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Fisheries Acoustics with Addition Mccartney, B Rapp. P.-v. Réun. Cons. int. Explor. Mer, 184: 25-33. 1984. Fish and other organisms as acoustic targets Lars Midttun Institute of Marine Research P.O. Box 1870 5011 Bergen-Nordnes Norway cause of the complexity in shape and acoustic properties Introduction of their component bones, tissue, and bladder, theoreti­ Knowledge of fish and other organisms as acoustic ob­ cal considerations are of importance both for the design jects is essential both for their identification or classifica­ of experimental work and for the interpretation of re­ tion as well as their sizing and abundance estimation. sults. Recently a new text book has been published by During nearly 50 years of acoustics in fisheries, a consid­ Clay and Medwin (1977) dealing with principles of acous­ erable amount of activity has been concentrated on the tic reflection from targets including processes affecting study of target strength of fish and to some extent also of the scattering characteristics of marine life. other marine organisms. The present paper intends to In some approaches fish or their swimbladders have review our state of knowledge on the subject. Since there been treated as simple geometric shapes (Haslett, 1970). is as yet no acoustic theory which can fully describe the Yudanov and Kalikhman (1981) and Mitson (Anon., process of scattering from complex targets such as fish 1978) have also reported some results along this line. So and other marine organisms, several practical ap­ far the method has given only a first approximation to the proaches and techniques have been applied to obtain solution. wanted information on target strength for applications. David Cushing (1973), himself a pioneer in the field, has written a historic review on the detection of fish, with a comprehensive list of references covering progress up to around 1970. Since then a number of conferences have Measurements of target strength been held to consider the state of development of our knowledge on target-strength matters. The philosophy behind the measurements can be re­ The situation by the end of 1972 is reflected in the garded as twofold: report from the last Bergen Symposium (Margetts, 1) To obtain knowledge of scattering from fish with 1977). respect to the acoustic wavelength, fish species, size, The FAO/ACMRR Working Party on Fish Target orientation, swimbladder condition, and generally with Strength met in Aberdeen in 1977 with the view of respect to all variables affecting the target strength. This assessing the state of and gaps in, current knowledge on type of measurement requires carefully controlled condi­ the subject and to specifying future research needs and tions where the variables can also be observed or re­ priorities and reported accordingly (Anon., 1978). corded together with the reflected signal. The results can In June 1979 another meeting was held, this time in then be used in models and applied to field conditions at Cambridge, Massachusetts, USA. Among the topics sea. were acoustic scattering characteristics of single and ag­ 2) For practical reasons, in situ observations on wild gregated fish, equipment calibration, and verification of fish have the advantage of being a direct way to measure results. A critical review from this conference seems to fish in their natural habitat. Such measurements can be me a bit too pessimistic to reflect the true state of the art used for calibration purposes, but also for verification of (Suomala, 1981). results obtained otherwise. Some disadvantages and as­ sociated problems are related to the transducer direc­ tivity and lack of control over the target being measured. Fish as acoustic targets Combined with underwater photography or fishing the method can be considerably improved. Theoretical considerations A great number of experimental exercises have been Although no acoustic theory has been fully formulated to performed to measure the target strengths of fish. They describe the scattering process from fish in the sea, be­ can be grouped into three methods. 25 Measurements on tethered fish and have been reported by Dunn (1978), Forbes et al. Such work has been conducted, among other places, in (1980), Edwards and Armstrong (1981), and MacLen- Japan by Hashimoto and Maniwa (1956) and Shibata nan (1981). Some of these British experiments also in­ (1971), in Norway by Midttun and Hoff (1962). Nakken clude studies of adaptation effects on target strength. and Olsen (1977), and Dalen et al. (1976), in the UK by The general drawback with this type of experiment on Harden Jones and Pearce (1958), Cushing et al. (1963), live caged fish is the lack of control, particularly over the and Haslett (1970), in the USA by Smith (1954), Diercks tilt-angle distribution of the caged fish. This may explain and Goldsberry (1970), Volberg (1963), and Love (1969, the reason why the sometimes large variances in ob­ 1971, 1977), and in the USSR by Yudanov, Gaukov, and served strength have seldom been explained. However, Shatoba (Yudanov, 1977) and Shishkova (1964). Mea­ there is one