Rapp. P.-v. Reun. Cons. int. Explor. Mer, 184: 7-18.

Fisheries acoustics: a review of general principles

D. N. MacLennan and S. T. Forbes Marine Laboratory. RO. Box 101, Aberdeen AB9 8DB, Scotland

The physical principles of echo sounding are discussed with particular reference to system design and the accuracy of the echo-integration method of biomass estimation. The velocity, absorption, and scattering of underwater sound are reviewed, especially the scattering properties of multiple targets. Many types of sonar have been applied in fisheries investigations. The various techniques are described and the factors which limit performance are critically examined.

Ce document traite les principes de sondage acoustique, particulièrement la précision d'écho-intégration pour évaluer une biomasse. Après avoir révisé les phénomènes de propagation et d’absorbtion du son dans l’eau, nous nous sommes attachés au pro­ blème des échos multiples. On applique de nombreuses types de sonar dans les études de la pêche. Nous décrivons les différentes techniques afin de faire une analyse critique de leurs limites.

Introduction Principles of echo sounding Echo sounders were first used to detect fish shoals some The echo sounder is a sonar which transmits in the verti­ 50 years ago, and since 1945 there has been rapid de­ cal direction. It was the first acoustical instrument to be velopment of acoustical techniques for the fisheries. The used in fisheries applications and remains an essential impact on the fishing industry has been enormous, es­ tool for fisherman and scientist alike. Moreover, the pecially on the pelagic fisheries. Sonars, echo sounders, echo sounder is the basis of the echo-integration method and netsounders are now used routinely and skilfully by of fish stock estimation. It is therefore important to have fishermen to eliminate some of the chance element in a clear understanding of what can and cannot be done their business. with this basic machine. In fisheries research, too, it has long been recognized The design of echo sounders has advanced over the that acoustics can provide information which is not ob­ years through more powerful transmitters, more stable tainable by other means. The acoustical method of fish receivers, better displays, and so forth. However, the stock estimation is the most important of the research information provided by the echo sounder depends upon applications. It is not difficult to state the problem - how fundamental principles which have not changed. The much fish of species X is in area Y at a given time? - but machine provides signals which contain amplitude and providing the answer is not so easy. The interpretation of time information. The conventional approach then ob­ acoustic data in terms of biomass is often difficult and tains the range of the target from the formula R = Vi ct, there is a continuing debate about the validity of stock where, c, is the sound velocity and, t, is the time delay estimates. Acoustical methods have also led to a better between the transmission pulse and the target echo. The understanding of fish behaviour, how fishing gears work, treatment of the amplitude information is more compli­ and the distribution of fish in the sea. cated. In order to derive a quantity which describes the The literature of underwater acoustics is extensive. target per se, it is necessary to correct the received echo The elementary principles are covered in many text for the effect of wave spreading, absorption loss, and the books, for example Urick (1975) or Clay and Medwin uncertain bearing of the target within the transmitted (1977), and the basic theory and practice of fisheries beam. The last of these corrections can only be done acoustics have been described by Forbes and Nakken through a statistical interpretation of many echoes based (1972) and Burczynski (1979). Fish-tracking applications on assumptions about the target distribution, but the have been reviewed recently by Urquhart and Hawkins basic echo sounder can provide no more than estimates (1982), and here we shall concentrate on acoustical meth­ of one coordinate (the range) and one scalar property of ods of fish detection and abundance estimation. the target, for example the target-strength or volume-

7 scattering coefficient. It is true of course that the display years has led to much improvement, in particular the of signals over a period of time, on such as a chart advent of cheap microprocessors which have made it recorder, may allow further deductions to be made about practical to model the exact form of the theoretical TVG the targets, but this is merely collation of data which does function (Hoare, 1978; Robinson, 1981). The limiting not represent an extension of the fundamental measure­ factor now is error in the TVG function itself, rather than ment capability of the system. error in the implementation, and this aspect is discussed later.

System design Frequency response The optimum choice of design parameters - frequency, pulse length, and so forth - is largely a matter of making The echo sounder may be conceived as a linear system in the best selection in the face of several competing re­ which the output signal is formed from the convolution of quirements. The following is a brief statement of the the input pulse spectrum and the frequency response main factors which are relevant to the choice of design function of the system which includes the contribution of parameters. The ability to detect a target depends ul­ the transducer and the target. The term “linear” in this timately upon the signal-to-noise ratio (SNR). The context implies that the output is the sum of contribu­ greater the transmitted power, the higher the SNR ex­ tions from each frequency component of the input signal, cept for targets close to the transducer where reverbera­ and in the case of a continuous input spectrum, the tion dominates the noise, since the reverberation level output may be described by an integral (sum) equation. also increases with the transmission power. The sampling The usual response function is that of a bandpass filter. volume, that within which targets are detected, increases In principle, the bandwidth of this filter should be with the beam width, but so does the reverberation level. matched to the input pulse spectrum in order to maxi­ Larger transducers transmit more power at greater cost, mize the SNR. It is often the case that the effect of and for a given size of transducer there is an upper limit to bandwidth is ignored in the analysis of echo-sounder the transmission power determined by the sound pres­ data, when the assumption is made that a negligible sure at which cavitation occurs. The effective range de­ proportion of the input pulse energy is blocked by the creases as the frequency goes up mainly on account of the bandpass filter. However, the matching condition which absorption loss which is roughly proportional to the maximizes the SNR implies that some proportion of the square of the frequency. On the other hand, high-fre­ input pulse energy will be blocked. It follows that in work quency systems are better able to discriminate between which demands a high degree of precision, such as echo close targets since the bandwidth, which determined the integration, the effect of the frequency response function range resolution, increases in proportion to the centre on the received signal energy should not be ignored. frequency. Low-frequency systems require large and ex­ The linear system model of echo integration was first pensive transducers, since the beam pattern depends considered by Stevenson (1974) whose theory has been upon the ratio of the transducer dimensions to the wave­ reviewed by Swingler and Hampton (1981). However, length. The larger the transducer, the smaller the beam- Stevenson’s model did not include a frequency-depen- width at a given frequency. dent response function for the target. The target strength and the acoustic cross section are normally defined in terms of the response to a single-frequency (CW) trans­ mission. In the case of a pulsed transmission, when the A ccuracy energy incident upon the target is spread over a fre­ When an echo sounder is used for quantitative estima­ quency spectrum, the normal definition will still apply if tions the accuracy depends upon the stability of amplifier the acoustic cross section is independent of frequency, gains, the transducer efficiency, and the correction for but not otherwise (Craig, 1981). spreading and absorption losses. The overall perfor­ Consider the various signals in a linear echo-sounder mance may be checked by calibration which is the subject system as shown in Figure 1. The output voltage, v0(t), is of another review (Blue, 1984). Here we shall confine related to the input voltage, v;(t), through the transfer ourselves to the factors which cause the performance to functions S(ct>), F(a>), and H(co). S and H are taken to drift and the consequent errors in the information de­ include the contribution of the transducer. These func­ rived from signal amplitude measurements. The correc­ tions are defined by the following equations which apply tion for spreading and absorption losses is normally done to CW transmission at frequency to: by applying time-varied gain (TVG) to the receiving amplifier. Until fairly recently, this was done by means of V |(t) = V exp(/iot) (1) analogue circuits which were temperature sensitive and P i(t) = (S/R)exp(-/coR/c) v,(t) (2) which, moreover could not in general represent the re­ P o(t) = (F/R)exp(—(ojR/c) p;(t) (3) quired TVG function to better than 1 dB or so. However, the rapid development of digital techniques in recent v0(t) = H p0(t). (4) Transmitter Receiver It is not immediately obvious how o should be defined in the general case. There are alternatives, but here we shall adopt the definition proposed by Foote (1982):

