Tomohito Imaizumi

Measuring the target strength spectra of fish using dolphin-like short broadband sonar signals

Dolphins identify their prey using broadband sonar signals. The broadband spectrum of the target strength (TS) of fish is believed to be a key factor in target discrimination. In this study, the TS spectrum was measured using sonar signals generated by two different dolphin species: finless porpoise and bottlenose dolphin. First, the broadband form functions of a tungsten carbide sphere and a copper sphere were measured in a water tank, and a close agreement between measurements and theoretical values was confirmed. Second, the TS spectra of anesthetized fish from three species were measured in a water tank. Although the results showed characteristics similar to previous measurements, they varied among species, individuals, and tilt angles. Third, the TS spectra of live fish suspended and tethered by nylon monofilament lines were measured at sea. The dolphin-like sonar signals were effective in obtaining the broadband TS spectra of the fish. Cross-correlation processing of the echo from a tungsten carbide sphere showed a further advantage of using the dolphin-like sonar signals: the signal-to-noise ratio increased by more than 10 dB. The variation of TS spectra with fish behavior provides useful information for target identification.

Frequent and prolonged nocturnal occupation of port areas by Yangtze finless porpoises (Neophocaena asiaeorientalis): Forced choice for feeding?

During the Yangtze Freshwater Dolphin Expedition 2012, Yangtze finless porpoises (Neophocaena asiaeorientalis) were acoustically monitored in 9 port areas at night. During 6566 min of nocturnal monitoring, porpoise sonar was detected for 488 min (7.43% of the total time). Of all 81 encounters, the longest echolocation span obtained was 102.9 min, suggesting frequent and prolonged porpoise occupation of the port areas. A combined total of 2091 click trains were recorded, with 129 (6.2%) containing minimum inter-click intervals (ICIs) below 10 ms (termed a buzz). Buzzes with a decrease in ICIs and search and approach phases that resembled feeding echolocation signals accounted for 44.2% (N=52) of all buzzes. Buzzes with an increase in ICIs, suggesting a mirrored prey capture phase, accounted for 20.2% (N=26) and could reflect attempts to locate escaped prey because they were followed by approach-phase feeding buzzes. Anecdotal evidence of porpoises fleeing the proximity of vessels was observed. The recordings indicating clusters of porpoises feeding near the port areas suggest a forced choice for feeding due to the relatively higher prey availability in the port areas compared to other areas in the Yangtze River that are probably overfished.

Analysis of the temporal structure of fish echoes using the dolphin broadband sonar signal

Behavioral experiments indicate that dolphins detect and discriminate prey targets through echolocating broadband sonar signals. The fish echo contains components from multiple reflections, including those from the swim bladder and other organs, and can be used for the identification of fish species and the estimation of fish abundance. In this paper, temporal structures were extracted from fish echoes using the cross-correlation function and the lowpass filter. First, the echo was measured from an anesthetized fish in a water tank. The number, reflector intensity, and echo duration were shown to be dependent on the species, individual, and orientation of the fish. In particular, the echo duration provided useful information on the fish body height and for species identification. Second, the echo was measured from the live fish suspended by nylon monofilament lines in the open sea. It was shown that this duration could be estimated regardless of whether or not the fish were moving.

Receiving Distance of the Stationary Ultrasonic Biotelemetry Receiver in the Temperate Ocean

The investigation by the ultrasonic biotelemetry currently performed in Asia is shallow ocean space where water temperature is high. Furthermore, there is also much underwater noise, such as Pistol shrimp (Alpheus lobidens), in this ocean space. Therefore, differing from the receiving range that a maker recommends is assumed. Then, the maximum detection distance of VR2 was measured in the tank. From that result, the simulation that shows the detection range of this receiver quantitatively was performed. Moreover, the receiving range was measured in real ocean space, and change by oceanic condition was verified. Consequently, receiving distance brought a result shorter than a makers simulation. Also, the receiving distance was sharply changed by wind and weather.

Classification of three tuna species in enclosures by using target strength spectra measured by a broadband split-beam system

The selective capture of tunas; bigeye tuna (Thunnus obesus), skip jack tuna (Katsuwonus pelamis) and yellow fin tuna ((Thunnus albacares) is important for Japanese seine net fisheries. Classification of the species composition by using acoustic information before catching them will significantly contribute for the selective catch. Target strength spectra (TSSP) of each species were measured by a broadband split beam system. Each tuna species was separately kept in a enclosure, which sized 8 × 8 m square and approximately 5 m in depth. This system was able to measure not only TSSP but also a swimming track of individual in 3D. The differences of TSSP among three species were confirmed. For example, the target strength value of begeye whose tilt angle was 0 degree about 10 dB higher than skip jack tuna one. Swimbladder sizes were measured for each species by soft X-ray because skip jack tuna are physoclist species, and others are physostome species. There were differences of the swimbladder shape between b...

