signatures in acoustic estimates of zooplankton are studied using data collected from the center of the Yellow Sea Bottom Cold Water with a two-frequency scientific echosounder (BioSonics DT-X series), combined with zooplankton net surveys. The acoustic-net surveys were done at 11 sampling stations (29~91 m depth) in April 2010. When acoustic signatures due to copepods were removed from acoustic data using the threshold level (？75 dB), diel patterns for major zooplankton species within the study area showed that the patches concentrate to thebottom or mid-depth during the day and move to the surface layer at night. Then, the upward and downward speed of the zooplankton patches was 0.74 and 0.87 m/s, espectively. Net surveys showed that, whereas nauplii and adult of E. pacifica and chaetognath randomly appeared at some station, E. pacifica eggs were abundantly distributed throughout all the stations. Zooplankton catch data showed that the major species of zooplankton having an influence on the acoustic signature were chaetognaths and E. pacifica.