H ( ^ > ° u tp u t> o = 4n f |g SHF|.2 da»/f |g SH|2 dco. (11) ^ v(t) Jj

7K-1 Transducer This leads to a gain expression analogous to Equation lp(t) (6):

T-WJ G e = (o/4jcR4) J |g SHpda)/ J |g|; du. ( 12)

The spectrum of a CW signal is a delta function. If this PM function is substituted for g(w), Equations (11) and (12) Target at reduce to Equations (5) and (6) respectively. Thus, the F(u) range R general theory outlined in Equations (7—12) includes the conventional CW theory as a special case. Note that if Figure 1. Linear system model of an echo sounder. |SH|2 and |g|2 are maximum at the same frequency, which is true of a well-tuned echo sounder, then the integral ratio in Equation (12) is less than the maximum |SH|2. The acoustic cross section is defined as: This represents the blocking effect of the bandpass filter as discussed above. a(co) = 4jt|F(co)|2. (5)

So the power gain of the complete system may be written Transducers as: Two kinds of transducer are commonly found in echo (6) sounders and sonars, in which the conversion from elec­ GP = |Vo|W = ö|SH|2/(4jiR4). trical to acoustical energy (and vice versa) is performed To extend this analysis to the general case of a finite by magnetostriction and électrostriction respectively. pulsed transmission, the above equations are rewritten in The magnetostrictive type has a relatively poor energy the form of integrals over the pulse spectrum. If the conversion efficiency, 25—40 % , and it cannot be used at normalized spectrum is defined as: frequencies much above 100 kHz. The electrostrictive type is more commonly described as ‘ceramic’ since this is the active material in such transducers. The latter are g(tt>) =J" (V|/2jiV)exp(-w)t) dt. (7) more efficient, generally around 75 %, and they are — oo suitable for use at all frequencies of practical interest. An important feature of ceramic transducers is that the en­ Then the input and output signals are: ergy conservation efficiency is less variable than that of the magnetostrictive type. Thus the ceramic transducer is better suited to applications which demand precise mea­ ViW-vJ g exp(iCL)t) dto (8) surements of the echo amplitude. — oo Fish targets are normally observed at ranges great

+ 00 enough to be in the ‘far field’ of the transducer, where the transmitted wavefront is nearly spherical and the energy v0(t) = (V/R2) J g SHF exp[i(u(t-2R/c)] do. (9) loss through spreading is inversely proportional to the square of the range. Close to the transducer, in the ‘near field’, the distribution of acoustic energy in range and The power gain is not a useful description of the general bearing is more complicated. If there is any choice in the system. Instead we use the energy gain which is: matter, fish targets should always be observed in the far field, to avoid the complications which make the inter­ Ge = J |v0|2 dt/J |Vi|: dt pretation of near-field echo data rather difficult. There is — 00 — 00 a gradual change from near- to far-field conditions as the range goes up, and the boundary between the two is

+ 00 + 00 about a2/X distant from the transducer, where, X, is the = J |g SHF]2 dco/(R4J |g|2 do). (10) wavelength and, a, is the largest dimension of the trans-

9 ducer face. Since the beamwidth depends upon the ratio volume is estimated, and the appropriate TVG function X/a, it follows that narrow beam systems have a relatively is now 40 Log R + 2aR. This function makes the ex­ large near-field region in proportion to the transducer pected signal from an individual target independent of size. range. In order to resolve the echo from one fish, the We conclude this section with a few remarks on the target must be separated from others by a minimum non-linear or parametric transmitter (Westervelt, 1963). increment of range. This minimum resolvable distance is This works by transmitting two or more frequencies si­ 'AC (x + Ô), where, x, is the pulse length and, Ô, is multaneously from one transducer. These primary fre­ proportional to the rise time of a single target echo, ô is quencies are generally in the MHz range and they are normally less than x and it is inversely proportional to the attenuated rapidly by absorption. However, the non­ bandwidth. The best resolution of an echo counter is linear elasticity of water causes an interaction between achieved with the shortest pulse length and the largest the primary waves such that secondary waves at the bandwidth (which implies the highest frequency) com­ difference frequencies are generated. In effect, the trans­ patible with the required range and signal-to-noise ratio. ducer size is augmented by the water volume which con­ Another limitation is the increase in the effective sam­ tains the low-frequency sources, and the technique offers pling volume with the individual target strength. This the possibility of obtaining narrow beams at low frequen­ difficulty may be overcome by computing the distribu­ cies from a transducer of modest size. Moreover, the tion of target strengths from the distribution of echo- difference frequency beam has no side lobes, and wide amplitude measurements, as first pointed out by Raitt band signals may be transmitted within a beamwidth that (1948). A well-known technique for recovering the tar- is substantially independent of the frequency. We shall get-strength distribution is that of Craig and Forbes return to the last-mentioned feature in the later discus­ (1969) which has been improved by Robinson (1978). The sion of constant beamwidth sonar. These advantages statistical problems in treating the signals from a single­ appear most attractive against the limitations of con­ beam transducer are much reduced by the dual-beam ventional sound generation methods, but the parametric technique which is discussed later. transmitter is relatively inefficient. Much of the transmit­ The echo-counting method is only useful in practice ted energy is lost in the absorption of the primary fre­ for the study of fish which behave independently, thus quencies, and the secondary wave source level is corre­ excluding the shoaling species, but a few successful ap­ spondingly low. The efficiency problem is the main plications have been reported (Yamanaka et al., 1977; reason why there has been little interest, at least in recent Lindem, 1981). We also note that echo counting is ap­ years, in using the parametric transmitter for fisheries plied in the measurement of in situ target strengths, to work. calibrate echo-integration systems.

