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Density Estimation of Antarctic Krill in the South Shetland Island (Subarea 48.1) Using Db-Difference Method

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sustainability Article Density Estimation of Antarctic Krill in the South Shetland Island (Subarea 48.1) Using dB-Difference Method Seok-Gwan Choi 1, Jinho Chae 2, Sangdeuk Chung 1, Wooseok Oh 3, Euna Yoon 4, Gunhee Sung 2 and Kyounghoon Lee 3,* 1 Distant Water Fisheries Resources Research Division, National Institute of Fisheries Science, Busan 46083, Korea; [email protected] (S.-G.C.); [email protected] (S.C.) 2 Marine Environmental Research & Information Laboratory, Gunpo 15850, Korea; [email protected] (J.C.); [email protected] (G.S.) 3 Division of Fisheries Science, Chonnam National University, Yeosu 59626, Korea; [email protected] 4 Fisheries Resources Research Center, National Institute of Fisheries Science, Tongyeong 53064, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-61-659-7124; Fax: +82-61-659-7129 Received: 21 May 2020; Accepted: 7 July 2020; Published: 15 July 2020 Abstract: This study is aimed to estimate the density of Antarctic krill that inhabit in the area around South Shetland Island by using a dB-difference method. An acoustic survey was conducted from 13 to 24 April in 2016 in the sea of South Shetland. Acoustic data on frequency 38 and 120 kHz were collected. The Antarctic krill echo was extracted for the SV120-38 kHz range of 0.4–14.3 dB, which was obtained by applying the size of the collected Antarctic krill in this study (25–60 mm). The mean 2 Antarctic krill density across the survey area was 33.65 g m− (CV = 45.97%). Keywords: Antarctic krill; acoustic survey; scientific echosounder; dB difference; biomass 1. Introduction Antarctic krill (Euphausia superba) play an important role as part of the Antarctic marine ecosystem, with recent research focusing on their potential as a future alternative food source for use in the development of various health foods and medications [1–5]. However, global warming, changes in the marine environment, and the overfishing of Antarctic krill resources have caused their decline, impacting the marine ecosystem itself. In an effort to resolve these issues, the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) was formed in 1982 for the sustainable conservation and logical use of marine organisms inhabiting Antarctic waters. At present, the CCAMLR has 25 member states. South Korea joined the CCAMLR in 1985, and, in collaboration with the other member states, has been conducting surveys and research, participating in international surveillance operations of the Antarctic marine ecosystem and marine organisms [6,7]. In particular, Antarctic krill are a major food source for marine animals, such as penguins, seals, and whales. Because of this, the CCAMLR has implemented systematic control measures for Antarctic krill resources since 1982, such as restricting total catch volumes. In addition, there have been collaborative research efforts to improve the management of Antarctic krill resources, with a strong focus on the countries that catch Antarctic krill. In order to manage Antarctic krill resources, research must be conducted to understand the distribution and abundance of Antarctic krill. Acoustic technology is often used to evaluate the spatiotemporal distribution and abundance of Antarctic krill, because it provides information at all depths, over a wide area, and in a short time [5,8–10]. The CCAMLR-2000 Antarctic krill Synoptic Survey was designed, planned, and implemented in 1995 Sustainability 2020, 12, 5701; doi:10.3390/su12145701 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 5701 2 of 14 Sustainability 2020, 12, x 2 of 15 conductto conduct acoustic acoustic surveys surveys of ofAntarctic Antarctic krill krill resour resourcesces [11]. [11 ].The The Subgroup Subgroup on on Acoustic Acoustic Survey Survey and and Analysis Method (SG-ASAM) has been in action since 2011. AntarcticAntarctic krill krill fishing fishing operations operations take take place place in in waters around the South Shetland Islands (Subarea(Subarea 48.1), 48.1), the the South South Orkney Orkney Islands (Subarea (Subarea 48.2), 48.2), South South Georgia Georgia (Subarea (Subarea 48.3), 48.3), and and South South SandwichSandwich (Subarea(Subarea 48.4).48.4). The The total total catch catch sizes sizes are are restricted restricted to 155,000 to 155,000 tons intons the in South the ShetlandSouth Shetland Islands Islands(Subarea (Subarea 48.1), 279,000 48.1), 279,000 tons in thetons South in the Orkney South Orkn Islandsey (SubareaIslands (Subarea 48.2), 279,000 48.2), tons279,000 in South tons in Georgia South Georgia(Subarea (Subarea 48.3), and 48.3), 93,000 and tons 93,000 in South tons Sandwich in South (Subarea Sandwich 48.4). (Subarea Information 48.4). onInformation the abundance on the of abundanceAntarctic