Kazuo Amakasu

Multilayer biological structure and mixing in the upper water column of Lake Biwa during summer 2008

We carried out a 24-h station experiment at Lake Biwa (Japan) to measure mixing events and concurrent biological signals using a free-fall microstructure profiler (TurboMAP-L), conventional hydrographic measurement device (F-probe), and the Tracker acoustic profiling system (TAPS). A clearly defined three-layer physical system was observed. Two layers were actively mixed: the surface-mixed layer and the subsurface-mixed layer. Both winds and night-time convection create the surface-mixed layer, and vertical shear due to a counterclockwise gyre maintains turbulence in the subsurface mixing layer. A strongly stratified layer between these two mixing layers is almost turbulence free, so no material flux is expected. A local oxygen maximum layer, a local oxygen minimum layer, and layers of increased chlorophyll and zooplankton abundance are all located in this strongly stratified layer. The data show the intricate influence of physical processes on the structure of biological systems and their combined influence on biogeochemical and trophic transfers in aquatic systems.

Exact Simulation of Fish School Echoes and its Applications

A numerical simulation method was developed to investigate fish school echoes. The simulation incorporates strict expressions for fish target strength model, directivity of a transducer, and echo waveform in order to realize high fidelity. The fish school echoes are constructed by synthesizing single echoes for each ping. Two application examples are shown. One is the confirmation of linearity between the distribution density offish and the simulated volume backscattering strength; the result showed a very good linear relationship. The other example is fish school feature extraction by the wavelet analysis. In the results, no apparent one-to-one correspondence between the given distribution pattern and the results of the wavelet analysis ,but a characteristic wavelet pattern is observed.

Quantifying Sea Bottom Surface Backscattering Strength and Identifying Bottom Fish Habitat by Quantitative Echo Sounder

A quantitative echo sounder (QES) was used to measure the bottom surface backscattering strength (SS) extensively off Southern Jawa Island, Indonesia. The average SS by the bottom echo integration method were plotted as a map with the depth contour to give a synoptic view of the topography. On the other hand, the results from the ring surface scattering (RSS) model gave us more detailed SS values in ping base. A simultaneous display of the fish volume backscattering strength and the bottom SS was useful to observe bottom fish habitats remotely. The bottom material estimated by the measured SS showed that the fish schools were abundant in the sand bottom in this area.

Measurement of sea bottom surface backscattering strength by quantitative echo sounder

Measurement theory and method of the bottom surface backscattering strength (SS) using a quantitative echo sounder (QES) are discussed and applied in the ocean near Java, Indonesia. The frequencies of the QES were 38, 70, and 120 kHz. The measurements of bottom echoes and sampling of bottom material by a dredge were done simultaneously. Bottom characterization was based on analysis of particle size distribution for bottom samples taken during the survey. The SS value increases with the increase of the mean diameter of the bottom particles. The SS decreased with increasing frequency. The effectiveness of QES for measurement of the SS along with observation of the depth topography were demonstrated.

Strong turbulent mixing induced by internal bores interacting with internal tide‐driven vertically sheared flow

We observed the formation of an internal bore interacting with the vertically sheared flow generated during the previous phase of the internal tide, which resulted in strong turbulent mixing. The rate of turbulent kinetic energy dissipation reached on the order of 10 W kg 1 during the event. Numerical simulations reproduced the observed interaction of internal bores with the sheared flow and verified the hypothesized breaking and mixing mechanism. The numerical results indicated that the Iribarren number, or the ratio of the topographic slope to the internal wave slope, plays a major role in the mixing intensity and types of internal bores. It was found that waves with low Iribarren numbers lead to bores that interact with vertically sheared flows induced by the previous phase of the internal tide and are more likely to produce strong wave breaking and mixing.

Design and concept of a Biointeractive Autonomous Underwater Vehicle “BA-1”

To keep the precious protein source, it is thus extremely important to conserve the biodiversity of the ocean and at the same time to make use of its limited space as much as possible without doing any environmental harm. With this broad objective in mind, Tokyo University of Marine Science and Technology (TUMSAT) and Mitsui Engineering & Shipbuilding Co., Ltd. have started a research and development project, “Pen-free Offshore Aquaculture System using Biointeractive Autonomous Underwater Vehicles.” This project is part of a more comprehensive project, “Marine Biotechnology Innovation,” which has been funded by the Japanese government since 2007. In this paper, we propose a concept of a biointeractive AUV that monitors and takes care of schools of fish just like a sheep dog in a ranch. In a future plan of this system, multiple biointeractive AUVs monitor the environment spatially and accurately, feed the fish, monitor the growth of the fish, guide them, and report the data through satellite to the land while charging their batteries by docking with buoys. A first model of the biointeractive AUV “BA-1” has launched in early 2009. The design of the biointeractive AUV “BA-1” is described in this paper. The basic test result of the interaction system for fish is also described briefly.

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.

Robust analysis of single echo trace obtained by split beam sounder

The theory and method of the echo trace analysis of single echoes (ETA), which simultaneously analyze the shape and level of single echoes, are almost established. But, the method has not been extensively used. The major reasons are high‐quality echoes with a high signal‐to‐noise ratio (SNR) necessary for the analysis are some times difficult to obtain; the error introduced by transducer motion is large; and conditions for the theory sometimes could not be fulfilled. Therefore, we apply the ETA to the split beam method, which is most advantageous for the ETA, and make the method robust by introducing the following: careful echo selection considering SNR, correction of the effect of transducer motion, smoothing of the trace by regression analyses, introduction of indexes checking reliability, etc. The useful results of our ETA are the track, the swimming speed and orientation, the target strength as a function of tilt angle, and the estimated size of individual fish. The robust ETA was applied to echoes of...

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...

Target strength and behavior of swimming juvenile southern bluefin tuna in a cage estimated by echo‐trace analysis

Echo‐trace analysis (ETA) was applied to echoes from juvenile southern bluefin tuna (Thunnus maccoyii, SBT) swimming in a cage. The experiments were conducted offshore Esperance in Western Australia, using a split‐beam echosounder operating at 70 kHz. The dimensions of a cylindrical cage were 7 m in diameter, 20 m in depth, and 8.5 cm mesh size of a net. A transducer was set at the center of a cage. The samples of SBT were caught by trolling near the study site. The age classes were 1 or 2 years old that were ranging 45 to 82 cm in fork length. Only one SBT was put into a cage at a time in order to obtain single echoes from one fish. As the swimbladder of juvenile SBT was not gas‐filled in this stage, its target strength (TS) was much lower than TS of a same‐size fish with gas‐filled swimbladder. Because of low TS values, there were some difficulties to obtain high quality echo‐traces continuously. So, preselections of high signal‐to‐noise and noninterference echoes from the echogram were needed before ap...