Echo integration and counting Here we compare two important applications of the echo Sound propagation in the sea sounder, echo integration and echo counting, in which Sea water is an imperfect medium for the transmission of measurements of the echo signals are used to derive sound. Acoustic waves are scattered by marine organ­ quantitative information about the targets. Both meth­ isms, gas bubbles, and other inhomogeneities. Acoustic ods depend upon the assumption that the targets will be energy is lost through chemical absorptions and viscous randomly distributed in the cross section of the transmit­ friction. When the fish density is to be estimated from ted beam. The data from many echoes are combined to echo measurements, the effect of these factors on the produce one result which is an average property of the signal must be determined. The contribution to the re­ target ensemble. This statistical treatment overcomes a ceived signal from factors other than reflection by fish fundamental limitation of the single-beam echo sounder, depends upon the frequency, the water temperature, and namely the uncertainty about the position of a particular the salinity. Knowledge of hydrographic conditions is target within the beam. However, it is still necessary to also necessary to determine the sound velocity which is know the beam pattern of the transducer. required for the calculation of the target range and the The method of echo integration is the more generally TVG function. applicable since it does not depend upon being able to The normal procedure on echo-integrator surveys is to resolve single target echoes. The received signal is cor­ decide upon single values of c and a, which are consid­ rected for spreading and absorption losses, then the sig­ ered to be unbiased averages for the area being surveyed. nal energy is summed or integrated over a period of time. If the assumed values are inaccurate, there will be a The signals are corrected by varying the receiver gain (in corresponding error in the fish density estimate. This dB) according to the TVG function 20 Log R + 2aR, problem has been examined by Simmonds and Forbes where, a, is the acoustic absorption. When it is possible (1980) and MacLennan (1981). Most of the error is con­ to resolve single-fish echoes, the alternative technique of tributed by uncertainty in the absorption coefficient. The echo counting may be considered (Forbes and Nakken, greater the range of the target, the more significant is the 1972). In this case the number of fish in a particular error.

10 former is based upon low-frequency data (25 kHz max­ The velocity of sound imum) and he recommends the more modern Fisher and The literature contains many empirical equations which Simmons (FS) equation as the better choice at the fre­ may be used to calculate the sound velocity at a given quencies normally used in fisheries work. The FS value temperature, salinity, and depth. A brief historical re­ of a is generally lower than that from the Shulkin and view will be found in MacKenzie (1981 a). Marsh (SM) equation. At 25 kHz and 4°C for example, Equations such as those of Wilson (1960) contain a the difference is 0-002 dB m !. Foote also makes the large number of terms, but simpler equations are ade­ important point that irrespective of the equation used, quate for fisheries applications. Urick (1975) recom­ hydrographic information should be collected during mends Leroy’s formula which is: surveys in order to provide accurate data for the calcula­ tion of a and to avoid bias in fish stock estimates. c = 1449-34 + 4-56 T - 0-046 T 2 + (1-38 - 0-01 T) x The FS equation was based on laboratory measure­ x (S - 35) + D/61. (13) ments of the absorption in simulated sea water and it should therefore be tested by comparison with experi­ Here, T, is the temperature in °C, S, is the salinity in parts mental data obtained in the sea. Fisher and Simmons per thousand, and D, is the depth in m, giving the sound (1977) made a few such comparisons. At frequencies velocity, c, in msH. More recently, MacKenzie (1981 b) from 20 to 142 kHz, the experimental a values mostly lie has published the following equation based on the work between those predicted by the FS and SM equations, of Del Grosso and Mader (1972) which is claimed to be and the evidence is not conclusive as to which equation is better supported by experimental data: superior. Fisher and Simmons claim only that their re­ sults “provide a new basis for evaluating oceanographic c = 1448-96 + 4-591 T - 0-05304 T2 + acoustic data” and they emphasize the need for further + 2-374 x 10-4 T3 + (1-34 - 0-01025 T) (S - 35) + work. + 0-0163 D + 1-675 x 10'7 D 2. (14) A relatively large error in a is tolerable when the range is short. At 50 and 500 m ranges, for example, an error of MacKenzie’s term in D 3 has been omitted from Equation 0-002 dB m "1 would result in biases around 5 % and 50% (14) since it is negligible (< 0-01 ms-1) within 1 km of the respectively in the stock estimates. The probable error in surface. In the continental shelf seas, or in depths less a is not significant for most surveys on the continental than 250 m, the D2 term is also negligible. The velocities shelf, but it does need to be considered in the case of fish predicted by Equation (14) are generally lower than such as the blue whiting which live in deeper water. those given by Equation (13). Considering a range of It is worth noting that nearly all the data on this subject conditions which cover most marine fisheries applica­ have been derived from horizontal sound transmission. tions, T = 4—16°C, S = 34-36 %o, and D = 0 -700 m. the The absorption effect in deep-water vertical echo sound­ maximum difference between the two equations is 0-8 ing has received very little attention. ms*1. An error of this magnitude would contribute about 0-2 % of bias to an echo-integration measurement, an insignificant amount in comparison with other errors. Scattering - general Although the difference between the two equations is not significant, at least as regards errors in echo integra­ An acoustic beam will fluctuate when it is transmitted tion, the more modern and better substantiated formula through an inhomogeneous medium because energy is of MacKenzie is preferred. scattered into or out of the beam. The physical features of the sea which cause scattering fall into two categories, continuous and discrete. Examples of the former are the temperature microstructure of the ocean (Skudrzyk, Absorption 1957; Medwin, 1974) and turbulence (Dunn, 1965). These Sound waves lose energy through molecular relaxation continuous features cause fluctuations which are mainly and water viscosity. The total absorption is measured by important in long-range communication. Discrete scat- the coefficient, a, which is the energy lost in dB per terers such as gas bubbles and fish are more important in metre travelled by the wave. fisheries applications. As in the case of the sound velocity, there are various A significant loss arises from the presence of gas bub­ empirical equations for the calculation of a as a function bles near the surface which are induced by the wind. of hydrographic parameters and the frequency. The Medwin (1977) has described the theory of sound propa­ equation of Shulkin and Marsh (1962) takes account of gation in bubbly water. Energy is lost from the transmit­ viscosity and the relaxation loss due to magnesium sulph­ ted beam through scattering and absorption by those ate, while the Fisher and Simmons (1977) equation also bubbles which are resonant near the transmitted fre­ contains a term in respect of boric acid relaxation. quency. Moreover, the medium is dispersive, so the du­ Foote (1981) has discussed the application of these two ration of a pulsed transmission increases with distance equations in fisheries acoustics. He points out that the along the ray path.

