Citation 長崎大学水産学部研究報告, v.29, pp.1-82; Issue Date Right.

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1 NAOSITE: Nagasaki University's Ac Title Study on Details of Ultrasonic Refl Author(s) Shibata, Keishi Citation 長崎大学水産学部研究報告, v.29, pp.1-82; 1970 Issue Date URL Right This document is downloaded

2 1 Study on Details of Ultrasonic Reflection from Individual Fish * Keishi SHIBATA Contents Introduction 1 Chapter 1. History of investigation 4 Chapter 2. Acoustic characteristics of fish flesh 8 Chapter 3. Construction of fish body 19 Chapter 4. Methods of measurement 31 Chapter 5. Back-scattering pattern of fish body 41 Chapter 6. Reflection loss of fish 57 Chapter 7. Field experiment on tunas 67 Summery and conclusions 74 References 79 Introduction Echo-sounder was first used only for sounding in fishing, around Since then it showed a progress so as to be used for the detection of fish concentration under the name of ultrasonic fish detector. The use of this equipment has now made it possible to detect not only fish concentration but also large individual fish at the depth of about 500 meters. Moreover, along with the remarkable development of military science in resent years, echosounder has been progressed to usual sonar, sector scanning sonar, frequencymodulated sonar and acoustic camera. The typical study concerning echo-sounder directly related to the fishery was started in England by RICHARDSON6) and CuSHING1,2,8) and in Japan by HASHIMOTO and other workers of the Fishing Boat Laboratory of Fishery Agency3-5,36-38) since On the other hand, echo survey of fishing ground has been proceeded earnestly by fishermen, where some purse sein fleet are operating at the fishing ground of mackerels in the western waters of Kyushu, not by the * Thesis submitted for the degree of Doctor of Fisheries at the Hokkaido University 1969., Sept.

K The nal air bladder cavity Accordingly X ray because the photograph shows it expanding seems clarify Reflection transformation that the bladder of the bladder measurement to Ultrasonic SHIBATA the

28 K The nal air bladder cavity Accordingly X ray because the photograph shows it expanding seems clarify Reflection transformation that the bladder of the bladder measurement to Ultrasonic SHIBATA the air bladder of is from at the given was live 27 Fish a done outside certain by target of body means abdomi pressure of the fish 導饗講 i霧1 懸欝じ評ll幽凝響灘藝欝驚議錨轟灘灘 Fig 7 Rentgen 1 2 photographs above of total length below Acoustic of cross section Funa air bladder C rea12 of carassius cm cm pa T subspp weight in two 31 8 grm 6 1 planes grm O 91311gg9 2S il 1t3 O 25 2 Funa Vab Z 7 z Lg of soft

75 Bull. Fac. Fish., Nagasaki Univ., No. 29 (1970) were considered to be duly available for practical use. The 9th and 10th columns in Table 17 shows the results of measurement on 6 target fishes with artificial air-bladder, and the 11th column shows the number of ping pong balls inserted, into body cavity of target fish. The reflection loss in the 10th column were obtained by Eq. (7.7). The reflection loss of fish body with artificial air-bladder in the abdominal cavity was not much difference from that of fish body without it. It seems that the contribution of the air-bladder to the acoustic reflection of larger fish should be clarified experimentally in near future. Entered in Fig. 33 are the observed values in this experiment and the reports on the reflection loss (target strength) at 30 khz by some workers which can be illustrated by line and curve. (1) MANIWA4) ; 1962, marine teleostean fishes in the adjacent water of Japan. (2) HASLETT31) ; 1965, sticlebacks, and guppies, converting the values by the scale model method into that at 30 khz. (3) CUSHING22) ; 1963, mainly cods. (4) measurement of Funa in a anechoic tank. (5) dorsal maximum signal of Funa. It seems that those results are in fair agreement and especially the results of this experiment are in considerable agreement to the results of HASLETT and MANIWA. The line (5) may coincide with HASLETT' s peak value. In this experiment, it failed to clarify the fluctuation of the reflection loss repeating the maxima and minima according to the body length as reported by HASLETT. Summary and conclusions 1. Summary Much knowledge was acquired by carrying out strict measurements on the acoustic back-scattering signal from an individual fish in the anechoic tank using Funa, measuring 2.8 cm to 17 cm in total length as the target fish. The characteristics of reflection loss of fish may be divided into two components, which are the shape and dimension of fish body and the boundary condition (acoustic impedance) between the medium and the fish body. Measurement was made at two frequencies, i. e. 50 khz and 200 khz, for the characteristics of acoustic reflection of 198 fishes, namely, Funa, C. carassius subspp. in the anechoic tanks which were provided at the Faculty of Fisheries, Hokkaido University and Nagasaki University. All of those

