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Article

Acoustic Characteristics of Largehead Hairtail (Trichiurus japonicus) Using Multi-Frequency Detection in the Southern Sea, South Korea

by
Hyungbeen Lee
,
Euna Yoon
*,
Seungjong Lee
and
Jeonghoon Lee
Fisheries Resources Research Center, National Institute of Fisheries Science, Tongyeong 53064, Republic of Korea
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(10), 1918; https://doi.org/10.3390/jmse11101918
Submission received: 3 August 2023 / Revised: 23 September 2023 / Accepted: 24 September 2023 / Published: 4 October 2023
(This article belongs to the Special Issue Fisheries Acoustics and Marine Science Technology)
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Abstract

:
This study was conducted to investigate the acoustic characteristics of largehead hairtails (Trichiurus japonicus) in the Southern Sea of South Korea from April 2022, using 18, 38, 70, 120, and 200 kHz split-beam echosounders. At two stations, juvenile (preanal length: 3.2 cm–5.8 cm) and immature (preanal length: 13.0 cm–26.0 cm) hairtails were caught by midwater trawling. Juvenile hairtails were distributed in layers without forming schools, while immature hairtails were detected as forming small, clustered schools. The mean volume backscattering strength (MVBS) of immature hairtails was more than 20 dB stronger than that of juvenile hairtails at both the 38 kHz and 120 kHz detection frequencies. Furthermore, the SV of juvenile and immature hairtails was high at 18 kHz and decreased as the frequency increased. We concluded that juvenile and immature hairtails can be distinguished according to their morphological characteristics because the distribution characteristics detected in the echograms differed according to their growth stage. Finally, these findings hold remarkable potential for applications within the field of hairtail identification. Acoustic backscatter information, as presented, offers valuable insights into age determination in this fish species.

1. Introduction

The hairtail (Trichiurus japonicus) is a migratory fish species of the order Perciformes, family Trichiuridae, that inhabits subtropical waters and has a wide range, which includes the coastal waters around South Korea and the East China Sea [1,2]. This species is one of the major commercial fish species in Asia and is caught by various methods, including set-net fishing, bottom trawling, angling fishing, and surround-net fishing [3]. Hairtails have a long spawning season from April to October, and they migrate to the Southern Sea coast and the southern parts of the West Sea for spawning [4,5]. Spent females and young, hatched hairtails are known to move southward from September to the southwestern waters of Jeju Island for wintering, which lasts from November to March [6]. Before the 1970s, the annual hairtail catch was approximately 60,000 tons in South Korea; however, with advances in fishing technologies, the annual catch grew continually, reaching 120,000 tons in 1981 before subsequently declining. As of 2020, the annual catch is approximately 65,000 tons [7].
In South Korea, studies on hairtails have included ecological studies on their age, growth, feeding behavior, spawning, and maturation, as well as research on trends in fish stocks, such as predicting catch size, assessing stocks, and school formation [1,8,9]. Despite continuous research, changes in the marine environment of the East China Sea and the seas surrounding South Korea have led to decreasing trends in both hairtail catch size and age. Accordingly, South Korea has been implementing resource management measures for hairtails, which include ascertaining their biological and ecological characteristics to determine closed seasons and minimum size limits, and it introduced the total allowable catch policy in 2022.
Acoustic surveys using scientific echosounders have been conducted worldwide. This technique enables the examination of the spatiotemporal distribution of pelagic fish in all layers of a vast ocean area in a relatively short time [10,11]. Numerous acoustic surveys have also taken place in South Korea. These include echosounder studies on the chub mackerel (Scomber japonicus) and jack mackerel (Trachurus japonicus) in the seas around Jeju Island, as well as studies on the Pacific anchovy (Engraulis japonicus) in the Southern Sea. Additionally, research has been conducted to investigate the distribution and changes in the distribution of pelagic fish in artificial reefs in regions such as Jeju Island, Asan Bay, and Gwangyang Bay [12,13,14,15].
Since 2021, the National Institute of Fisheries Science (NIFS) has been using research vessels to perform acoustic studies in the coastal waters of the Southern Sea that combine echosounders with midwater trawling. Since the coastal waters of the Southern Sea include a confluence of different water masses, they are nutrient-rich habitats that support diverse fish species. A pelagic fish school signal was detected in an acoustic survey in the Southern Sea coastal waters in the spring of 2022. When a midwater trawl vessel was sent to the area, the dominant fish in two catches were juvenile and immature hairtails at stations (St.) 1 and 2, respectively. Owing to the long spawning season of the hairtail, individuals at different growth stages can be found simultaneously. In addition, hairtails show characteristic vertical movement during the daytime from the lower epipelagic or upper mesopelagic zone toward the surface for feeding at night [16]. In the acoustic studies on hairtails, the target strength has been measured both ex situ and in situ [17,18,19]. Kao et al. [16] and Kang et al. [20] calculated the spatiotemporal distribution and standing stocks using a frequency of 38 kHz. However, acoustic studies using multiple frequencies are required to differentiate hairtails from other species.
In this study, we aimed to differentially identify the hairtail signal in the coastal waters of the Southern Sea, which contains spatiotemporally fluctuating populations of diverse anadromous pelagic fish species. To this end, we used multiple frequencies, namely 18, 38, 70, 120, and 200 kHz, to analyze the acoustic characteristics of schools of juvenile and immature hairtails. Additionally, we analyzed differences in the mean volume backscattering strength (∆MVBS) per frequency.

