Donhyug Kang

Ex situ target strength of rockfish (Sebastes schlegeli) and red sea bream (Pagrus major) in the Northwest Pacific

This study determined the ex situ target strength (TS) of rockfish (Sebastes schlegeli) and red sea bream (Pagrus major) in an artificial seawater tank as a means of helping to estimate fishery resources in coastal areas. TS experiments were conducted at frequencies of 38 kHz (split beam), 120 kHz (split beam), and 200 kHz (dual beam). The species were examined under two conditions: first, live fish confined to a small, net cage; and, second, as free-swimming fish inside a large tank. The study examined 21 rockfish and 20 red sea bream. The data were used to obtain expressions for TS against length and weight for the two species. The relationships between TS and fish length were as follows: for rockfish, TS 38 kHz = 20 log 10 (L) − 67.7 (r = 0.80), TS 120 kHz = 20 log 10 (L) − 74.3 (r = 0.61), TS 200 kHz = 20 log 10 (L) − 72.8 (r = 0.41); and for red sea bream, TS 38 kHz = 20 log 10 (L) − 66.8 (r = 0.86), TS 120 kHz = 20 log 10 (L) − 74.0 (r = 0.65), TS 200 kHz = 20 log 10 (L) − 74.1 (r = 0.83). The TS equations for rockfish and red sea bream as a function of fish weight at 38 kHz were TS 38 kHz = 6.75 log 10 (W) − 56.0 (r = 0.78) and TS 38 kHz = 4.08 log 10 (W) − 49.9 (r = 0.89), respectively. For comparison, calculations using the Helmholtz-Kirchhoff ray-approximation model based on swimbladder morphology were compared with the measured TS. When the tilt angle of the fish is zero, the mean TS from the model is 3-10 dB higher than the experimental results, although the maximum TS values were only 3-4 dB different.

Density and sound speed contrasts of the Japanese common squid Todarodes pacificus and their influence on acoustic target strength

In this study, density and sound speed contrasts were measured for the Japanese common squid Todarodes pacificus. Target strength (TS) data derived from an acoustic scattering model based on measurements of these two parameters were compared with TS values based on acoustic measurement data to determine whether the measured parameters are reasonable values for theoretical scattering model. Density contrast (g) was measured from the displacement volume and wet weight, while sound speed contrast (h) was measured from the acoustic measurements of travel time (time-of-flight method). The Kirchhoff ray mode model, which represents the squid body as a set of fluid-filled cylinders, was used to calculate theoretical TS. Mean g- and h-values of the squid were 1.029 and 1.041, respectively. Comparison with previous data showed that g was similar, whereas h was relatively high. The difference in the TS between the theoretical model using measured parameters and the acoustic measurement was within 3 dB. Thus, the measured g- and h-values are acceptable for theoretical models of squid.

Acoustic target strength of Japanese common squid, todarodes pacificus, and important parameters influencing its TS: swimming angle and material properties

The squids, one of the cephalopods, is important species in fisheries and ecology. If acoustic properties of squid can be well examined, especially target strength (TS), acoustic method can be provided a good tool for squid survey. The purpose of the study is to investigate the TS property of the Japanese common squid, Todarodes pacificus, and parameters influencing TS: swimming angle and material properties, density (g) and sound speed contrast (h). In the swimming angle measurement, the mostly head part of the squid stands below the horizontal axis, and averaged swimming angle was -17.7deg (s.d.:plusmn12.7deg). Based on the tethered TS measurements, the mean standardized TS values (b20) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b 20 were expressed as: b20=-73.3+0.48middotthetas+0.0122middotthetas2 +0.00016middotthetas3 for 38 kHz and b20=-72.6+0.53middotthetas+0.0134middotthetas2 +0.00014middotthetas3 for 120 kHz, where thetas is the negative tilt angle in degrees. The material properties, g ranged between 1.0209 and 1.0396 (mean 1.0288), and mean h for individual, multiple and tilted squid were 1.041, 1.042, and 1.039, respectively. In the model estimation, the estimated TS data from the measured h (1.04) were higher than those of previous studied h (1.007) and were close to maximum TS data that were obtained from live squid. The results suggest that further work is needed to conduct the TS measurement for various sized squid, and to decide suitable acoustical parameter for model estimation. These data can help in making acoustics a quantitative survey tool for biomass estimation of the squids

Overview of the KIOST-HYU Joint Experiment for Acoustic Propagation in Shallow Water Geological Environment