76 K. SHIBATA : Ultrasonic Reflection from Fish 75 target fishes underwent soft Roentgen photograph after being anesthesized in order to clarify the inner structure of fish body. Determination was made on the density of fish flesh and fish body without air-bladder, and on the sound velocity spreading through the flesh of various kinds of fish. And then the total reflectivity of fish flesh, amplitude coefficient of reflection at a plane interface in water, which contributes the majority of acoustic reflection of fish body, was determined dy calculation from the density and sound velocity of fish flesh on the experimental results. The target fish was submerged in a fresh water across the acoustic axis at a range of 100 cm from the receiving and transmitting transducers which were set horizontally in the anechoic tank. The echo strength of target fish was measured under various conditions; live fish under anesthesia, after removing air-bladder, further removing other viscera and other, rotated about three different axis of fish body; on the roll plane, pitch plane and yaw plane. The outline of the experimental results was as follows : 1) The dimension of target fish, Funa, were very similar to those of marine teleostean fishes in the coastal waters of Japan. Hence the use of Funa as experimental target fish was deemed approximate. 2) The long axis of the air-bladders of Funa forms a dip-angle of about 10 to the swimming axis. The dip-angle aftects to the echo strength and back-scattering pattern, especially, on the dorsoventral direction, the dorsal maximum signal was observed in an angle of 10 to the tail (in direction of turning the head downwards on the pitch plane), as reported by MIDTTUM and HOFF, and on the roll plane the lateral maximum signal was observed in the direction of 5 to 10 to the ventral side and to the tail on the yaw plane. 3) The density of fish flesh was within the range of 1.04 to 1.09 and even within the same species it greatly varied individually according to the size and region of the fish body even in the same fish. The density of air-bladder-removed body was to It was likely that the fresh water fish and some of marine teleost fishes might keep the neutral buoyancy by adjusting the size of their air-bladder to the condition of surrounding water. Therefore, the seasonal and individual change of its density were due to the change of the airbladder volume. 4) The sound velocity spreading through the fish flesh was observed 1,500 to 1,600 meters per second. 5) At a range of frequencies used for the commercial echo-sounder, the acoustic absorption in the fish flesh might be very little, but it might

77 76 Bull. Fac. Fish., Nagasaki Univ., No. 29 (1970) affect at a high frequency or in larger fish. This tendency was arose in those fishes with harder scales or dermis. 6) The total reflectivity of fish flesh determined from sound velociy and density was to 0.062, and majority in the range of to And also the value of demersal fish showed smaller than pelagic fish. 7) The target fish supported on the acoustic axis in the anechoic tank and back-scattering polar diagram of fish was obtained by measuring the echo amplitude on all drections, at the rotated angle of every 2.5, 5 or 10 in three measuring planes. Generally the polar diagrams showed the complicated fluctuation which repeated the maxima and msnima, and the number of lobes increased with the ratio of the fish size and acoustic wave length and also the difference between maxima and minima indecreased as the above ratio. Application of the Rayleigh region for the acoustic reflection from a fish seems to be because the acoustic polar diagrams of a amall individual fishes up to the acoustic wave length in fish size, were observed to be non-directional on the roll plane measurement. 8) The characteristics of back-scattering strength from a fish in a range of to in total length, can be approximated to that of a short cylinder. Empirical equations approximating target strength of the dorsal and lateral maximum signal of fish are : i) dorsal maximum : Ts = -Lp = 28.0 log10 L -8.0 log ii) lateral maximum : Ts = -Lp = 25.7 log10 L -5.7 logio for of Funa at 50 khz an 200 khz, and of tunas at 28 khz, where Ts is the target strength in db, Lp is the reflction loss, L is the total length of fish in cm and is the acoustic wave length in cm. 9) The echo strength from an individual fish is basically the vector sum of from each parts which forms the fish body (HASLETT29)), but it is very complicated because of the phase difference between them. For example, the dorsal maximum signal may be given at the same phase detween them. The estimating target strength of fish in given by following eauations : where, back-scattering cross section of fish body without airbladder and vertebral centa, on the dorsal maximum signal,