2. Materials and Methods

2.1. Collection of Acoustic Data

We conducted an acoustic survey of pelagic fish in the Southern Sea near the coast of South Korea in April 2022, using the NIFS research vessel Tamgu No. 23. Before the acoustic survey, on April 13th, near Saryangdo, Tongyeong-si, Gyeonsangnam-do (34°48.69′ N, 128°11.87′ E; depth: 20 m), an EK80 echosounder (SIMRAD, Kongsberg Maritime AS, Norway) was calibrated to the frequencies of 18, 38, 70, 120, and 200 kHz using a calibration sphere (38.1 mm tungsten carbide) [21]. During calibration, the water temperature and salinity were 14.0 °C and 33.8 PSU, respectively, and the sound velocity was 1502.3 m/s.
For the acoustic survey, ten parallel transect lines were defined in the coastal waters of the Southern Sea (Figure 1). For stable data collection, the research vessel followed the transects at a speed of 11 knots as it collected and stored the acoustic data of continuous wave signals at 18, 38, 70, 120, and 200 kHz during daytime hours (7 a.m. to 7 p.m.).

2.2. Midwater Trawl Survey

For the precise differentiation of the species detected during the acoustic survey, a midwater trawl (length: 106 m; hand rope: 97 m; net pendant: 97 m; square-mesh codend: 2 mm) was used to perform two Station (St.) 1 and 2 surveys (Figure 1a). To catch the fish detected by the echosounder, the midwater trawl was fitted with a depth sensor (SS4 depth sensor, Scanmar AS, Norway), the depth of the trawl net was monitored in real time, and the net was towed for 15 min and 35 min at St. 1 and St. 2, respectively, while trawling at a speed of 3.7 and 4 knots (Table 1).
The dominant species (>90%) caught in both midwater trawl surveys was the hairtail. Juvenile hairtails were caught at St. 1, and immature hairtails were caught at St. 2. The preanal length (PL, cm) and wet weight (g) were measured per individual (Figure 1b). For accurate measurement, the juvenile hairtails were frozen and weighed at a fishery resources research center at the NIFS.

2.3. Analysis of Acoustic Data

To investigate the acoustic characteristics of juvenile and immature hairtails, the 18, 38, 70, 120, and 200 kHz acoustic data in the planned trawl depth range was analyzed using analysis software for echosounders (Echoview Ver 11.0, Echoview software Pty. Ltd., Hobart, Australia). From the raw acoustic data, a clean echogram was generated by removing surface and sub-seabed data based on the 38 kHz signal, and the acoustic characteristics of juvenile and immature hairtails were analyzed. For the juvenile hairtail signal at St. 1, the horizontal and vertical cell intervals for the volume backscattering strength (SV) at each frequency were set to 10 pings and 0.5 m, respectively, and the mean volume backscattering strength (MVBS) was extracted for each frequency. For the immature hairtail signal at St. 2, a school detection algorithm at 38 kHz was applied to the clean echogram, and only signals identified as hairtail schools were included (Table 2). For the identification of hairtail schools, the maximum vertical and maximum horizontal linking distances were determined. School candidates that were larger than the minimum candidate length and height were selected as hairtail schools. For the identified schools, the MVBS at each frequency was extracted by selecting the horizontal and vertical cell intervals, using the same method as for the juvenile hairtail signal. To compare the frequency characteristics of juvenile and immature hairtails, the ∆MVBS relative to 38 kHz was calculated from the MVBS for each frequency [22].