A broad band dolphin mimetic sonar—inspiration and modification from the nature

Broadband techniques are getting popular for underwater sensing methods because of its high spatial resolution and target discrimination abilities. We have been developing a broadband split beam system to locate and identify each species in the ocean. Our system initially learned from dolphins and then modified architecture appropriately. For example, biological sonar sound was effective for the short range sensing to locate individual target, chirp sound provided clear target image for the long range up to 200 m. Multi angle scanning of a target was proved to be essential for the species discrimination in our system. It was just like a finless porpoises rolled their body possibly to enlarge sensing volume and change beam incident angles to a target. Unlike dolphins, the split beam system was not able to change transmitting beam directions. The sound incident angle to a fish was calculated using the body movement vector and the position of a target fish in the beam. Reconstruction of target strength spect...

Automatic detection and localization of croaker’s fish calls using beamforming

Many kinds of fish, including croaker, produce species-specific low-frequency sounds associated with courtship and spawning. A recording system was used to monitor underwater fish call sounds. We have proposed the method to detect croaker calls from data measured by a single hydrophone. However, it was difficult to detect the desired calls at a high detection rate because of a low signal-to-noise ratio. We proposed the method using beamforming to improve the detection rate. At first step, fish calls are detected from data measured on one hydrophone using the previous method, which detects calls automatically using the acoustic features, that is, duration and periodicity. At second step, additional calls are detected by beamforming the four-channel data. At third step, detected calls are localized by using the time differences of arrivals. It was clarified that the detection rate using the proposed beamforming method was higher than that under the previous method with a single data channel. In addition, it...

Size estimation of walleye pollack (Theragra chalcogramma) by using a broadband split-beam system

Recently, broadband quantitative echo sounders have been developed. These systems can measure single echo traces from individual fish with high range resolution. They can be used for the accurate size distribution by measuring the target strength. On the other hand, walleye pollack is one of the important species for stuck management in Japan. They form a school at juvenile stage. We compared the captured fish size using bottom trawl net with the acoustically estimated values. We conducted the survey in July 2015 in east side of off Hokkaido, Japan, using the fisheries research vessel Kaiyo-maru No. 5 (495 tons), which is equipped with narrowband echo sounder transducers (EK-60, Kongsberg, 38, 70, 120, and 200 kHz). The broadband split-beam transducer (FSS-SBX, Furuno) was deployed at the ship sideboard. All of the clock of the system were synchronized for comparison of data. More single echo traces could be measured using the broadband system than those measured by the narrowband system. Fork lengths wer...

Passive acoustic monitoring of fishes and a crustacean using a towed hydrophone in Tateyama bay

Passive acoustic monitoring is useful to observe presence of marine organisms such as marine mammals, fishes and crustaceans. We aimed to estimate the distribution of fishes and crustaceans using a towed hydrophone. A range-wide passive acoustic monitoring was conducted in Tateyama bay in Japan using R/V Takamaru (61t) on July, September and November in 2015 of day and night. The towed hydrophone (Towed Aquafeeler, AquaSound Inc., Japan) was used for the observation. The vessel cruised 5 knot on 5 lines to cover the focal survey area (N 35°020’—N 35°02’ and E 139°46—E 139°51). Recorded data with frequency from 100Hz to 20 kHz were analyzed. Recorded fish sounds have dominant frequency between 200 Hz and 1500 Hz and duration between 0.1 s and 2.1 s. Crustacean sound shows dominant frequency as 3 kHz–20 kHz with the duration of 10 ms—20 ms. Using these features, number of fish sounds was counted automatically using custom-made MATLAB program. Large number of the sounds were recorded in the nighttime than in...

Localization of individual call among croakers’ chorus using a stereo recording system

Passive acoustic monitoring (PAM) is widely used in cetacean census in the ocean. In these years, PAM has been applied for various species out of mammals such as fish and crustaceans. For detection, classification and density estimation in PAM, isolation of each biological sound and reliable detection is essentially needed. Choruses of fish sounds were recorded by a stereo sound monitoring system on the seafloor in off Choshi, Chiba prefecture, Japan in May 2013. Numbers of croaker sounds were recorded and the simple separation in time domain was not possible in chorus situation. A blind source separation method based on independent component analysis was applied to separate mixed sounds in order to obtain individual sounds and to localize them. The directions of sound arrivals could be estimated by using the cross correlation function of the separated signals and the estimated directions were plotted. Using proposed methods, individual call sounds could be localized and inter-pulse intervals could be acc...