Tohru Mukai

Estimation of Fish School Volume Using Omnidirectional Multi-Beam Sonar: Scanning Modes and Algorithms

Previous studies have resulted in a quantitative omnidirectional sonar (FSV30 research version, Furuno) using 64 multi-beams and a 24 kHz center operating frequency. The practical applications of this sonar are currently being studied with regard to fisheries resource assessment. This paper presents one part of this research, i.e., methods for estimating fish school volume using different sonar scanning modes. To determine its suitability for various purposes, we conducted a simulation to measure scatter aggregation; the simulation was validates by measuring the volume of a balloon filled with seawater. A practical survey was also simulated for spherical scatter aggregation with a 20 m diameter, and its volume was examined using different scanning modes. Our results indicate that a semicircular cruise mode is more suitable than a circular mode for measuring fish school volume, and that using an anchored instrument scanning mode provides greater accuracy than a cruising scanning mode.

Observation of Marine Animals Using Underwater Acoustic Camera

An underwater acoustic camera enclosed in a pressure-resistant case was constructed to observe underwater marine animals. This enabled the measurement of the size, shape, and behavior of living marine animals in the detection range up to 240 cm. The transducer array of the acoustic camera was driven by 3.5 MHz ultrasonic signals, and B-mode acoustic images were obtained. Observations were conducted for captive animals in a water tank and for natural animals in a field. The captive animals, including fish, squid and jellyfish, were observed, and a three-dimensional internal structure of animals was reconstructed using multiple acoustical images. The most important contributors of acoustic scattering were the swimbladder and vertebra of bladdered fish, and the liver and reproductive organs of invertebrate animals. In a field experiment, the shape, size, and swimming behavior of wild animals were observed. The possibilities and limitations of the underwater acoustic camera for fishery applications were discussed.

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.

Verification of mean volume backscattering strength obtained from acoustic Doppler current profiler by using sound scattering layer

Acoustic Doppler current profilers (ADCPs) have recently been used to estimate the dynamic characteristics and biomass of sound scattering layers (SSLs) or swimming speed of fish schools and to analyze SSL spatial distribution or various behavior patterns. This paper shows that it is necessary to verify mean volume backscattering strength (MVBS, dB) values acquired from each beam for quantitative analysis of the spatial distribution or the biomass estimates of such specific targets as SSL or a fish school when using an ADCP. In this study, the SSL was selected to be a homogeneous density layer over a large area and two methods were used to verify the MVBS values from each beam of the ADCP. First, a mutual comparison among four beams was conducted after calculating MVBS from the measured echo intensity. Second, the MVBS values were verified using comparison between the calculated MVBS from the 153.6 kHz ADCP and MVBS from three frequencies of a well-calibrated scientific echosounder. Moreover, the dominant scatterers (euphausiids) were collected by a framed midwater trawl. From these samples, biological data were used to identify the different frequency characteristics between two systems, using a distorted-wave Born approximation (DWBA) theoretical backscattering model in order to assess the averaged target strength and target strength TS differences for the three frequencies.

Measurements of swimming angles, density, and sound speed of the krill euphausia pacifica for target strength estimation

In recent years, acoustic technology has been used extensively to estimate krill abundance. Acoustic assessment is required to obtain precise estimates of krill target strength (TS). However, predictions of target strength from theoretical scattering models are often influenced by the swimming angle, density, and sound speed contrasts between krill and seawater. Density and sound speed contrasts are known to show annual cycles. In this study, swimming angles and seasonal variations of the specific density and sound speed contrasts of Euphausia pacifica are presented. Biological sampling was carried out during twilight, when the sound-scattering layer migrates up to the surface. Sound speed measurements were performed on a vessel 2 hours after net sampling, using a T-shaped velocimeter with two transducers mounted at the ends of a horizontal tube. The swimming angles and specific densities of E. pacifica were measured at a laboratory within 48 hours of net sampling. The specific densities of the krill were measured using a series of glycerol solutions of variable densities. Moreover, the total lipid content, lipid composition, and fatty acid composition of the krill were analyzed. The swimming angles of the krill varied from 30 to 50 degrees in low illumination condition (0.1 lx). The density and sound speed contrasts changed with the season. These variations depended on the lipid content in the krill body. In particular, the density of the krill changed with the wax ester content. The variations of TS estimated by the distorted-wave Born approximation (DWBA) method will be discussed

