The target strengths of fish

J. Sound Vib. (1969) 9 (2), 181-191

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TARGET

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OF

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R. W. G. HASLETT

Kelvin Hughes, a Division of Smiths Industries Limited, New North Road, Hainault, Essex, England (Received 30 October 1968) Knowledge of the acoustic target strength of a fish (or of a shoal of fish) is required to enable the performance of present and future sonar equipments to be determined for fish targets. Also, it is hoped that measurement of the strengths and characters of the echoes received from fish will give a reliable guide to the size of the fish and, possibly later, an indication of at least gross differences between various types of fish. As a result, a variety of workers have been interested in the acoustic scattering properties of fish, initially at the frequencies of existing equipments, but later over a much wider range of frequencies, pursuing a more fundamental approach to understanding the formation of fish echoes and their relation to the physical structure of the fish. Over the last 12 years, a programme of research in this field has been directed by the author. Recently, the White Fish Authority has supported the design of apparatus operating at 30, 97, 170, 278 and 547 kHz and measurements have been made on cod fish at these frequencies in a large water tank. This field is also related to a number of other subjects such as the design of new sonar equipments, the identification, counting and behaviour of fish, and telemetry from the trawl. Over 50 references are given. Further work remains to be done. 1. INTRODUCTION The acoustic target strength of a fish is a measure of the signal intensity of the echo received f r o m it (in dB) compared with, for example, that which would be received from a perfectly reflecting sphere of radius two yards, if it were substituted for the fish. Thus, if all the parameters of a given sonar equipment are measured, the strength of the echo to be expected from a fish of a known target strength at a certain range can be calculated for ideal conditions. Alternatively, if the acoustic target strengths of fish of various sizes at different operating frequencies are known, then the magnitude of the echo from an unknown fish gives some knowledge about the fish itself. Various workers [1, 2, 4, 5, 7, 15, 18] have found that reliable measurements to within 1 or 2 dB can be made at sea when the acoustic conditions are good, particularly in vertical propagation. The well-known sonar equation can then be used to allow for transmitted acoustic power, the attenuation of sound in the sea water in the course of the propagation of the pulse to the fish and back, and the spreading of the transmitting and receiving beams, each according to an inverse square law. The target strength of the fish can then be calculated from the measured echo. Following on some early values of fish target strengths (or related parameters) obtained in Japan [5] and in the U.K. [7, 8], interest spread to the investigation of the manner in which the echo from a fish was formed [12] and its dependence on fish size and frequency [10, 11]. t Presented at the meeting of the British Acoustical Society on "Sonar as a research and commercial tool", Weymouth, 16 May 1968. 181

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This field now has a considerable literature and a selection of some 50 references is attached. 2. METHODS OF MEASUREMENT There are several ways in which the back-scattering of acoustic waves from a fish can be measured, for example: (i) at sea; (ii) in a large water tank in the laboratory; (iii) in a scalemodel in a small water tank in the laboratory. One difficulty of working at sea is that the fish and the apparatus on the ship move with respect to one another and this sets a limit to the degree of accuracy which can be achieved. Another is that the fish type or size is often unknown. These embarrassments are overcome when measurements are made on a fish mounted on a support in a laboratory tank. Another variation is that different types of fish can be used in the experiments: (i) live fish; (ii) dead fish; (iii) scale-model fish. In the case of live fish, movement upsets the readings, but although dead fish can be mounted in a fixed position, an artificial swimbladder [12] may need to be inserted into the fish to maintain the correct shape of this structure. The swimbladder is normally gas-filled and lies underneath the vertebral column. Artificial swimbladders made of various kinds of foamed plastic are often used [54]. Apart from convenience, the advantages of the scale-model method are that recently-killed scale-model fish such as sticklebacks and minnows can be used and that the distance between fish and transducer can be much larger (in terms of fish length) than in measurements on full-size fish in a tank. (It may be noted that the target strength of a fish changes when the transducer is near the fish.) 3. SOME TYPICAL POLAR DIAGRAMS The polar diagram of back-scattering of sound from a fish is rather complex (as seen in Figure 1 obtained from a scale model [23]) and depends on the length of the fish in relation to the wavelength in water at the operating frequency (2). The upper diagrams are for a fish of length 42, the middle ones for about 162 and the lower figures for 302. The number of lobes in the diagram increases with fish length. On the left, the fish is rotated about a vertical axis (i.e. yaw plane) while on the right, it is turned about its longitudinal axis (i.e. roll plane). The amount of sound reflected by the fish is seen to depend on the direction (or "aspect") in which it is viewed. This complicated structure of lobes exists in three dimensions around the fish. In general, the amounts of sound reflected by the fish are low in head and tail aspects, high near broadside aspect and intermediate in dorsal aspect. Diagrams like these, showing the fine detail of the variation of target strength, have been obtained from scale-model observations [21, 22, 26] and are confirmed by measurements on full-size fish under carefully-controlled conditions [12, 25, 20]. Other measurements have been made at sea by various workers using fish suspended on lines [7, 14, 16, 28, 31, 32]. 4. PHYSICAL STRUCTURE OF A FISH When the physical structure of a fish is examined (Figure 2), the swimbladder, the vertebral column and the fleshy body are found to give significant contributions to the echo. The dimensions of these structures as compared with the overall length of the fish have been determined [24] and together with measurements on the acoustic reflectivities of fish bone and tissue [22], enable the target strengths of the various parts to be calculated.