krill of Antarctic is required krill to is set required an upper to set limit an to upper the total limit allowed to the total catch allowed volume. catch The volume. abundance The abundanceof Antarctic of krill Antarctic populations krill populations in these waters in these is continually waters is continually assessed and assessed reported and based reported on acoustic based onsurveys. acoustic In thesurveys. present In study, the wepresent used acousticstudy, we data used collected acoustic through data CCAMLR collected analytical through methodsCCAMLR to analyticaldetermine methods the distribution to determine and density the distribution of Antarctic an krilld density inhabiting of Antarctic the waters krill near inhabiting the South the Shetland waters nearIslands the (SubareaSouth Shetland 48.1). Islands (Subarea 48.1). 2.2. Materials Materials and and Methods Methods 2.1.2.1. Survey Survey Area Area and and Sampling Sampling Sites Sites TheThe survey areaarea coveredcovered the the west west of of South South Shetland Shetland Islands, Islands, Elephant Elephant Island, Island, and Bransfieldand Bransfield Strait Straitin the in south. the south. The survey The survey area consisted area consisted of the of coastal the coastal area, formed area, formed by the by continental the continental shelf within shelf within200 m of200 the m islands,of the islands, the tidal the front tidal atfront the at interface the interface of diff oferent different types types of water, of water, and theand o theffshore offshore area areawhere where the water the water became became deeper. deeper. TheThe survey survey was was conducted conducted using using a a commercial commercial fishing fishing vessel vessel (Gwangja-ho), (Gwangja-ho), from from 13 13 to to 24 24 April April 2 2016.2016. The survey area was 90,700 km 2,, within within which which 24 24 survey survey lines lines and and seven seven sampling locations locations were established (Figure (Figure 11).). Acoustic Acoustic data data were were reco recordedrded while while travelling travelling along along the the survey survey lines lines at at a speeda speed of of<10< 10knots, knots, and and organisms organisms were were collected collected at the at sampling the sampling points points using using a midwater a midwater trawl. trawl. The towingThe towing speed speed of the of trawl the trawl was wasmaintained maintained at 2–3 at knots. 2–3 knots. Figure 1. Survey lines and sampling points for acoustic and net surveys to determine the density of Figure 1. Survey lines and sampling points for acoustic and net surveys to determine the density of Antarctic krill. Antarctic krill. 2.2. Acoustic System Setup and Data Collection Sustainability 2020, 12, 5701 3 of 14 2.2. Acoustic System Setup and Data Collection For the acoustic survey system, we used a split-beam scientific echosounder (EK60, Simrad, Norway) attached to the bottom of the Gwangja-ho at frequencies of 38 and 120 kHz. The parameters of the system during acoustic surveys were set according to the criteria presented by the CCAMLR (Table1). Before the survey, the echosounder was calibrated at the coast using 60 mm at 38 kHz and 23 mm at 120 kHz copper spheres following the method of Foote et al. [12] (62◦28.70 S, 59◦42.40 W). The results of calibration are shown in Tables1 and2. Table 1. Scientific echosounder set up to collect acoustic data. Parameters Setting Frequency (kHz) 38 120 Power setting (w) 2000 250 Ping duration (ms) 1.024 1.024 Ping interval (s) 2 2 Data collection range (min.-max.) (m) 0–1100 0–1100 Bottom detection range (min.-max.) (m) 5–1100 5–1100 Display range (min.-max.) (m) 0–1100 0–1100 Table 2. Results of calibration at frequencies of 38 and 120 kHz. Frequency (kHz) 38 120 Two-way beam angle (dB) 20.6 21.0 − − Receiver bandwidth (kHz) 2.43 3.03 Transducer gain (dB) 26.82 27.64 3-dB Beam angle (athwart/along) (deg.) 7.08/7.03 6.47/5.60 1 Absorption coefficient (dB km− ) 9.8 24.7 1 Sound speed (m s− ) 1448.9 1448.9 2.3. Antarctic Krill Sampling The Antarctic krill were collected with a midwater trawl used on a commercial fishing ship. The total net length was 167.6 m, the mesh size was 15 mm, the net height was 40 m, and the net width was 72 m. The biomass of each sample was measured after hauling. At each sampling point, 200 Antarctic krill were randomly selected, and the length was measured at 1 mm intervals from the anterior margin of the eye to the tip of the telson, excluding the terminal spines. 2.4. Analysis of Acoustic Data The collected acoustic data were analyzed using acoustic analysis software (Echoview V 8.0, Echoview Software, Australia). Noise from the ship and from electric signals was removed using the methods of De Robertis and Higginbottom and Wang et al. [13,14]. Figures2 and3 present a flowchart used for noise reduction and an example echogram following processing.
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