Acoustic Detection of Krill Scattering Layer in the Terra Nova Bay Polynya, Antarctica

Myounghee Kang,Wuju Son,Rina Fajaryanti,Hyoung Sul La,Jeong-Hoon Kim

doi:10.3389/fmars.2020.584550

Abstract

Krill play a crucial role in the transfer of energy in the marine food web, connecting primary producers and the upper trophic levels in the Terra Nova Bay polynya (TNBP), which is part of the Ross Sea marine protected area. Despite the substantial ecological importance of krill, there are few studies on their distribution and abundance in the TNBP. An acoustic survey was conducted on 7‒14 January 2019 in the TNBP, Ross Sea, using a Simrad EK60 echosounder (38 and 120 kHz) aboard the icebreaker RV Araon. The most commonly used range of the difference of the mean volume backscattering strength (2–16 dB) was applied to distinguish krill. The acoustic data (120 kHz) were extracted to examine the krill distribution characteristics. The study area was divided into low-value areas and high-value areas based on the third quartile of the nautical area scattering coefficient. The results showed that the krill aggregations were distributed in three layers at depths of 0‒30 m, 70‒110 m, and 270‒300 m. The interpolated environmental parameters associated with the backscattering strength were compared. High-value areas of krill coincided with relatively low temperature, low salinity, and high chlorophyll, although very weak correlations were found. The primary goal of this study was to understand the vertical and horizontal distributions of krill acoustic biomass and to relate the observed patterns to the dominant environmental conditions.

Highlights

Krill are considered keystone species in the Southern Ocean marine ecosystem (Atkinson et al, 2004) and in commercial fisheries (Nicol et al, 2012)
The echograms, which were processed with various noise removal algorithms and krill identification techniques, indicated the presence of acoustic scattering layers composed of krill
Postprocessing methods have been utilized to filter out noise, including a 7 × 7 convolution filter for investigating krill swarms (Klevjer et al, 2010) and a twosided comparison filter for handling the acoustic data collected from fishing vessels and research vessels (Anderson et al, 2005; Ryan et al, 2015)

Summary

Introduction

Krill (euphausiids) are considered keystone species in the Southern Ocean marine ecosystem (Atkinson et al, 2004) and in commercial fisheries (Nicol et al, 2012). They play an important role in the biogeochemical carbon cycle (Belcher et al, 2017; Cavan et al, 2019). The Ross Sea continental shelf is the most productive region in the Southern Ocean (Arrigo et al, 2008; Smith et al, 2014) and produces massive plankton and krill blooms that support huge numbers of fish, seals, penguins, birds, and whales (Ainley, 2010; Ballard et al, 2011). The Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) has strongly encouraged scientific research to protect and assess the Ross Sea marine ecosystem and in the future

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Conclusion