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

Hyungbeen Lee,Yangjae Im,Wooseok Oh,Kyounghoon Lee,Junghwa Choi,Kangseok Hwang

doi:10.3390/w14020203

Hyungbeen Lee, Yangjae Im + Show 4 more

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https://doi.org/10.3390/w14020203

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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.

Highlights

The East Sea is a semi-enclosed deep marginal sea in the northwestern Pacific with seasonal varying water conditions such as deep water circulation, a polar front, upwelling, and eddies due to the warm and saline equatorial waters of the Tsushima Current and cold coastal water of the North Korean Cold Current [1]
The vertical structure of water temperature and salinity represent the seasonal variations along the coastal southwestern East Sea from April to July 2010 (Figure 2)
We examined the characteristics of the multi-frequency data to identify E. pacifica that dominated the 120 kHz acoustic backscatter for all seasons in the East Sea, and whether they changed between seasons

Summary

Introduction

The East Sea is a semi-enclosed deep marginal sea in the northwestern Pacific with seasonal varying water conditions such as deep water circulation, a polar front, upwelling, and eddies due to the warm and saline equatorial waters of the Tsushima Current and cold coastal water of the North Korean Cold Current [1]. Frequent coastal upwelling and eddies caused by the well-defined oceanographic processes were observed in the coast of the East Sea [2]. The southwestern region is one of the most productive regions in the East Sea, with extensive phytoplankton blooms consistently detected during spring and summer via satellite-based ocean color sensors [3]. The region supports high primary and secondary production, making it an important nursery ground for many invertebrates and fish stocks [4]. It is important to understand pelagic food web dynamics in this region

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