3. Results

3.1. Midwater Trawl Catch

In the midwater trawl survey in the coastal waters of the Southern Sea in April 2022, in both St. 1 and St. 2, the dominant species was the hairtail. The relative abundance by population was 98.1% and 95.7% at St. 1 and St. 2, respectively, and the relative abundance by biomass was 81.9% and 93.6%, respectively (Table 3). The PL distribution of the hairtails caught by the midwater trawl was 3.2–5.8 cm (mean ± SD, 4.6 ± 0.6) at St. 1 and 13.0–26.0 cm (19.5 ± 2.4) at St. 2, demonstrating a clear difference in the size of individuals caught at each station (Figure 1b). The difference in PL between the two stations was statistically significant (t-test, p < 0.05). Based on the PL, the hairtails caught at St. 1 were juveniles, and those at St. 2 were immature [23,24,25].

3.2. Hairtail Echograms and Frequency Characteristics

The echosounder data collected during trawl fishing revealed significant differences between the acoustic signals from juvenile and immature hairtails (Figure 2). The signal for juveniles detected at St. 1 indicated that they did not form schools; instead, they appeared as a long layer with a scattered distribution, resembling a sound scattering layer. The SV for juveniles was detected at depths ranging from 60 to 80 m, with values between −65 and −60 dB. Comparing frequency characteristics, the SV was stronger at 38 kHz than at 120 kHz (Figure 2a,b). The water temperature where juvenile hairtails were observed was 12.98–14.63 °C (Figure 2c). In contrast, the immature hairtails detected at St. 2 formed schools in the shape of small, non-continuous clusters (Figure 2d,e). These schools were primarily distributed at depths of 50–65 m and were relatively small, with heights and widths ranging from 0.5 to 4.0 m and 0.7 to 13.9 m, respectively (Table 4). Despite variations in the SV of immature hairtail schools across frequencies, the schools exhibited a strong SV of more than −40 dB, which was 20 dB stronger than that of the juvenile hairtails. The water temperature ranges in the survey region, where immature hairtails were found, were similar across all layers and also comparable to the range of water temperatures at St. 1, where juvenile hairtails were observed (Figure 2f).
When we compared the acoustic signals collected from juvenile and immature hairtails at the commonly used echosounder frequencies of 38 kHz and 120 kHz, the juvenile hairtails caught at St. 1 showed SV ranges (mean ± SD) from −85.00 to −52.95 dB (–69.96 ± 4.31 dB) and −85.00 to −61.01 dB (–79.12 ± 4.02 dB) at 38 kHz and 120kHz, respectively. The immature hairtails caught at St. 2 showed an SV range (mean ± SD) of −85.00 to −32.29 dB (–48.10 ± 10.63 dB) at 38 kHz and −85.00 to −37.80 dB (–51.52 ± 9.87 dB) at 120 kHz. Compared with the juvenile hairtails, which were distributed across a layer, the immature hairtails exhibited a greater number of school signals and produced a markedly stronger acoustic signal that was 21.86 dB–27.60 dB stronger at both frequencies (Figure 3).
Regarding the differences in the signals between frequencies for the juvenile and immature hairtails, at both growth stages, the difference was largest for △MVBS18–38 and smallest for △MVBS200–38. This indicates that hairtails, which have swim bladders, show stronger SV values at low frequencies than at high frequencies (Figure 4, Table 5). In the comparison of △MVBS between growth stages, similar values were observed for all frequency intervals, with juvenile hairtails showing higher values than immature hairtails.