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Figure 1. Acoustic back-scattering polar diagrams for scale-model fish (sticklebacks) [23]. Planes of observation: (a), (b) and (c)-yaw plane. (d), (e) and (f)-roll plane. Frequencies: (a) and (d)--360 kHz. (b), (c), (e) and (f)-1.48 MHz. L = actual length of fish, L' = equivalent full-size length of fish at 30 kHz and A = wavelength in water. (a) L = 4-4A (0.72 in.) L' = 8.74 in. (b) L = 14.6A (0.57 in.) L' = 28.7 in. (c) L = 30.4A (1.20 in.) L' = 59.9 in. (d) L = 4.4A (0-72 in.) L' = 8.74 in. (e) L = 18.0A (0.71 in.) L' = 35.4 in. (f) L = 30.4A (1.20 in.) L' = 59.9 in. 5. T H E UNIVERSAL G R A P H All these results can be plotted on a universal graph (Figure 3) [45] in which the acoustic back-scattering cross-section of the fish (in dorsal aspect) divided by the square of its length, is plotted against the ratio o f its length to the wavelength. At sea, due to the m o v e m e n t o f the fish and of the vessel, the reading obtained is averaged over a few degrees in aspect and the fine structure showing m a x i m a and m i n i m a in the polar

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diagrams, tends to be lost. Thus, although all the readings are, generally, in agreement, there are wide variations between the values of target strength for a given length of fish which are partly due to the different manners of measurement.

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Figure 2. A scale drawing of an average fish (whiting) showing the parts which contribute to its acoustic echo. (All dimensions are given in terms of the overall fish length L.) [24] v = vertebral column, s = swimbladder, g = gut. This figure also shows the graphs obtained for various theoretical values [23, 37]. For example, in the geometrical region where the fish is large compared with the wavelength, A is for the swim bladder, E for the backbone and J, G, K and H for various approximations to the body of flesh. On the other hand, in the Rayleigh scattering region where the fish is small compared with the wavelength, B, C and D are different approximations to the swimbladder, F is for the backbone and Q and R are for the body of flesh. From this universal diagram [45], the target strengths of fish over a range of sizes can be calculated for different frequencies. It may be of interest to note that this diagram applies to other underwater bodies of similar shape, for example a submarine [54] which is found to lie at the upper right-hand corner and in agreement with the graph based on measurements on fish between 0.4 and 1.2 in. long. 6. RESULTS AT LOW FREQUENCIES Much interest has been aroused by work (particularly at the National Institute of Oceanography [13, 41]) on a possible resonance of the swimbladder at very low frequencies, for example, a cod fish at 250 Hz. McCartney has read a paper on this subject [56]. One of the difficulties of working at a low frequency is the size of the transducer array required to give a sufficiently narrow beam and establish that the echoes are from fish situated under the head-line of the trawl. 7. FISH SHOALS Another matter which has received some attention is the target strength of a shoal of fish [31, 47, 39, 40, 41, 42, 43] and its relation to the density of the shoal and the size offish. This