11 Dalen and Løvik (1981) have measured the extra at­ sity shoals are consistent with coherent scattering, but it tenuation when echo sounding through wind-induced is not claimed that the evidence is conclusive. We con­ bubbles. The transmission loss increases with both fre­ sider that the argument should be conducted on different quency and wind speed. Since the bubbles are concen­ lines. At the frequencies of interest in fisheries research, trated near the surface, the problem is partly overcome a few tens of kHz or more, the wavelength is a few cm or by lowering the transducer. In a 20 knot wind and at 38 less. Thus the existence of coherent scattering implies kHz for example. Dalen and Løvik report that the extra that shoaling fish are able to maintain station with a attenuation can be as much as 2-5 dB, but it reduces to precision of a few mm. As noted by Swingler and 0-3 dB when the transducer is 10 m deep. The empirical Hampton (1981), this is simply not a realistic proposi­ relationship with wind speed is a crude but convenient tion. At frequencies around and above 38 kHz, reason­ way of estimating the extra attenuation. Probably it able assumptions about the random component of fish would be more accurate to estimate the bubble density distribution lead to the conclusion that the scattering is and hence the attenuation from reverberation measure­ incoherent. ments. On the other hand, multiple scattering effects cannot The scattering properties of fish are considered in be ignored. Multiple scattering occurs when the sound another review (Midttun, 1984) and we confine attention field at a target is influenced by the scattering of several in this paper to the physical problem of how the signal other targets. Clearly the significance of multiple scatter­ from a shoal or aggregation relates to that from one fish ing depends upon the density of the scatterers, and it may in isolation. It is commonly assumed in the analysis of be supposed that there is a critical density above which echo records that the energy returned from a shoal the first order scattering approximation and the models (which means strictly the average or expected value of discussed above will fail (Weston, 1967). It is important the energy) is the sum of the contributions which would to have enough information to decide whether the den­ arise from the individual fish in isolation. This implies sity of an observed shoal is above or below this critical that the fish density is low enough for multiple scattering value. One noticeable feature of multiple scattering is effects to be ignored, and furthermore that the fish dis­ the lengthening of the echo which is caused by the time tribution is sufficiently random to avoid coherent scatter­ required for the scattering to occur before all the energy ing. Under these conditions the sound field is described is either absorbed or radiated out of the shoal. Examina­ by first order scattering theory which has been the basis tion of the echo-sounder trace should indicate the pre­ of several mathematical models of the echo-integrator sence of severe multiple scattering since the lengthened process (Ehrenberg, 1971; Lozow and Suomala, 1971; echo appears as a diffuse extension to the mark of the Moose, 1971; Stevenson and Shepard, 1973; Bodholt, shoal. A good example of this effect will be found in 1977). The various models have been compared by Figure 4 of Røttingen (1976). However, it does not neces­ Swingler and Hampton (1981) who found that once the sarily follow that multiple scattering is insignificant when different treatments were converted to comparable there is no such evidence on the echo-sounder trace. forms, for which purpose certain simplifying assump­ The density dependence of the echo energy has been tions had to be made, the various expressions for the investigated in experiments with caged fish (Røttingen, mean and variance of the integrator output were very 1976; Burczynski, 1979). The two workers (who were similar. Thus Swingler and Hampton conclude that the studying different species) both report that the inte­ theory of echo integration is well established within the grated energy is proportional to the fish density up to a limits of the assumptions made about fish density and certain value, then the energy increases more slowly and distribution, but they stress the need for experiments to it may even decrease as the density goes up. Foote (1978) confirm the theory and the extent to which the stated suggests that the observed form of the energy—density assumptions will hold in practice. curve might be explained by an inverse relationship be­ tween the density and the variance of the fish orientation distribution. However, the main cause of the non-lin- Multiple scattering and coherency earity is the so-called ‘shadow’ effect. When the density is very high, the fish nearest the transducer attenuate the The distribution of fish in a shoal may be conceived as a acoustic energy so that more distant fish contribute less set of random displacements superimposed upon a per­ to the reflected signal. This shadowing is a consequence fectly ordered lattice, rather similar to the thermal mo­ of ‘second order’ multiple scattering and the end of the tion of atoms in a crystal. For coherent scattering to linear region of the energy—density curve may be identi­ occur, the random displacements must be small relative fied with the onset of multiple scattering. In the case of 12 to a wavelength and if the target ranges increment by cm sprat at 38 kHz and 0-6 ms pulse length, for example, integer multiples of half the wavelength, the energy re­ Røttingen determined the critical density at which non- turned would vary as the square of the number of targets. linearity becomes evident to be qc = 1800—2000 fish/m3. Yudanov and Kalikhman (1981) have discussed this pos­ Multiple scattering is often discussed in terms of the sibility. They say that observed differences in the auto­ volume density of targets. This is unhelpful at least in the correlation function of signals from low- and high-den- case of the shadow effect when the controlling parameter

12 is the number of fish insonified per unit cross section area MN, the total number of fish. It may be shown by ex­ of the beam. It is obvious that two shoals of different panding the series in Equation (18) that the correspond­ heights but having the same fish density by volume will ing area density is approximately: not suffer shadowing to the same extent. In the case of Røttingen’s experiment, the sprat cage was 2-4 m high, nc = O-l/o. (19) so the result quoted above was equivalent to a critical area density, nc = 4300-4800 fish/m2. It will be interesting to compare predictions from the The rigorous theory of multiple scattering is very com­ above equations with Røttingen’s experimental result for plicated (Foldy, 1945). It has been necessary to make sprat which was quoted earlier. Taking c = 1490 ms“1, simplifying assumptions of one kind or another to obtain Equation (16) gives nc = 310 fish/m2. The wavelength at useful analytical expressions. In much of the literature, 38 kHz is 3-9 cm, and the acoustic cross section of a 12 cm multiple scattering is discussed only in qualitative terms, sprat is about 7 cm2 (Nakken and Olsen, 1977). Substitut­ although approximate expressions for the critical density ing these values in Equations (17) and (19), the respec­ have been developed. Yudanov and Kalikhman (1981) tive results are nc = 166 000 and 143 fish/m2. There is a apply the theory of scattering in turbid media which factor of 1000 difference between the extreme predic­ suggests that multiple scattering becomes important tions, and all three are more than an order of magnitude when the ‘acoustical thickness’ of insonified targets is removed from Røttingen’s experimental result, about about 0-2 rayleighs. This leads to the result: 4500 fish/m2. We conclude that the present understand­ ing of the multiple scattering effect leaves much to be qc = l/(0-6 c x L2) (15) desired. Clearly there is need for further work to deter­ mine the limits within which the first order scattering where, L, is the fish length. The validity of applying the models are valid. turbid media theory to the case of discrete fish scatterers is doubtful. However, Yudanov and Kalikhman report experimental results with artificial targets which support their theory. For a shoal of height, h, the above equation is equivalent to: Sonar systems nc = h/(0-6 c x L2). (16) Many kinds of sonar are used to obtain information about remote objects in the sea by means of the transmis­ Lozow and Suomala (1971) have considered the case of sion and reception of acoustic signals. We confine atten­ isotropic scattering by discrete point sources. They as­ tion here to the active sonars which themselves generate sume that multiple scattering is insignificant if the propa­ acoustic signals. Passive sonars are used to detect sounds gation velocity is within a few per cent of the ‘empty- produced by the objects under investigation, submarines water’ value, then they apply Foldy’s theory to derive the for example, but they have seldom been applied in fish­ following expression: eries work. oc = ji3/2/(2 X2o 1/2). (17) Searchlight sonar Again we may suppose that the equivalent nc is Qch. Another theory has been proposed by Love (1981) who The basic echo sounder provides very limited informa­ describes a model in which the shoal is constructed from tion about the bearing of targets. It can be said that a N layers in height each containing M fish. He shows that detected signal comes from a target that is probably the echo energy corresponds to that from I independent within the main lobe of the transmitted beam, which fish, where might be 15 degrees wide, but little else. The searchlight sonar was an early development in which the directional N - l properties of transducers were used to better effect. This I = M {1 + 2 [ 1 + v2 p(p+l)/2] ( 1 - v)2p) (18) is done by supporting the transducer on a mounting p=i which may be rotated mechanically in the horizontal and vertical planes. The horizontal and vertical bearing of the v is a reflection coefficient, equal to oN2/h2, which is transducer is monitored, so the bearing and depth of a assumed small enough to neglect terms of order v4 and target may be estimated from the direction and range higher. Love’s model is perhaps too simple to represent corresponding to the maximum echo strength. The adequately the complicated structure of a fish shoal, but searchlight sonar is widely used in commercial fishing. it does describe the shadowing effect correctly in terms of Fishermen who purse seine or trawl for pelagic species the area density. Shadowing is important when the term rely on this instrument for advance information about within the braces in Equation (18) is significantly dif­ the location of fish so that they can plan their tactics ferent from N, say 0-9N, since I is then 10 % less than accordingly.