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

Estimation of the variation in target-strength of fish under pressure condition

This paper reports a newly developed measurement system of fish swimbladder shape under pressure condition and theoretical modeling of bluegill (Lepomis macrochirus) under pressure condition. A pressure tank with a pressure device is used in combination with the existing soft X-ray machine. Observed swimbladder images taken from the lateral aspect are digitized to obtain the fish morphological parameters. Based on these parameters, target strength is calculated using deformed cylinder scattering model. Swimbladder images indicate that the swimbladder is compressed from the ventral side to the backbone of the fish. This fact supports that the swimbladder length is less sensitive to the pressure and the swimbladder height is sensitive to the pressure. Maximum TS in the calculated TS pattern at 59 m depth becomes 5.3 dB lower than one at the atmosphere condition at the frequency of 38 kHz. Averaged TS, assuming normal distribution of tilt angle with mean of -5.0 degrees (head down) and standard deviation of 15.0 degrees, becomes 4.8 dB lower.

Development of broadband transducer using multilayer piezoelectric elements

A Langevin-type broadband transducer was developed using a multilayer piezoelectric actuator (hereafter referred to as a multilayer piezoelectric element). The frequency range of the broadband transducer was aimed to cover the frequency range from 38 to 120 kHz typically used for sensing fish and zooplankton. The 10-mm long multilayer piezoelectric elements whose resonance frequency is 138 kHz were chosen for covering the high-frequency region. Acrylic disks of 13-mm thick as the front and rear masses were used for realizing the resonance at the low-frequency region. Also the diameter of 11 cm was determined for realizing suitable beamwidths 5–20°. Thirty elements were set in circular-shaped arrays on the disk and the two acrylic disks were fastened with four bolts. The peaks of the transmitting sensitivity were measured at 46 and 135 kHz, and the values were 168.3 and 165.3 dB re 1 μPa/V, respectively. Although ripples were observed, useful band around 46 and 135 kHz were 26–65 kHz (39-kHz bandwidth) and 110–142 kHz (32-kHz bandwidth), respectively. The transmitting sensitivity function from 70 to 105 kHz was flat, but the sensitivity of 158.0 dB re 1 μPa/V was lower than those of the above frequency range. The peaks of the receiving sensitivity were measured at 28 and 135 kHz, and the values were −188.5 and −173.6 dB re 1 μPa/V, respectively. The receiving sensitivity function was not flatter than the transmitting sensitivity function and the sensitivity at 70 kHz of −207.9 dB re 1 V/μPa was considerably low. The measured beam patterns at 38, 70, and 120 kHz were almost the same as that of the 11-cm diameter circular piston source. The broadband acoustic system was constructed using commercially available equipments and the measurement of the target strength spectra of a 38.1-mm diameter tungsten carbide sphere using a 20–150 kHz linear-frequency-modulated signal. The measured target strength spectra of the 38.1-mm sphere were in good agreement with the predictions except for the region of the low signal-to-noise ratio.

Measurement of fish school abundances in shallow sea using omnidirectional multibeam sonar

Previous studies had been constructed in the use of commercial scanning sonar to estimate the abundance of pelagic fish school. However, a relatively narrow dynamic range and lack of linear signal processing limited the application of sonar. In this research, quantitative omnidirectional multi‐beam sonar (FSV30 research version, 24 kHz, Furuno) was used to investigate a fish school in a shallow area. The raw volume backscattering strength signal (RSV, output signal with 20 log r TVG and coefficient of multiple echo) of the fish school echoes were recorded and then its biomass was confirmed using purse seine capture. In postprocessing, the fish school echoes were extracted from the sonar echogram using the image processing method (polygon and a low‐pass filter). The relationships between a school’s biomass, its echo area, the maximum, and the average RSV of each ping were considered for various species. Results showed a positive correlation between the sonar‐measured school area and school biomass, b...

Target strength pattern measurement of krill (Euphausiapacifica) and measuring limits in a small tank

Euphausia pacifica is an extremely important species for Japanfs northeast coastal fisheries and a key species for ecosystems in the northwest Pacific Ocean. Target strength (TS) of krill information have been required for acoustic surveys; they constitute a scale factor that is used to convert acoustic estimates of abundance. A precise TS pattern measurement system was constructed in a small tank. Target strength patterns of a target are measurable in the angle range of 60 deg at 1‐deg steps. First, TS values of the thinnest fishing line with 53‐mm diameter were measured as it was planned for use in tethering krill. TS measurement limits were also confirmed by measuring echo signals and noise signals. Maximum TS of the fishing line was −95 dB and the measurement limit was −98 dB at the frequency of 200 kHz with 10 dB of signal‐to‐noise ratio (SNR). Second, we confirmed the validity of the target suspension method. The thinnest fishing line was penetrated into the krill body to tether it between two verti...