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Figure 3. Acoustic back-scattering cross-sections a of fish of various lengths L (in dorsal aspect). Results from several observers are plotted on the universal diagram: [45] u, Hashimoto; A, Harden-Jones and Pearce; e, Cushing; ©, Hashimoto and Maniwa; x + *, Haslett; O, McCartney. These results are compared with the following graphs representing calculated or observed values of g for various parts of a fish. Geometrical region: A swimbladder E core of backbone G equivalent ellipsoidal body of flesh in fresh water H as for G but in sea water J body of flesh in fresh water K as for J but in sea water. Rayleigh scattering region: B spherical bubble of same volume as swimbladder C swimbladder if it were rigid (with its length in geometrical region) D swimbladder if it were rigid (and completely in Rayleigh scattering region) F core of backbone (with its length in geometrical region) Q equivalent eUipsoidal body in fresh water R as for Q but in salt water. perhaps applies more to pelagic fishing rather than sea-bed trawling where single fish are more frequently met. 8. WORK AT KELVIN HUGHES The author's C o m p a n y has been investigating the acoustic properties of fish since 1956 [12, 22, 23, 24, 26, 28, 20, 37, 38, 48] and reports have been written to the White Fish Authority on this subject and various related topics such as the acoustical methods of fish identification [34], methods of observing fish behaviour [35], forward-searching techniques [36], analysis

186

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HASLETT

of fish echo-traces [51], automatic fish counting [52] and acoustic telemetry from the trawl [53]. It is particularly important to estimate the sizes of the observed fish (in addition to their position relative to the net) since in accordance with the regulations for conservation, the meshes are made large enough to allow fish shorter than 12 in. to pass through. Recently, the W.F.A. sponsored the development of apparatus to measure fish target strengths at five frequencies: 30, 97, 170, 278 and 547 kHz in the laboratory water tank (20ft long and 11 ft deep) at Hainault [54]. The object was to take a large number of readings on cod fish between 12 and 45 in. long and so extend the knowledge of the acoustic properties of these fish under carefully-controlled conditions, especially at higher frequencies. This is relevant to a number of projects, for example, forward-searching for fish from a trawler. o*

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Figure 4. Target strengths of a fish taken at 30° intervals in the yaw plane (0°) and at tilt angles of 15° and 30° to this plane [54]. Fish length: 45 in.; frequency: 97 kHz; pulse length: 160/~sec. The development of such a versatile piece of apparatus, including a satisfactory support for the fish, the design of five sets of transducers and absolute calibration, involved a considerable amount of work and it was therefore disappointing that the time and budget remaining for the subsequent measurements restricted the number of readings which could be obtained. A few representative graphs are seen in Figures 4 to 8. Most of the measurements were taken in the yaw plane as in Figure 4 (and to a lesser extent in the roll plane) at intervals of exactly 30 °, starting from head (or dorsal aspect), respectively. Also planes tilted at 15° and 30 ° (upwards) from the yaw plane were explored in a similar way since these were particularly applicable to forward searching. Separate diagrams were obtained at each frequency for fish of each size and, where possible, for pulse lengths between 30 and 500/zsec. Altogether, over 100 polar diagrams were produced in a short time, including a few taken at 5 ° intervals as in Figure 5 which is for a fish of length 29"5 in. at 97 kHz. Near dorsal and ventral aspects, the interval was reduced to 1°. One or two plots were made entirely at 1o intervals, for example, that in Figure 6 for a 22 in. fish in the yaw plane at 278 kHz. The smaller the angular interval, the more timeconsuming the process of measurement becomes, but the greater the degree of detail of the lobes. At a frequency of 547 kHz, there are about 400 lobes in the polar diagram for rotation of the fish of length 45 in. in the yaw plane. (In this case, the fish is about 4002t long.)

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likely to be met at sea but the maximum range available in a water tank is obviously restricted. A similar effect occurs in the waves incident on the fish due to the nearness of the transducer. This causes the incident wavefront to be curved whereas the far-field target strength assumes that plane waves are used.

