MEASUREMENTS OF FISH TARGET STRENGTH: A REVIEW

RICHARD H. LOVE'

ABSTRACT

The concept of target strength and its application to the quantitative assessment of fishery resourcesare discussed. Methods of determining the echo characteristics of fish are reviewed and a number ofresults presented. Among the more important of these results are: (1) practically every case of in­terest to the fishing industry is in an acoustic region in which the target strength varies widely withfish size and aspect and acoustic frequency, (2) the major contributors to target strength in this regionhave been determined to be the swim bladder, flesh, and skeleton, ancl (3) the average maximum side­aspect and dorsal-aspect target strength of an individual fish have been determined for this region.

(1)

(3)

(4)

(2)10

7'

10

where 10 is the intensity of the sound strikingthe target and II is the intensity of the reflectedsound measured at 1 m from the acoustic centerof the target. If IT is the intensity of the re­flected sound measured at some distance r fromthe target, then, assuming that the sound spreadsspherically, and there are no losses, IT will bedirectly proportional to 10 :

T = 10 log (~) .

Now if Is is the intensity of the projected sound1 m from the source,

U' is defined as the acoustic cross-section of thetarget, and 47Tr2 is the spherical surface areathrough which all the incident energy is reflected.(T depends on the size, shape, and orientationof the target, and, in general, will vary with theangle between the incident direction and thedirection of the receiver. In all present fish­eries work, this angle is zero and therefore inthis paper (T will be the acoustic cross-sectionof a target for the case in which the source andreceiver are located at the same point.

By letting r in equation (2) be equal to 1 mand combining equations (1) and (2),

Active sonars project acoustic energy into thewater in an effort to detect objects by the echoesthey return, the intensity of the echo dependingon the proportion of the sound reflected back tothe receiver. The target strength of the echo­producing object is a quantitative measure of itsreflecting characteristics and is defined as

T = 10 log (~:) ,

TARGET STRENGTH

Quantitative assessment of fishery resources isa difficult task, and many groups have turned toacoustic techniques to conduct assessment sur­veys. Present acoustic techniques can give anestimate of the number and dimensions of fishschools in a geographic area and, by estimatingthe density of the fish in a school, the number ofindividual fish in the area can be approximated.Measurements of fish target strength are beingmade by various investigators in an effort to en­able the direct acoustic estimation of the numberand size of individuals in a school and to enablethe direct identification of those individuals byacoustic methods. This paper discusses theconcept of target strength and its applicationto the quantification and/or identification of fishschools, reviews target strength measurementtechniques, and discusses some results whichhave been obtained utilizing these techniques.

1 U.S. Naval Oceanographic Office, Washington, D.C.20390. and therefore,

Manuscript accepted April 1971.FISHERY BULLETIN: VOL. m, NO.4, 1971.

703

FISHER Y m:LLETIN: VOL. 69, NO.4

10 log IT = 10 log (~) + 10 log Is - 40 log r.

(6)

Defining the echo level (E) as 10 log IT and thesource level (5) as 10 log Is, and rearranging,

T = E - 5 I- 40 log r. (7)

Equation (7) can be used to compute targetstrength in an ideal medium. In a real medium,however, the actual transmission loss will in­clude effects due to absorption, scattering, re­fraction, and the boundaries. Hence, in a realmedium

where H is the one way transmission loss. Htakes into account spreading loss, absorption,and any anomalies, In actual practice H cannotalways be reliably predicted and must be mea­sured unless the ranges involved are small.Equation (8) is known as the active sonar equa­tion and is always used in the computation oftarget strength since it involves only directlymeasurable quantities-echo level, source level,and transmission loss.

It is impossible for more sound to be reflectedfrom a target than is incident upon it, and it istherefore seemingly impossible for any objectto have a positive target strength, yet many largetargets do. This is a consequence of the refer­ence distance being 1 m, and the measurementsbeing made at greater distances, with sphericalspreading assumed in order to calculate thetarget strength. However, the spreading lossvery close to a target is less than the sphericalspreading loss which is assumed, and hence pos­itive target strengths can be obtained for largetargets.

The importance of the target strength of a po­tential target is obvious from the sonar equation(equation (8». The maximum range at whicha target can be detected in any given environ­ment depends on its target strength and thecharacteristics of the transmitting and receiving

systems. Therefore, an estimate of targetstrength is essential to the effective design andoperation of any active sonar system.

The quantification of a fish school, knowing itstarget strength, is possible because the targetstrength of a school depends on the average size,number, distribution, and aspect of the individ­uals in the school. In order to quantify fishschools using target strength information, it isfirst necessary to determine the size and numberof individuals required to produce a given targetstrength. The initial step in this process is thedetermination of the target strength of an in­dividual fish. The application of this knowledgeto studies on the acoustic interactions of arraysof scatterers will eventually produce accuratepredictions of school target strength. The greatmajority of work done up to this time has beenon individual fish, and quite a bit more must bedone before this initial step is completed. De­finitive work on the quantification and identifi­cation of fish schools utilizing target strengthinformation awaits completion of this step.