4. Discussion

4.1. Biological Characteristics

The mature PL of hairtails in the seas surrounding Jeju Island, the South China Sea, and the East China Sea has been reported to be 25.0–28.5 cm [2,4,5,9,23]. In our survey, the PL of juvenile and immature hairtails caught in the coastal waters of the Southern Sea in April 2022 was 3.2–26.0 cm, meaning that the body length of most individuals was shorter than that of mature individuals. Additionally, when calculated using the von Bertalanffy growth function, the age of the juvenile hairtails was estimated to be 0+ years, while that of the immature hairtails was 1–2 years [2]. Hairtails living near South Korea have a long spawning season, from April to October [4,5]. The juveniles caught in our study would have been spawned the previous year, and the immature hairtails would have hatched two years before the study. Because of their long spawning season, South Korean hairtail individuals of various body lengths can co-exist. Hence, to detect schools and calculate standing stocks with an echosounder, it is essential to understand the acoustic characteristics with respect to the distribution of individuals of diverse body lengths.

4.2. Acoustic Characteristics

Methods of identifying schools of fish acoustically can be classified into three types: frequency characteristic measurement, ecological distribution characteristic measurement, and echo tracking [26,27]. When we compared the acoustic characteristics of juvenile and immature hairtails in this study, although the frequency characteristics were similar, there were differences in the shapes of the detected schools. In a previous study, the walleye pollock (Gadus chalcogrammus) showed age-specific acoustic characteristics and unique echogram characteristics between schools depending on age, demonstrating the potential for classification based on ∆MVBS patterns [28]. Although the walleye pollock and hairtail are different species, they both have swim bladders and a similar range of ∆MVBS. Owing to their school-forming characteristics, ∆MVBS patterns could potentially be used to classify juveniles and immature fish. As the hairtail grows, discernible variations in both the morphological characteristics and ecological distribution patterns within the fish school become evident. These observations support the hypothesis that a distinction can be made between juvenile and immature hairtail specimens. Fish and zooplankton are typically isolated using ∆MVBS with only two frequencies, 38 kHz and 120 kHz. After obtaining the acoustic signal for fish only, in survey areas where diverse fish species co-exist, species are commonly differentiated based on the morphological characteristics of the schools, their spatiotemporal distribution, which includes the depth, and their SV characteristics [29,30,31]. In April, various fish species migrate from adjacent waters to the coastal waters of the Southern Sea to spawn, and in the trawl survey in the present study, in addition to hairtails, we observed Pacific anchovy, Pacific sardinellas (Sardinella zunasi), and dotted gizzard shads (Konosirus punctatus). In particular, given that Pacific anchovies are one of the major commercial fish species in South Korea, it is essential to be able to distinguish them from hairtails using acoustic methods to accurately measure their spatiotemporal distribution and the standing stocks of each species. The juvenile and immature hairtails identified in our study showed a ∆MVBS120–38 kHz of −9.20 ± 5.39 dB and −6.29 ± 5.39 dB, respectively, while Pacific anchovies show −16.9 < ∆MVBS120–38 kHz < 13.0, meaning that hairtails are included within the ∆MVBS range of Pacific anchovies [32]. However, the mean height and length of the immature hairtail schools were 1.7 m and 5.8 m, respectively, and they formed small schools. Conversely, Pacific anchovies in the south of the East Sea and in the East China Sea have been reported to form large schools more than twice this size, with mean heights and lengths of 3.4–4.4 m and 13.8–22.7 m, respectively [32,33].
The main fish species inhabiting the pelagic waters of the South Sea during the spring season are Pacific anchovies [34,35]. Hairtail schools also occupy the pelagic region during this season [34,35], and they constitute a prominent catch within the surveyed area. Despite the overlapping presence of hairtail and Pacific anchovy schools in shared waters, we verified the potential of differentiating the two species based on distinctive morphological characteristics. Moreover, even within the same species, there were differences in the echogram distribution characteristics depending on the stage of growth, allowing differentiation based on echogram appearance. For a more precise species identification and calculation of standing stocks, it will be necessary to perform catch surveys in parallel with echosounder surveys.