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(Average taken over three tilt angles, 12 aspects and five frequencies) [54]. Slope of best straight line = 2-4. Most of the readings were taken at 30 ° intervals and represent a kind of average (between peak and minimum values of the lobes) which is probably nearer to observations made at sea. It is useful to note the variation of target strength as a power law of fish length. Over the range of length from 14 to 45 in., the target strength in broadside aspect was found to increase -IC

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THE TARGET STRENGTHS OF FISH

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The frequency responses of four fish of different sizes in broadside aspect are seen in Figure 8 and the average increase of target strength is between 0 and 2 dB per decade in frequency. (This is much less than previous prediction.) The lengths of the fish covered by this work were from 6~ (at 30 kHz) to 416A (at 547 kHz), the latter entering the geometrical region more deeply than hitherto.

9. CONCLUSIONS REGARDING THE FUTURE It may be helpful to summarize the present position and suggest avenues that are worth following in the future. Although there has been a considerable amount of work in this field, it has mainly concerned the yaw plane and much still remains to be done. Further readings are needed in the roll and pitch planes which are comparatively unexplored. This would undoubtedly be assisted by using a mechanized apparatus (as did Volberg [32]) in which the fish is rotated at constant speed in synchronism with a pen recorder. At the same time a comparison between readings taken on large fish in the far-field region in open water and those made in the near-field region in the laboratory tank, would establish the corrections, if any, to be applied to the latter. Also measurements on large fish at much higher frequencies (e.g. 1-2 MHz) would be useful to determine the performance of highresolution systems such as the Underwater Acoustic Camera [46, 49] (which can give an acuity of 0.1 degree). The observations could be extended to shoals of fish where the average density and the fish type and size is known. Another matter requiring investigation is the change in the target strength caused by the removal of the swimbladder from fish of various sizes at different frequencies. Other related subjects are the measurement of the noise levels found on typical fishing vessels, against which the fish echoes must be detected, also of the scattering strengths (at various frequencies) of the sea beds over which fishing operations are conducted. It may be possible to find a difference in the acoustic properties between, on the one hand, a sea bed alone, and on the other, a sea bed with fish in close contact. This would be important to sea-bed trawling, especially for flat fish.

ACKNOWLEDGMENTS The author is grateful to the Directors of Kelvin Hughes for permission to publish this paper. The work under reference 54 was conducted largely by Mr. W. Burgess and acknowledgments are due to the White Fish Authority for the release of some of the results (including Figures 4 to 8). Figures 1 and 3 are reproduced from the British Journal o f Applied Physics and Figure 2 from the Journal du Conseil lnternational pour L'Exploration de la Mer.

REFERENCES 1. O. SUND 1941 Rapp. P-v. RJun. Cons. perm. int. Explor. Mer-Ann. Biol. 1, 62. The fat and small herring on the coast of Norway. 2. A. yon BnANDTand J. SCH~,R~ 1950 Protok. FischTech. 2, 4. Zur quantitativen Auswertung der Echolot-Beobachtungen. 3. W. C. HO~SON 1950 Fishery Invest. Lond. 2, 17, 4. Echo sounding and the pelagic fisheries. 4. D. H. CUSHINGand I. D. PdCHARDSON1952 J. Cons. perm. int. Explor. Mer 18 (1), 45. Echo sounding experiments on fish. 5. T. HASHIMOTO1953 Rept. Fishg Boat Lab. No. 1.