This paper presents an overview of the geological environment investigation and underwater acoustic measurements for the purpose of “Study on the Relationship between the Geological Environment and Acoustic Propagation in Shallow Water”, which are jointly carried out by KIOST (Korea Institute of Ocean Science & Technology) and Hanyang University in the western shallow water off the Taean peninsula in the Yellow Sea in April-May 2013. The experimental site was made up of various sediment types and bedforms due to the strong tidal currents and coastal geomorphological characteristics. The geological characteristics of the study area were intensively investigated using multi-beam echo sounder, sub-bottom profiler, sparker system and grab sampler. Acoustic measurements with a wide range of research topics in a frequency range of 20~16,000 Hz: 1) low frequency sound propagation, 2) mid-frequency bottom loss, 3) spatial coherence analysis of ambient noise, and 4) midfrequency bottom backscattering were performed using lowand mid-frequency sound sources and vertical line array. This paper summarizes the topics that motivated the experiment, methodologies of the acoustic measurements, and acoustic data analysis based on the measured geological characteristics, and describes summary results of the geological, meteorological, and oceanographic conditions found during the experiments.

Ex situ echo sounder target strengths of ice krill Euphausia crystallorophias

Ice krill is the keystone species in the neritic ecosystem in the Southern Ocean, where it replaces the more oceanic Antarctic krill. It is essential to understand the variation of target strength (TS in dB re 1 m2) with the different body size to accurately estimate ice krill stocks. However, there is comparatively little knowledge of the acoustic backscatter of ice krill. The TS of individual, formalin-preserved, tethered ice krill was measured in a freshwater test tank at 38, 120, and 200 kHz with a calibrated split-beam echo sounder system. Mean TS was obtained from 21 individual ice krill with a broad range of body lengths (L: 13–36 mm). The length (L, mm) to wet weight (W; mg) relationship for ice krill was W=0.001218×103 × L3.53 (R2 =0.96). The mean TS-to-length relationship were TS38 kHz =−177.4+57log10 (L), (R2 = 0.86); TS120 kHz = −129.9+31.56log10 (L), (R2 =0.87); and TS200 kHz =−117.6+24.66log10 (L), (R2 =0.84). Empirical estimates of the relationship between the TS and body length of ice krill were established at 38, 120, and 200 kHz and compared with predictions obtained from both the linear regression model of Greene et al. (1991) and the Stochastic Distorted Wave Born Approximation (SDWBA) model. This result might be applied to improve acoustic detection and density estimation of ice krill in the Southern Ocean. Further comparative studies are needed with in situ target strength including various body lengths of ice krill.

Hydroacoustic Survey of Fish Distribution and Aggregation Characteristics in the Yongdam Reservoir, Korea

국내 내수면은 하천의 수위 유지 및 농업용수 공급을 위한 보 부터 수자원의 효율적인 관리 및 발전소 운영을 위한 다목적 댐 에 이르기까지 다양한 규모의 수리 구조물이 축조되어 있다. 그 중 용담호는 2001년 용담댐이 준공되어 생성된 인공호수로 서 해안 지역의 효율적인 수자원으로 사용되고 있다. 2000년 이전 용담호는 얕은 수심 및 빠른 유속의 계류성 하천이었으나, 용담 댐 완공 이후 완만한 유속 및 정체로 인한 오염 물질의 퇴적 현 상으로 생태계 큰 변화가 진행되고 있는 수역이다(Lee et al., 2009). 특히 용담댐 건설 이후 서식지의 수몰로 인한 고유 어종 의 서식지 감소와 블루길과 배스 등 외래종의 유입으로 생태계 교란이 진행되고 있다(Yang et al., 2012). 국내에서 육상 담수역 및 해양에서 어류의 시·공간적인 분포 파악을 위한 조사는 1960년 이후 현재까지 주로 어구어업으로 진행되고 있다(Choi, 1971; Huh, 1986; Cha et al., 2004). 용담 호 또한 2000년 이후 낚시, 투망, 족대, 일각망 등의 어구 조사 를 통해서 어류의 종조성 및 분포 특성에 대한 연구가 진행되고 있다(Lee et al., 2009; Yang et al., 2012). 이러한 전통적인 채집 방법은 정성적인 자료 측면에서 많은 장점이 있지만, 한정된 채 집 면적 및 수층 제한성이라는 측면에서 정량적인 자료 도출에 오차를 만들어 낼 가능성이 높다. 정성 분석이 치우친 어구 채집 방법의 문제점을 극복하기 위한 방법 중 하나인 생물 음향 기법 은 1980년대 초 제시되어 국내에서는 1990년대 이후 주로 해양 생물의 분포 및 자원량 조사에 활발히 적용되고 있다(Kang et al., 2003; Lee et al., 2014). 생물 음향조사는 국외에서 해양 생태계뿐만 아니라 호수 및 저수지와 같은 담수 환경에서 어류 분포의 정량적인 평가와 대 상 어류의 모니터링 및 생태학에 관한 연구에 적용되고 있다 (Godleska and Jelonek, 2006; Knudsen and Larsson, 2009). 생물 음향 기법의 특징은 연구자의 주관을 배제시키고 사전에 설정된 음향 조사 정선으로부터 연속적이며 전 수층에 대한 객 수중음향기법을 이용한 용담호의 어류 분포특성 연구 이형빈·이경훈*·김성훈·김인옥·강돈혁