5. Conclusions

To identify hairtail schools using an echosounder, we analyzed the acoustic characteristics of juvenile and immature hairtails living in the coastal waters of the Southern Sea in April 2022. Juvenile hairtails did not form schools but were distributed in layers, whereas immature hairtails formed schools. The SV of immature hairtails was stronger than that of juvenile hairtails at both 38 kHz and 120 kHz, with a difference of around 20 dB. The MVBS was greatest at 18 kHz and decreased with increasing frequency. This pattern was consistent for both juveniles and immature hairtails. As juvenile and immature hairtails showed differences in their distributional characteristics on the echogram depending on their growth stage, we deduced that these two groups could be differentiated based on morphological differences in the echogram.

Author Contributions

Conceptualization, H.L. and E.Y.; methodology, H.L. and E.Y.; software, H.L. and E.Y.; investigation, H.L. and E.Y.; data curation, H.L. and E.Y.; writing—original draft preparation, H.L. and E.Y.; writing—review and editing, H.L., E.Y., S.L., and J.L.; visualization, H.L.; supervision, J.L.; project administration, S.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a grant from the National Institute of Fisheries Science (R2023001).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

We thank the researchers and crew of the Tamgu 23 fishery resource research vessel (NIFS) for the survey.

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 11 01918 g001
Figure 1. Study area in the Southern Sea of South Korea (a). The acoustic survey transects are shown as black lines. The squares represent midwater trawl surveys for fish catches. The hairtail (Trichiurus japonicus) preanal length–weight relationship (b) in the survey area. White bars and black bars represent the length–frequency distribution (c) of juvenile (St. 1) and immature (St. 2) T. japonicus, respectively.
Figure 1. Study area in the Southern Sea of South Korea (a). The acoustic survey transects are shown as black lines. The squares represent midwater trawl surveys for fish catches. The hairtail (Trichiurus japonicus) preanal length–weight relationship (b) in the survey area. White bars and black bars represent the length–frequency distribution (c) of juvenile (St. 1) and immature (St. 2) T. japonicus, respectively.
Jmse 11 01918 g001
Jmse 11 01918 g002
Figure 2. Examples of acoustic echograms in the midwater trawl transects at 38 kHz (a,d) and 120 kHz (b,e) and the vertical distribution of water temperature (c,f) in the Southern Sea of South Korea at St. 1 (a–c) and St. 2 (d–f).
Figure 2. Examples of acoustic echograms in the midwater trawl transects at 38 kHz (a,d) and 120 kHz (b,e) and the vertical distribution of water temperature (c,f) in the Southern Sea of South Korea at St. 1 (a–c) and St. 2 (d–f).
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Figure 3. Scatter plot of volume backscattering strength (SV) at 120 kHz against 38 kHz (a). The white and gray dots indicate SV from juvenile hairtails at St. 1 and immature hairtails at St. 2, respectively. The black line and dashed line represent the linear relationship between the SV at 38 kHz and the SV at 120 kHz with 95% confidence limits at St. 1 and St. 2, respectively. The regression equation is SV120 kHz = 0.73 SV38 kHz−28.39 (r2 = 0.56) at St. 1 and SV120 kHz = 0.93 SV38 kHz−9.89 (r2 = 0.70) at St. 2. Histograms of the SV distribution at (b) 38 kHz and (c) 120 kHz. The white and gray bars indicate the whole SV of stations 1 and 2, respectively.
Figure 3. Scatter plot of volume backscattering strength (SV) at 120 kHz against 38 kHz (a). The white and gray dots indicate SV from juvenile hairtails at St. 1 and immature hairtails at St. 2, respectively. The black line and dashed line represent the linear relationship between the SV at 38 kHz and the SV at 120 kHz with 95% confidence limits at St. 1 and St. 2, respectively. The regression equation is SV120 kHz = 0.73 SV38 kHz−28.39 (r2 = 0.56) at St. 1 and SV120 kHz = 0.93 SV38 kHz−9.89 (r2 = 0.70) at St. 2. Histograms of the SV distribution at (b) 38 kHz and (c) 120 kHz. The white and gray bars indicate the whole SV of stations 1 and 2, respectively.