78 K. SHIBATA : Ultrasonic Reflection from Fish 77 TR is the total reflectivity of fish body, TRf is the value of the fish flesh, and TRb is the value of wet bone. And also B, SL and H is the body width, standard length and head length, respectively. : back-scattering cross section of the air-bladder of Funa, : back-scattering cross section of the vertebral centra of Funa, The observed target strength and calculated one from the above equations often coincided, and full coincidence was seen at 50 khz. The highest agreement at 50 khz were found within ± 3 db on the maximum dorsal and maximum lateral signal, but the observed value of small fish was larger than the calculated one. While at 200 khz, the number of lobes in the polar diagrams were much more than that at 50 khz in the same sized fish, so that, the reading taken at 200 khz, greatly varied with an error of a few degrees on the suspending posture of a target fish. Therefore, such larger difference between the calculated and the observed target strength might be ascribed to the minute difference of suspending system of target fish. If the dimensions, weight and density of fish body are known, the estimation of target strength of fish up to length of fish and up to 200 khz, might be possible to reach the considerable higher accuracy which were good enough for practival use. The influence of various fins on the echo strength from a fish, is estimated to be negligible. Because of 2 percent on the echo amplitude of a live fish, on the acoustic determination, the body length of fish must be considered not as the total length but as the standard length excluding the caudal fin. The overall mean echo amplitude from a fish around 360 rotating by a series of measurements on each target fish in all measuring postures and conditions was given and the ratio of the overall average of live fish was obtained. The air-bladder rejection reduced the average echo amplitude by 43 percent on the roll plane measure-

79 78 Bull. Fac. Fish., Nagasaki Univ., No. 29 (1970) ment on 140 live Funa, and further rejection of viscera by 22 percent. The body width and visceral contents varies according to feeding and spawning conditions and concequently cause more changes of echo amplitude even in the same individual fish. 12) experiments on the target strength were carried out. The 25 target fishes were all frozen tunas without air-bladder, viscera and gills. After defrosting fish body in sea water for 12 hours, the target fish was suspended in sea water in a swimming posture from the side of the research boat. As the result of these experiment the observed target strength coincided well with the calculated one. However, the observed target strength of tuna with artifieial air-bladderr inserted into digestive cavity, showed a slight lower value than the calculated one. 2. Further probrems on the reflection loss of fish. 1) One of the desires of fishermen to the echo-sounder in recent years is "foward detection of fish school". This is known by the fact that Sonar has become popular among fishing boats in Japan. In this thesis, one of the three dimensions, body length, body height and body width was used for the rotating axis at a time, but such a measurement may not be sufficient in the sonar technique applied for fisheries, because on the fish detection by using Sonar, the echo from an individual fish may be received in the oblique direction with a certain dip-angle against the fish body, and it may show some difference from the experimental results in this report. Therefore, it is required in the future studies to carry out the acoustic measurement of fish body on the yaw plane and pitch planes with a certain dipangle against the transmitting axis of Sonar transducer. 2) The next interst for fishermen is to identify the fish species and to estimate the quantity of catch by using the acoustic devices. Currently the identification of fish species has been made empirically by comparing the echograms with the hauls of the fishing operation. However, an acoustic appratus for direct identification must be developped. Attempt has already been made to identify the species of fish and to estimate the body length from its swimming behavior which is given by the FM Sonar. The estimation of fish stocks which is directly related to the prediction of fish catch is the most important point in the fisheries but no sufficient results have been obtained. The acoustic devices for estimating fish stocks are also being developing in Europe. However it has not been clarified vet on the reflection loss of concentrated fish school. The author

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