13 cross section of the target and the transducer beam func­ Netsounders tion. It follows that if it is required to measure the target To complement the information obtained from acoustic properties, it is necessary to have some means of remov­ instruments on board the vessel, it is useful to locate the ing the effect of the beam function on the observed fish relative to the trawl. This is done by means of one or signals. One solution to this problem is the dual-beam more transducers attached to the net. Signals are trans­ technique which was first proposed by Ehrenberg (1979). mitted between the net transducers and the ship by a The method works by transmitting a narrow-beam cable connection or an acoustic link. The netsounder pulse and receiving signals on both narrow- and wide- display helps the fishermen to decide how to control the beam transducers. In principle, measurement of the two position of the net in order to have the best chance of signals should provide sufficient information to eliminate catching fish. In commercial fishing applications the nor­ the effect of the beam function. In practice, however, mal practice is to use one transducer unit which is at­ difficulties may arise due to the side lobes of the narrow- tached to the centre of the net headline. This unit will beam pattern and noise in the received signals. contain one transducer the beam of which is directed When the two receiving beams are achieved by using downwards across the netmouth, and possibly a second independent transducers as in Ehrenberg’s original sys­ transducer directed upwards for the purpose of measur­ tem, little can be done about the side-lobe problem. ing the net depth. More complicated netsounders have Simmonds et al. (1984) have described a more sophisti­ been developed, the multi-netsounders, in which addi­ cated system based on one multi-element transducer, the tional transducers are used to observe fish ahead of the elements of which are arranged in concentric rings, and net, between the wingends, or elsewhere in the vicinity of the signals from each ring are filtered and combined in a the gear (Horn, 1971). manner which determines the beam pattern. This beam- forming technique is known as “shading”. Simmonds et al. have shown how the side-lobe problem may be mini­ Side-scan sonar mized by the correct choice of the shading function which The transducer of a side-scan sonar is again no different describes the filtering required on the various rings. in principle from that of the simple echo sounder, a single-beam device, but it is mounted in a towed body so that the beam axis is perpendicular to the ship’s track and Scanning sonars directed on or below the horizontal plane. As the ship proceeds, successive echo signals are displayed on a chart Under this heading we shall discuss various techniques recorder thus building up a two-dimensional picture of a for determining the bearing as well as the range of tar­ rectangular area bounded by the ship’s track and the gets, so that the distribution of targets may be viewed in maximum range of the equipment. The simplest side- two dimensions. scan sonars do not provide an isometric display, but it is The searchlight sonar, as described above, is capable not difficult in principle to obtain such a display through of a crude form of scanning by mechanical movement of signal processing (Walker, 1978). The transducer beam is the transducer. However, the scanning rate is very low designed to be wider in the vertical plane than in the since each transmission contributes data relating to only horizontal, to cover more of the water column while one bearing angle and the scanning rate is limited by the optimizing the horizontal resolution. The side-scan sonar acoustic travel time out to the maximum range and back is well suited to the task of surveying the seabed and again. Yuen (1971) has described an elegant system which detecting objects such as wrecks and pipelines, but it has permits higher scanning rates, the Continuous Transmis­ not been applied much in fisheries work. Rusby (1977) sion Frequency Modulated (CTFM) sonar. This works has described an experimental study of herring using a by transmitting continuously at a frequency which low-frequency (6-4 kHz) side-scan device. He showed changes linearly with time, so the signal from a target is that herring shoals could be tracked at ranges up to 13 km received at a frequency that differs from the transmission from the survey vessel. Sadly, this interesting research on frequency by an amount proportional to the target range. fisheries applications of side-scan sonar does not appear An important feature of the CTFM sonar is the continu­ to have been followed up. One snag is the sheer size of ous availability of target signals, whereas the con­ the transducer which is consequent upon the low fre­ ventional pulsed sonar provides no information about a quency. Rusby’s towed body weighed several tonnes and particular target for a large proportion of the time be­ any vessel using such a device would require expensive tween transmissions. Thus the CTFM technique offers special purpose handling equipment. the possibility of observing fast moving targets on a time scale much shorter than the acoustic travel time to the target and back. Moreover, by receiving on a narrow beam while transmitting over a wide sector (Yuen used Dual-beam systems separate transducers to achieve this), the receiving trans­ When a point target is insonified by a single-beam trans­ ducer may be rotated rapidly to locate the bearing of ducer, the received signal is determined by the acoustic targets.