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Figure 4. Means and standard deviations of the mean volume backscattering strength difference (∆MVBS) estimated for juvenile and immature hairtails (Trichiurus japonicus) during acoustic-trawl surveys in the Southern Sea of South Korea.
Figure 4. Means and standard deviations of the mean volume backscattering strength difference (∆MVBS) estimated for juvenile and immature hairtails (Trichiurus japonicus) during acoustic-trawl surveys in the Southern Sea of South Korea.
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Table 1. Details of the midwater trawl survey in the Southern Sea of South Korea, conducted in April 2022.
Table 1. Details of the midwater trawl survey in the Southern Sea of South Korea, conducted in April 2022.
Trawl
(Station)		DateLocationTrawl Depth
(m)		Speed
(Knots)
Latitude (N)Longitude (E)
117 April 202234°6.3′128°14.0′63–903.7
221 April 202234°11.8′127°57.9′50–714.0
Table 2. Parameters for the school detection algorithm for hairtails (Trichiurus japonicus).
Table 2. Parameters for the school detection algorithm for hairtails (Trichiurus japonicus).
ParameterValue (m)
Minimum total school height2
Minimum candidate length5
Minimum candidate height1
Maximum vertical linking distance5
Maximum horizontal linking distance10
Maximum total school length10
Table 3. Species composition of fish collected using a midwater trawl in the Southern Sea of South Korea.
Table 3. Species composition of fish collected using a midwater trawl in the Southern Sea of South Korea.
Trawl
(Station)		SpeciesIndividual (n)Individual (%)Total Weight (kg)Total Weight (%)Remarks
1Trichiurus japonicus109698.47 1.3381.60
Todarodes pacificus141.260.042.45
Other30.270.2615.953 species
2Trichiurus japonicus165495.66165.5493.63
Sphyraena pinguis231.331.781.01
Pennahia argentata150.873.331.88
Trachurus japonicus130.750.640.36
Pampus argenteus110.641.931.09
Other130.753.592.038 species
Table 4. Aggregation characteristics of immature hairtail (Trichiurus japonicus) schools in the Southern Sea of South Korea.
Table 4. Aggregation characteristics of immature hairtail (Trichiurus japonicus) schools in the Southern Sea of South Korea.
CharacteristicRangeMean ± SD
Height (m)0.5–4.01.7 ± 0.9
Length (m)0.7–13.95.8 ± 3.7
Thickness (m)0.8–4.11.9 ± 1.0
Perimeter (m)2.5–62.222.6 ± 15.3
Area (m2)0.5–39.78.2 ± 10.3
Table 5. Summary of the ∆MVBS (dB) estimated for juvenile and immature hairtails (Trichiurus japonicus).
Table 5. Summary of the ∆MVBS (dB) estimated for juvenile and immature hairtails (Trichiurus japonicus).
Taxon∆MVBS18–38 (dB)∆MVBS70–38 (dB)∆MVBS120–38 (dB)∆MVBS200–38 (dB)
Juvenile2.17 ± 4.84−6.99 ± 3.01−9.20 ± 2.87−11.35 ± 3.32
Immature3.23 ± 5.23−5.12 ± 5.21−6.29 ± 5.39−9.13 ± 4.99
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MDPI and ACS Style

Lee, H.; Yoon, E.; Lee, S.; Lee, J. Acoustic Characteristics of Largehead Hairtail (Trichiurus japonicus) Using Multi-Frequency Detection in the Southern Sea, South Korea. J. Mar. Sci. Eng. 2023, 11, 1918. https://doi.org/10.3390/jmse11101918

AMA Style

Lee H, Yoon E, Lee S, Lee J. Acoustic Characteristics of Largehead Hairtail (Trichiurus japonicus) Using Multi-Frequency Detection in the Southern Sea, South Korea. Journal of Marine Science and Engineering. 2023; 11(10):1918. https://doi.org/10.3390/jmse11101918

Chicago/Turabian Style

Lee, Hyungbeen, Euna Yoon, Seungjong Lee, and Jeonghoon Lee. 2023. "Acoustic Characteristics of Largehead Hairtail (Trichiurus japonicus) Using Multi-Frequency Detection in the Southern Sea, South Korea" Journal of Marine Science and Engineering 11, no. 10: 1918. https://doi.org/10.3390/jmse11101918

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