experiment which, in this respect, may claim surements are also reported by Johannesson and Losse to be properly conducted since a television camera could (1977). These series of observations have usually been observe and record the tilt-angle distribution (Foote, made with stunned or dead fish over a wide range of 1983). Foote’s experiment also consisted of target- frequencies, species, and sizes of fish. strength measurements of tethered, anesthetized indi­ The experiments were different, both in their perfor­ viduals. The TS functions thus measured, together with mance and their data analysis. A more detailed discus­ behaviour information, constituted a basis for calculat­ sion will be made later in this paper. However, it can ing the expected echo from the encaged fish. The direct already be concluded that target strengths of fish vary echo measurements from the encaged fish were in excel­ with size, orientation, species or group of species, and lent agreement with the calculated values. Among other frequency of sound. The validity of the results has been things, this experiment concluded that observations on questioned since they are derived from dead or stunned anesthetized fish are valid and representative for live, fish (Anon., 1978). free-swimming fish. Measurements on live fish in controlled systems In situ measurements Buerkle and Sreedharan (1981) measured the target There are certain requirements involved in using this strength of live cod supported in the centre of the acous­ method of observing fish in their natural habitats. First, tic beam and rotated in varying combinations of pitch to ensure single-fish signals, the pulse lengths of echoes and roll. McCartney and Stubbs (1971) measured six are normally tested and second, the transducer direc­ gadoid species with four frequencies from 4 to 20 kHz. tivity must be removed from the received signals. Craig The fish could swim within a bag 1 m long and 40 cm in and Forbes (1969) have given one method which should diameter. A hydrophone measured both the incident and be well known to everyone working in this field. Cushing the reflected signals of the same transmitted pulse so that (1968) regarded all the echoes as observed at a mean calibration errors could be avoided. On the other hand, angle from the transducer axis. Midttun and Nakken the fish could pitch by an unknown angle. The authors (1971) observed fish traces, i.e., successive echoes from conclude “there is no guarantee that the absolute max­ the same fish, and regarded the maximum echo in a trace imum value in the pitch plane has been recorded, es­ as being observed on the transverse axis of the beam. pecially at high L/X”. Because of variations in the scat­ They then calculated target strengths by reducing the tered signals, the maximum value of eight pulses was maximum echoes according to the known transverse di­ used. rectivity pattern of the transducer. Ehrenberg (1972) in­ Several experiments have been performed with caged, troduced another signal-processing technique, but but otherwise free-swimming fish. The method was in­ Ehrenberg and Lytle (1977) conclude that in many cases troduced by Johannesson and Losse (1977) in some FAO the reduction method does not work well, and suggest a projects for calibration purposes. Groups of fish of dual-beam technique to be applied for in situ measure­ known number were used. Volume backscattering ment of target strength. Some results are published by strengths were measured and the average target strength Traynor and Nelson (1981). Robinson (1976), in a pre­ of individuals calculated in addition to fulfilling the main liminary experiment with a deep-towed transducer, mea­ purpose of integrator calibration. Results from target- sured the target strength of blue whiting. Blue whiting strength measurements by this method are also reported have also been measured by Monstad and Midttun by Aglen et al. (1981) and by Johannesson and Vilchez (1981). (1981) who sometimes improved the method with a TV In situ observations have also been used as a direct camera monitoring the fish distribution from beneath the method for integrator calibration. Dispersed con­ cage. centrations are simultaneously integrated and counted Goddard and Welsby (1975), using a 2 x 2 m and 1 m on a paper recorder. According to this direct method, deep cage, observed individuals and groups of fish in described by Nakken and Dommasnes (1975), inac­ dorsal and 22!/: degrees aspect with TV-monitoring from curacies involved in conventional calibrations of the beneath the cage. Similar experiments on caged groups echo-sounder system are circumvented. of fish have been conducted in Scotland for several years 26 another model for averaging with respect to different Consideration of results orientation distributions and beamshapes of the trans­ The validity of the results obtained on stunned, freshly ducer. Footes’s model gives higher values compared with killed, or anesthetized fish can be trusted provided the those of Nakken and Olsen, especially for medium-sized experiments are properly conducted.
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