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6. A. B. WOOD and D. A. SENIOR 1954 Admiralty Research Lab. Rep. N.1/ABW/DAS Echofishing--an enquiry into the possibilities of further research and development into acoustic techniques. 7. D. H. CUSmNO and I. D. RICrtARDSON1955 Fishery Invest. Lond. 2, 18, 4. Echo sounding experiments on fish. 8. D . H . CUSmNG 1955 J. Cons.perm. int. Explor. Mer 20 (3), 266. Some echo-sounding experiments on fish. 9. R. E. CRAIG 1955 Rapp. P-v. Rdun. Cons. perm. int. Explor. Mer 139, App. 2, The use of the echo-sounder in fish-location. 10. T. HASHIMOTOand Y. MANIWA 1956 Rep. Fishg BoatLab., 8, 113. Study of reflection loss of ultrasonic wave on fish body by millimetre wave. 11. D. H. CUSHING and I. D. RICHARDSON1956 Fishery Invest. Lond. 2, 20, 1. A triple frequency echo sounder. 12. F. R. HARDEN-JONESand G. PEARCE 1958 J. exp. BioL 35, 437. Acoustic reflexion experiments with perch to determine the proportion of the echo returned by the swimbladder. 13. M.J. TucKER and A. R. STtmBS 1958 National Institute of Oceanography Rep. A.12. The reflexion of sound by fish. 14. I. D. RICHARDSON,D. H. CUSHING,F. R. HARDEN-JONES,R. J. H. BEVERTONand R. W. BLACKER 1959 Fishery Invest. Lond. 22, 9. Echo sounding experiments in the Barents Sea. 15. M.J. TUCKERand A. R. STUBBS1959 NationalInstitute of Oceanography Rep. A. 13. The absolute calibration of echo-sounders used for the detection of mid-water organisms. 16. Y. MANIWA 1959 Rep. Fishg Boat Lab., 13, 81. Study on ultrasonic waves reflection loss at fish body. 17. D. H. CUSHING 1959 Modern Fishing Gear of the Worm London: Fishing News (Books). 525, The quantitative use of the echo sounder for fish surveys. 18. R. W. G. HASLETT1961 J. Br. Instn Radio Engrs 22 (1), 33. The quantitative evaluation of echo sounder signals from fish. 19. R.B. MITSONand R. J. WOOD 1961 J. Cons.perm. int. Explor. Mer 26, 281. An automatic method of counting fish echoes. 20. J . E . L . SOTHCOTT1961-62 Kelvin Hughes, unpublished work. Measurement of the target strengths of fish at 15 kc/s and 30 kc/s. 21. R. W. G. HASLETT1962 Proc. Phys. Soc. 79, 3, 509, 542. The back-scattering of acoustic waves in water by an obstacle. I. Design of a scale model and investigation of its validity. 22. R. W. G. HASLETT1962 Proe. phys. Soc. 79, 3, 509, 559. The back-scattering of acoustic waves in water by an obstacle. II. Determination of the reflectivities of solids using small specimens. 23. R. W. G. HASLETT1962 Br. J. appl. Phys. 13, 349. Determination of the acoustic back-scattering patterns and cross sections of fish. 24. R . W . G . HASLETT1962,/. Cons.perm. int. Explor.Mer 27 (3), 261.Measurement of the dimensions of fish to facilitate calculations of echo-strength in acoustic fish detection. 25. L. MIDTTUN and I. HOFF 1962 FiskDir., Skr. 13, 3. Measurements of the reflection of sound by fish. 26. R . W . G . HASLETT1962 Br. J. appL Phys. 13, 611. Determination of the acoustic scatter patterns and cross sections of fish models and ellipsoids. 27. D. H. CUSmNG 1963 The Uses of Echo Sounding for Fishermen London: Her Majesty's Stationery Office. 28. D. H. CUSHINO, F. R. HARDEN-JONES,R. B. MIX'SON,G. H. ELLIS and G. PEARCE 1963 d. Br. Instn Radio Engrs 25 (4), 299. Measurements of the target strengths of fish. 29. G. FREYTAG 1963 Int. Fishg Gear Conf. 2 Paper No. 83. Bio-acoustical detection of fish-possibilities and future aspects. 30. T. HASmMOTOand Y. MANIWA 1963 Int. Fishg Gear Conf. 2 Paper No. 19. Frequency analysis of marine sounds. 31. E. V. SmSHKOVA1963 Int. Fishg Gear Conf. 2 Paper No. 74. Study of acoustical characteristics of fish. 32. H. W. VOLBERG1963 Proceedings 14th Pacific Tuna Conf., Lake Arrowhead, California. Acoustic properties of fish. 33. M. NISHIMARA1963 Modern Fishing Gear of the World--2 London: Fishing News (Books). Page 383, Echo-detection of tuna. 34. R. W. G. HASLETT1964 Kelvin Hughes Rep. to W.F.A. Acoustical methods offish identification. 35. R . W . G . HASLETTand J. E. L. SOTHCOTT1964 Kelvin Hughes Rep. to W.F.A. Methods of observing fish behaviour.

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