14 Yet higher scanning rates are achieved by means of Conventional transducers have a Q factor (ratio of electronic beam steering which is the basis of the “within centre frequency to bandwidth) around 10, and there is a pulse” sector scanner (Voglis and Cook, 1966). The re­ practical difficulty in realizing transducers with much ceiver is a linear array of transducer elements. A phase lower Q factors which would be required to obtain a shift rep is applied to the signal from element r, numbering bandwidth of, say, an octave in frequency (Small, 1971). the elements sequentially along the array, then the There is also a more fundamental problem which con­ phase-shifted signals are combined to form the array cerns the beamwidth. In the case of a conventional trans­ output. The beam direction of the array is controlled in ducer which works like a rigid piston with a plane face in one plane by varying cp and this can be done at such speed contact with the water, the beamwidth and hence the that a sector of say 30° may be scanned within a pulse sampling volume depends upon the frequency. The length of 100 us. The variable phase shift may be gener­ higher the frequency, the narrower the beam. It is impor­ ated by a frequency modulation technique (Voglis, 1971) tant that the volume sampled by a sonar should be de­ which is in effect a hardware representation of the Fast fined, and it follows that it will only be useful to widen the Fourier Transform. Tucker et al. (1958) have described bandwidth if it is possible to realize a transducer the another scheme which achieves the same end by means beam pattern of which is reasonably independent of fre­ of a swept frequency and a tapped delay line. The poten­ quency. tial of the electronic scanning sonar for fisheries research Much of the early work on constant beamwidth sonar has long been recognized, and in 1969 a scanner of the was done by a team at Birmingham University (Berktay Voglis type was installed on the UK RV “Clione” . This et al., 1968). They developed a system which could be 305 kHz equipment scanned a 30° sector with an angular used over a decade of frequencies (6 to 60 kHz), based on resolution of 0-33° (Voglis, 1972). Various design im­ separate transmit and receive transducers which provements have been effected over the years (Mitson et achieved the constant beamwidth by different means. al., 1979) and the system has proved useful in many The transmitter was a parametric source in which the applications (Cushing and Harden Jones, 1979). Cushing transmitted frequency was derived from the difference (1977) used the “Clione" scanner to study the packing between two primary frequencies around 500 kHz. The density of fish shoals for example, and Margetts (1971) beamwidth and the bandwidth are determined by the has observed trawls in action. With a transponding primary frequencies, so the bandwidth is a large propor­ acoustic fish tag the scanner allows the tagged fish to be tion of the much lower difference frequency. The receiv­ tracked at ranges up to some 200 m (Mitson and Store- ing transducer consisted of 169 elements arranged in ton-West, 1971). This technique has been applied in esti­ straight lines on the surface of a hyperbolic paraboloid. mating the catching efficiency of a demersal trawl Each line contained 13 elements and it was skewed by (Harden Joneses«/., 1977). Applications in stock estima­ 1-5° relative to its neighbours. Dunn (1975) has reported tion have been considered theoretically by Ehrenberg experimental measurements of the system performance. (1980). He showed that the variance of biomass estimates He found that the receiving array beamwidth varied be­ could be reduced by sector scanning instead of echo tween 10 and 25 degrees over the frequency range 10-50 sounding. kHz, in broad agreement with the calculated variation. However, the main problem with the Birmingham sys­ tem, at least as regards fisheries applications, was the low Constant beamwidth sonar source level (about +85 dB re 1 ptbar at 1 m) which was It is a general principle of signal processing that the consequent upon the inefficiency of the parametric trans­ information content of a signal increases with the system mitter. bandwidth. An important consequence of this principle, Rogers and Van Buren (1978) have proposed another as applied to sonar systems, is that the ability to resolve solution to the constant beamwidth problem, the spheri­ targets at nearly the same range improves as the band­ cal cap transducer. They develop the theory of beam width increases. What we mean here is the resolution of forming for this transducer shape and they show how a ‘point’ targets, but the principle applies equally to a particular shading function will result in a constant beam target which has shape, in the sense that the bandwidth pattern with no side lobes. Since the theory applies determines the amount of information in the echo which equally to the beam pattern when transmitting or receiv­ might be used to investigate the target structure. ing, separate transducers are not necessary in this case. However, increasing the bandwidth will generally result Although the theory of the spherical cap transducer is in a reduced signal-to-noise ratio (SNR), and the system well established, and development work on a practical designer must find a balance between competing require­ device is in progress, it remains to be seen whether the ments. On the one hand, the SNR must be good enough ideas of Rogers and Van Buren will lead to a working to discriminate between signal and background noise constant beamwidth sonar the performance of which which implies an upper limit on the bandwidth. And on matches the claims of theory. the other hand, a lower bandwidth limit is implied by the It is not entirely clear what advantages a very wide specification of what information has to be extracted bandwidth would have in fisheries investigations. The from the signal. emphasis on providing information about the target

15 shape seems hardly relevant here, although doubtless an receive, and one is redundant. The equipment is suffi­ important aspect in other fields. It has been argued that ciently small and portable to be used from a small boat, the variation of fish target strength with tilt angle, an the maximum working range is 500 m and the bearing important cause of uncertainty in converting echo energy resolution is better than 2°. An important requirement of to biomass, might be reduced if a greater range of fre­ the transducer design is that the acoustic coupling of the quencies was transmitted, on the assumption that the array elements via the support mounting must be as small variation arises from interference effects rather than as possible, to avoid distortion of the signal phase shifts. change in aspect of the fish. The results of Nakken and The phase comparison technique is limited in practice Olsen (1977) provide some support for this assumption to the detection of sparse targets in low noise conditions insofar as their observed target-strength variation was and when signal amplitude information is not required. more than could be explained by aspect changes alone. The electronic scanning technique does not have these However, very little is known about the characteristics of limitations, but it is relatively complicated and much fish echoes in response to broad-band transmissions. more expensive to apply in practice.

Phase comparison sonar Doppler sonar The phase difference cp between two elements in a trans­ When an echo is received from a target that is moving ducer array depends upon the target bearing. If, d, is the relative to the sonar transducer, the echo frequency dif­ distance between the two elements and, 0, is the angle fers from the transmitted frequency by an amount pro­ between the echo wavefront and the line joining the two portional to the radial target velocity. This is the well- elements, then sin 0 = Xcp/2jc d. The sector scanner makes known Doppler effect which has many applications, for use of these phase differences to generate a two-dimen­ example the “Doppler log” which is a particularly accur­ sional (range and bearing) display of all received signals. ate instrument for measuring ship velocity relative to the While the phase comparison sonar has the same kind of seabed or to the water. transducer, a linear multi-element array, in this case the Echoes from fish shoals have been shown to exhibit only targets displayed are those detected by an initial Doppler frequency shifts. Holliday (1977) has described processing of the phase differences to discriminate be­ how useful information about the size and swimming tween target and noise signals (Nairn, 1968). The ele­ speed of fish may be deduced from the frequency spec­ ments are connected to hard-limited (clipping) ampli­ trum of shoal echoes. He observed Doppler shifts even fiers the outputs of which switch rapidly between high when the shoal was insonified in side aspect, normal to and low states at times determined by the signal phases. the swimming direction, which he attributed to tail-beat Thus the system provides range and bearing data, but motions. Holliday derived an empirical relationship be­ signal amplitude information is lost. The technique over­ tween fish length and the maximum side aspect Doppler comes the difficulties normally associated with the large shift. However, he was unable to compare his predictions dynamic range of sonar signals, and the target location with length data from direct sampling, and it appears that data are obtained in a form which greatly simplifies digi­ experimental verification of this potentially useful tech­ tal encoding. On the other hand, the SNR must be very nique remains incomplete. good to ensure the correct operation of a phase compari­ son sonar. Another limitation is the inability of the tech­ nique to detect simultaneously more than one target at the same range. Nevertheless, Creasey and Braithwaite List of symbols and units (1969) demonstrated that the phase comparison sonar a Largest dimension of transducer face (m) could be used to observe fish shoals. c Velocity of sound (ms-1) Dunn (1977) proposed a novel fish counter in which d Distance between adjacent transducer elements signals from a transducer array were processed in parallel (m) through two channels. The reference channel was a phase D Depth below sea surface (m) comparison network as described above and the m ea­ F(co) Transfer function of the target (m) surement channel was a conventional linear echo-detec- g(w) Normalized spectrum of the transmitter input tion system. The reference channel output was used to signal (s) block signals from targets outwith a small range of bear­ Gp Power gain of an echo sounder ing angles, and the effect was a directional response close Ge Energy gain of an echo sounder to the ideal of constant sensitivity inside and zero sen­ h Height of a fish shoal (m) sitivity outside a well-defined beam. H(co) Transfer function of the receiving transducer and A phase comparison sonar has been developed re­ amplifier (V m2 N"‘) cently for the tracking of 70—80 kHz acoustic fish tags i Square of root of —1 (McQueen et al., 1981). The transducer array consists of I Effective number of fish insonified five elements of which one is used to transmit, three to L Fish length (m)