Target Signal Simulation in Synthetic Underwater Environment for Performance Analysis of Monostatic Active Sonar

Active sonar has been commonly used to detect targets existing in the shallow water. When a signal is transmitted and returned back from a target, it has been distorted by various properties of acoustic channel such as multipath arrivals, scattering from rough sea surface and ocean bottom, and refraction by sound speed structure, which makes target detection difficult. It is therefore necessary to consider these channel properties in the target signal simulation in operational performance system of active sonar. In this paper, a monostatic active sonar system is considered, and the target echo, reverberation, and ambient noise are individually simulated as a function of time, and finally summed to simulate a total received signal. A 3-dimensional highlight model, which can reflect the target features including the shape, position, and azimuthal and elevation angles, has been applied to each multipath pair between source and target to simulate the target echo signal. The results are finally compared to those obtained by the algorithm in which only direct path is considered in target signal simulation.

The diel vertical migration of the sound-scattering layer in the Yellow Sea Bottom Cold Water of the southeastern Yellow sea: focus on its relationship with a temperature structure

Using the hydroacoustic method with a 200 kHz scientific echo sounding system, the diel vertical migration (DVM) of the sound-scattering layer (SSL) in the Yellow Sea Bottom Cold Water (YSBCW) of the southeastern Yellow Sea was studied in April (spring) and August (summer) of 2010 and 2011. For each survey, 13–27 hours of acoustic data were continuously collected at a stationary station. The acoustic volume scattering strength (Sv) data were analyzed with temperature profile data. In the spring of both 2010 and 2011, the SSL clearly showed the vertical migration throughout the entire water column, moving from the surface layer at night to near the bottom during the day. Conductivity, temperature, and depth data indicated that the entire water column was well mixed with low temperature of about 8°C. However, the SSL showed different patterns in the summers of 2010 and 2011. In the summer of 2010 (≈28°C at the surface), the SSL migrated to near the bottom during the day, but there were two SSLs above and below the thermocline at depth of 10–30 m at night. In the summer of 2011 (≈20°C at the surface), the SSL extended throughout the entire water column at night, possibly owing to an abrupt change in sea weather conditions caused by the passage of a Typhoon Muifa over the study area. It was concluded that the DVM patterns in summer in the YSBCW area may be greatly influenced by a strengthened or weakened thermocline.

Laboratory experiment to measure 5-MHz volume backscattering strengths from Red-tide causing microalgae Chattonella antiqua

An acoustic laboratory experiment using 5-MHz signals was conducted to measure the volume backscattering strengths of red-tide causing microalgae, Chattonella antiqua, which is one of the species of harmful algal blooms in the coastal waters of Korea and Japan. The measured backscattering strengths increased with cell abundance, with a slope of approximately 10 dB per decade increase in cell numbers. The density and sound speed ratios of the Chattonella cell to the water medium were estimated via the density gradient centrifugation method and the time-travel difference method, respectively. Finally, the measured backscattering strengths were compared to those predicted by a fluid-sphere scattering model, in which the estimated sound speed and density contrasts were used as input parameters.

Comparison of bacterioplankton communities between before and after inoculation with an algicidal material, Ca-aminoclay, to mitigate Cochlodinium polykrikoides blooms: assessment using microcosm experiments

We investigated the ecological responses with a focus on the diversity of bacterioplankton communities with regard to the effects of artificial clay (Ca-aminoclay) on suppressing harmful algal bloom species (HABs) of Cochlodinium polykrikoides in indoor microcosms. The Ca-aminoclay induced cell lysis in the HABs within a few minutes, but had negligible impact on the non-harmful phytoplankton. However, the findings showed that applying Ca-aminoclay could have negative environmental effects. Specifically, it increased nutrient and dissolved organic carbon concentrations and caused anoxic conditions to emerge. The bacterioplankton abundance increased and the dominant species changed from α-proteobacteria and Bacteroidetes to γ-proteobacteria. It was noted that Phaeobacter caeruleus (α-proteobacteria) was strongly associated with the blooming stage of C. polykrikoides, suggesting that the α-proteobacteria are intimately linked to the development of C. polykrikoides blooms. In contrast, Alteromonas macleodii (γ-proteobacteria) was associated with the termination of HABs and hypertrophic environmental conditions due to the algicidal material. The abundance of heterotrophic flagellates and ciliates increased rapidly with the increase in bacterioplankton. The information obtained in this study should be important when incorporated into our understanding of the interactions between bacterioplankton communities and environmental changes due to algicidal materials.