Spatial–Temporal Distribution of the Euphausiid Euphausia pacifica and Fish Schools in the Coastal Southwestern East Sea

Abstract

The spatial and temporal distribution of euphausiid krill Euphausia pacifica (Crustacea: Malacostraca) and of fish schools were observed along acoustic transects at the southwestern East Sea coastline of Korea. Two-frequency (38- and 120-kHz) acoustic backscatter data were examined from April to July 2010. A decibel identification window (SV 120–38) and school detection algorithm identified Euphausia pacifica and fish schools in the acoustic backscatter, respectively. The E. pacifica was regularly observed in middle of southern waters, where phytoplankton was abundant during spring, and irregularly during summer, when phytoplankton was homogeneously distributed. Using the distorted-wave Born approximation model, the acoustic density we calculated of E. pacifica was higher in spring (April: 75.9 mg m−2, May: 85.3 mg m−2) than in summer (June: 71.4 mg m−2, July: 54.1 mg m−2). The fish schools observed by the acoustic data tended to significantly increase from spring to summer. Although major fish species, such as anchovies and herring, fed on copepods and euphausiids in the survey area, the temporal and spatial distribution of E. pacifica was weakly correlated with the distribution of the fish schools. These findings aid in our understanding of the temporal and spatial distribution dynamics of euphausiids and fish schools in the food web of the coastal southwestern East Sea.