16 M Number of fish in a horizontal layer sector scanning sonar, Lowestoft, 18—19 December 1979, pp. nc Critical number of fish per unit area (m-2) 1 —10. Institute of Acoustics. Edinburgh. Dalen, J., and Løvik, A. 1981. The influence of wind-induced N Number of fish layers in a shoal bubbles on echo integration surveys. J. acoust. Soc. Am., P i(t) Sound pressure incident upon the target (N m“2) 69(6): 1653-1659. p0(t) Echo pressure incident upon the transducer face Del Grosso, V. A., and Mader, C. W. 1972. Speed of sound in (N m“2) sea water samples. J. acoust. Soc. Am., 52: 961—974. Dunn, D. J. 1965. Turbulence and its effect upon the transmis­ Q Quality factor of a transducer sion of sound in water. J. Sound Vibr., 2: 307—327. r Sequence number of a transducer element Dunn. J. R. 1975. A note on wideband sonar trials. Proc. Conf. R Distance between the transducer and the target Acoustic surveying of fish populations, Lowestoft, 17 Decem­ (m) ber 1975. Institute of Acoustics, Edinburgh. Dunn,W. 1.1977. A controlled beamwidth echo sounder for fish S Salinity (parts per thousand) counting. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: S(co) Transfer function of the transmitting amplifier 162-166. and transducer (N n r 1 V-1) Ehrenberg, J. E. 1971. Derivation and numerical evaluation ofa t Time between the transmission pulse and the general expression for fish population estimates using an echo integrator. Univ. Washington Sea Grant Publication WSG received echo (s) 74-4. T Water temperature (°C) Ehrenberg, J. E. 1979. A comparative analysis of in situ methods V |(t) Transmitter input voltage (V) for directly measuring the acoustic target strength of individ­ v0(t) Receiver output voltage (V) ual fish. IEEE J. Oceanic Engng, 4: 141—152. Ehrenberg, J. E. 1980. Echo counting and echo integration with V Amplitude of the transmitter input voltage (V) a sector scanning sonar. J. Sound Vibr.. 73(3): 321-332. a Acoustic absorption coefficient (dB n r1) Fisher, F. H., and Simmons, V. P. 1977. Sound absorption in sea Ô Bandwidth-dependent resolution time (s) water. J. acoust. Soc. Am., 62: 558—564. X Wavelength (m) Foldy, L. L. 1945. The multiple scattering of waves. I. General theory of isotopic scattering by randomly distributed scat- v Reflection coefficient of a fish shoal terers. Phys. Rev., 67: 107—119. tp Phase difference between adjacent transducer Foote, K. G. 1978. Analysis of empirical observations on the elements scattering of sound by encaged aggregations of fish. FiskDir. q Number of fish per unit volume (m”3) Skr. Ser. HavUnders., 16: 423-456. Foote, K. G. 1981. Absorption term in time-varied gain func­ qc Critical value of o (m-3) tions. FiskDir. Skr. Ser. HavUnders., 17: 191-213. a Acoustic cross section of a fish (m2) Foote, K. G. 1982. Optimising copper spheres for precision 0 Angle between the echo wavefront and the line calibration of hydroacoustic equipment. J. acoust. Soc. Am., joining two transducer elements 71: 742-747. Forbes, S. T., and Nakken, O. (Eds.) 1972. Manual of methods T Pulse duration (s) for fisheries resource and appraisal. Part 2: the use of acoustic (0 Angular frequency (radians s“1) instruments for fish detection and abundance estimation. FAO Man. Fish. Sei., 5: 138 pp. Harden Jones, F. R., Margetts, A. R., Greer Walker, M., and Arnold, C. P. 1977. The efficiency of the Granton otter trawl References determined by sector scanning sonar and acoustic transpond- Berktay, H. O., Dunn, J. R., and Gazey, B. 1968. Constant ing tags. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: beamwidth transducers for use in sonars with very wide fre­ 45-51. quency bandwidths. Appl. Acoust., 1: 81—99. Hoare, D. W. 1978. A microprocessor controlled time-varied Blue, J. E. 1984. Physical calibration. Rapp. P.-v. Réun. Cons. gain amplifier. Proc. Conf. Acoustics in fisheries, Hull, int. Explor. Mer, 184: 19—24. 26-27 September 1978. Institute of Acoustics, Edinburgh. Bodholt, H. 1977. Variance error in echo integrator output. Holliday, D. V. 1977. Two applications of the Doppler effect in Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: 196—204. the study of fish schools. Rapp. P.-v. Réun. Cons. int. Explor. Burczynski, J. 1979. Introduction to the use of sonar systems for Mer, 170: 21-30. estimating fish biomass. FAO Fish. tech. Pap., No. 191. 89 Horn, W. 1971. New types of multi-netsonde equipment. In pp. Modern fishing gear of the world: 3, pp. 389-394. Ed. by H. Clay, C. S., and Medwin, H. 1977. Acoustical oceanography: Kristjönsson. Fishing News (Books), London. principles and applications. John Wiley, New York. 544 pp. Lindem. T. 1981. The application of hydroacoustical methods in Craig, R. E. 1981. Units, definitions and symbols in fisheries monitoring spawning migrations of whitefish (Coregonus lav- acoustics. In Meeting on hydroacoustical methods for the aretus) in Lake Randsfjorden, Norway. In Meeting on hydro­ estimation of marine fish populations, 25—29 June, 1979, 2, acoustical methods for the estimation of marine fish popula­ pp. 23—31. Ed. by J. B. Suomala. The Charles Stark Draper tions, 25 —29 June, 1979, 2, pp. 925—939. Ed. by J. B. Laboratory, Inc., Cambridge, Massachusetts, USA. 964 pp. Suomala. The Charles Stark Draper Laboratory, Inc., Craig, R. E., and Forbes, S. T. 1969. A sonar for fish counting. Cambridge, Massachusetts, USA. 964 pp. FiskDir. Skr. Ser. HavUnders., 15: 210-219. Love. R. H. 1981. A model for estimating distributions of fish Creasey, D. J., and Braithwaite, H. B. 1969. Experimental school target strengths. Deep-Sea Res., 28A, 7: 705-725. results of a sonar system with a digital signal processing unit. Lozow, J. B., and Suomala, J. B. 1971. The application of Appl. Acoust., 2(1): 39-57. hydroacoustical methods for aquatic biomass measurements. Cushing, D. H. 1977. Observations on fish shoals with the ARL Charles Stark Draper Laboratory Rep. R-712, 92 pp. The scanner. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: Charles Stark Draper Laboratory, Inc., Cambridge, Mas­ 15-20. sachusetts, USA. Cushing, D. H., and Harden Jones, F. R. 1979. Sector scanning MacKenzie, K. V. 1981 a. Discussion of sea water sound speed sonar and the fisheries since 1969. Proc. Conf. Progress in determinations. J. acoust. Soc. Am., 70: 801-806.

2 Rapports et Procès-Verbaux 17 MacKenzie, K. V. 1981 b. Nine-term equation for sound speed Simmonds, E. J., Forbes, S, T., and Stevens, P. J. 1984. Design, in the oceans. J. acoust, Soc. Am., 70: 807-812. construction and testing of a multi-beam transducer for in situ MacLennan, D. N. 1981. Acoustic absorption and errors in TVG target-strength measurement. FAO Fish. Rep., 300: 18—26. functions. Working Paper No. 81/10, 9 pp. Marine Labora­ (Contribution No. 81). tory, Aberdeen. Skudrzyk, E. 1957. Scattering in an inhomogeneous medium. J. Margetts, A. R. 1971. Sector scanning sonar used for observing acoust. Soc. Am., 29: 50-60. deep-sea trawling. In Modern fishing gear of the world, 3, pp. Small, D. J. 1971. Design of a low-Q sonar transducer. Ultra­ 137-140. Ed. by H. Kristjönsson. Fishing News (Books), sonics, 9: 154-157. London. Stevenson, E. A. 1974. A theory for multiple target scattering. McQueen, P. D., Dunn, J. R .,Creasey, D. J.,andUrquhart, D. Ph.D. thesis, Mississippi State University. Mississippi. 1981. A sonar to track the position of acoustic tags for fisheries Stevenson, E. A., and Shepard, S. W. 1973. A system simula­ research. Proc. Conf. Electronics for ocean technology, Bir­ tion for multiple target scattering. Mississippi State Univ. mingham, 8—10 September 1981. 1ERE Conf. Proc., 51: Rep. EIRS-ASE-73-3. 197-204. Swingler, N., and Hampton, I. 1981. Investigation and compari­ Medwin, H. 1974. Sound phase and amplitude fluctuations due son of current theories for the echo integration technique of to temperature microstructure in the upper ocean. J. acoust. investigating fish abundance and of their verification by ex­ Soc. Am., 56: 1105-1110. periment. In Meeting on hydroacoustical methods for the Medwin, H, 1977. Counting bubbles acoustically: a review. estimation of marine fish populations, 25 - 29 June, 1979, 2, Ultrasonics, 15(1): 7-13. pp. 97—156. Ed. by J. B. Suomala. The Charles Stark Draper Midttun, L. 1984. Fish and other organisms as acoustic targets. Laboratory, Inc., Cambridge, Massachusetts, USA. 964 pp. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 184: 25-33. Tucker, D. G., Welsby, V. G., and Kendeil, R. 1958. Electronic Mitson, R. B., Shreeve, E. G., and Hood, C. R. 1979. A sector sector scanning. J. Br. Instn. Radio Engrs, 18: 465. scanning sonar: 10 years of use. Proc. Conf. Progress in sector Urick. R. J. 1975. Principles of underwater sound. McGraw scanning sonar, Lowestoft, 18-19 December 1979, pp. 11-19. Hill, New York. 384 pp. Institute of Acoustics, Edinburgh. Urquhart, G. G., and Hawkins, A.D. 1982. Tracking fish in the Mitson, R. B., and Storeton-West, T. J. 1971. A transponding sea. In Marine biology at sea. Ed. by A. MacDonald and A. acoustic fish tag. Radio Electron. Engr., 41: 438-439. G. Priede. Academic Press, London. Moose, P. H. 1971. A simplified analysis of the statistical charac­ Voglis, G. M. 1971. A general treatment of modulation scanning teristics of the fish echo integrator. Univ. Washington Sea as applied to acoustic linear arrays. Ultrasonics. 9(1): Grant Publication WSG 71-2. 142-153 and 9(2): 215-223, Nairn. D. 1968. Clipped-digital technique for the sequential Voglis, G. M. 1972. Design features of advanced scanning sonar processing of sonar signals. J. acoust. Soc. Am., 44: based on modulation scanning. Ultrasonics, 10(1): 16-25 1267-1277. and 10(2): 103-113. Nakken, O., and Olsen, K. 1977. Target-strength measurements Voglis, G . M., and Cook, J. C. 1966. Underwater applications of of fish. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: an advanced acoustic scanning equipment. Ultrasonics, 4: 52-69. 1-9. Raitt, R. W. 1948. Sound scatterers in the sea. J. Mar. Res., 7: Walker, C. D. T. 1978. Development of a ground speed cor­ 393-409. rected side-scan sonar display system. Ultrasonics, 16(3): Robinson. B. J. 1978. In situ measurement of fish target 108-110. strength. Proc. Conf. Acoustics in fisheries, Hull, 26—27 Westerveit, P. J. 1963. Parametric acoustic array. J. acoust. Soc. September 1978. Institute of Acoustics, Edinburgh. Am., 35: 535-537. Robinson, B. J. 1981. Improvements in signal processing for Weston, D. E. 1967. Sound propagation in the presence of acoustic assessment of fish stocks. Proc. Conf. Electronics for bladder fish. In Underwater acoustics, pp. 55-58. Ed. by V. ocean technology, Birmingham, 8-10 September 1981.1ERE M. Albers. Plenum Press, New York. Conf. Proc., 51: 313-319. Wilson, W. D. 1960. Equation for the speed of sound in sea Rogers, P. H., and Van Buren, A. L. 1978. New approach to a water. J. acoust. Soc. Am., 32: 1357. constant beamwidth transducer. I. acoust. Soc. Am., 64: Yamanaka, H., Yukinawa, M., and Morita, J. 1977. Acoustic 38-43. fish counting system for tuna and related species. Rapp. P.-v. Røttingen. I. 1976. On the relation between echo intensity and Réun. Cons. int. Explor. Mer, 170: 174-184. fish density. FiskDir. Skr. Ser. HavUnders., 16: 301—314. Yudanov, K. L., and Kalikhman, I. L. 1981. Sound scattering Rusby, J. S. M. 1977. Long range survey of a herring fishery by by marine animals. In Meeting on hydroacoustical methods side scan sonar. Rapp. P.-v. Réun. Cons. int. Explor. Mer, for the estimation of marine fish populations, 25 -29 June, 170: 7-14. 1979,2, pp. 53 — 95. Ed. by J. B. Suomala. The Charles Stark Shulkin, M., and Marsh, H. W. 1962. Sound absorption in sea Draper Laboratory, Inc., Cambridge, Massachusetts, USA. water. J. acoust. Soc. Am., 42: 270-271. 964 pp. Simmonds, E. J., and Forbes, S. T. 1980. Some range-depen­ Yuen, FI. S. H. 1971. An evaluation of a continuous transmis­ dent variations affecting acoustic fish stock estimations. sion frequency modulated sonar in fishery research. In Mod­ Working Paper No. 80/12,18 pp. Marine Laboratory, Aber­ ern fishing gear of the world, 3, pp. 141 — 144. Ed. by H. deen. Kristjönsson. Fishing News (Books), London.

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