Reflection of sound from an object in wateroccurs when the object has an acoustic impedancewhich differs from that of the water. Acousticimpedance is defined as the product of the den­sity (p) of a substance and the velocity of sound(c) in that substance. The proportion of soundreflected from or transmitted into an object inwater depends on the magnitude of the imped­ance mismatch between the object and the water.The simplest case of reflection occurs when aplane wave is normally incident upon a planeboundary between two semi-infinite media. Thepressure amplitude reflection coefficient is de­fined as the ratio of the reflected pressure ampli­tude to the incident pressure amplitUde, and for

this case it is found to be P2C2 - PICI, whereP2C2 + PICI

Pt"! is the impedance of the medium in whichthe incident wave is traveling and P2CZ is theimpedance of the medium upon which the waveis incident. If the second medium is reduced tofinite thickness and a third medium is placedbehind it (the third medium mayor may not bethe same as the first), the problem becomesslightly more complicated. When the incidentWave arrives at the first boundary, some energy

(G)

(8)

Is]"4'IT = (~)

T =-.= E - 5 + 2H

or in logarithmic form,

704

LOVE: MEASUREMENTS OF FISH TARGET STRENGTH

is reflected and some is transmitted. After thistransmitted portion reaches the second boundary,some of the energy is transmitted into the thirdmedium and some is reflected back into the sec­ond medium, where it is again partially reflectedfrom the first boundary. This process continuesuntil a steady state is reached. The solution ofthis problem is relatively easy, but it is inter­esting because the resultant amplitudes of thetransmitted and reflected waves depend on thephases of the component waves. The componentwaves add vectorially and whether the amplitudeof the initially reflected wave is increased or de­creased depends on the thickness of the secondmedium and the wavelength of the incident wave.

The reflection of sound from an infinite platepoorly approximates the reflection from a fish,and it is useful to examine the reflection from afinite object such as a sphere. When the sphereis large compared to the acoustic wavelength,the echo originates from specular reflection, inwhich the part of the sphere near the pointwhere the sound wave is normally incident pro­duces a coherent reflected wave. When the sizeof the sphere is comparable to a wavelength, in­terference effects similar to those mentioned forthe plate with finite thickness will cause theacoustic cross-section to vary. When the sphereis small compared to a wavelength, scatteringtakes place and the acoustic cross-section of asphere of radius a is proportional to a6 />-4 where>- is the wavelength. This solution was obtainedby Lord Rayleigh and hence this region is calledthe region of Rayleigh scattering.

For objects other than spheres, analysis be­comes difficult, if not impossible. However, aslong as the object is not highly compressible,the concept of the regions of Rayleigh scattering,interference effects, and geometric reflection isvalid. For a fish, the distinctions between theseregions becomes unclear because of the fish'sinternal structure. When the fish is very smallcompared to the acoustic wavelength, Rayleighscattering can be expected. However, if the fishhas a gas-filled swim bladder the gas bubble willresonate at some wavelength in this region,greatly increasing the target strength over thatpredicted by Rayleigh scattering. When the sizeof the fish is comparable to the wavelength, in-

terferences will occur among the fish flesh andorgans, the skeleton, the gas in the swim bladder,and the boundaries of the fish. When the fishis larger than the wavelength, the dimensions ofmany of these parts will be comparable to thewavelength and the region of interference effectswill be greatly extended into what would be theregion of geometric reflection for a homogeneousbody.

Cushing et al. (19G3) have assumed that theregion of interference effects extends from L/>­= 8 to L/>- = 100, where L is the fish length,and >- is the acoustic wavelength, and they sug­gest that for quantitative results this regionshould be avoided. Neglecting the fact that theyhave ignored the effects of swim bladder reso­nance, the limits they have placed on the inter­ference region will now be examined. For a rigidsphere of radius a the limits of the interferenceregion are approximately 1 :(; 27Ta/>- :(; 10, andfor any other object these limits will probablybe farther apart. Measurements on individualfish indicate that interference effects occur atvalues of L,.>- 0.7 (Love, 1971) and this canbe taken as a lower limit. (This is not to saythat it is the lower limit, only that this is as lowas measurements have been made.) Haslett(1962a) examined a small number of whiting todetermine their "standard dimensions." Hefound that the diameter of the backbone wasabout 0.01 the length of the fish. Assuming thatthe backbone of a fish is the smallest part of afish which contributes to its echo, this meansthat if interference effects occur in the back­bone until its circumference is something near10 times as large as the wavelength, as in thecase for the sphere, then the upper limit of theinterference zone for a fish will be at least L/1I.= 200. Again, measurements have been madewhich indicate that the upper limit will be atleast this high (Haslett, 1969). Therefore, itmay be assumed that the limits of the interfer­ence region are at least 0.7 :(; L/1I. :(; 200.

If it is assumed that fish of interest to com­mercial fishermen range from 10 em to 150 em,and that fish-finding sonars have frequenciesranging from 10 kHz to 200 kHz, then the rangeof interest for fisheries applications will be 0.7 :(;L/>- :(; 200, the limits set for the interference

705

region. Therefore, although it would be advan­tageous to avoid the interference region, it isapparent that this is the region in which thework must be done.

METHODS OF TARGET STRENGTHMEASUREMENT

Analytical methods are of limited value in thisinterference region and experimental methodsmust be utilized to obtain any valid answers.It is possible to COJl(luct the needed experimentseither at sea or in the laboratory, with each typeof measurement having its limitations. Whetherthe measurements are made at sea or in the lab­oratory, target strength will be determined fromthe sonar equation, meaning that the source levelof the transmitter, the sensitivity of the receiver,and the propagation loss must be known. Thecalibration of the transmitting and receivingsystems is a standard procedure, but propagationloss is much more difficult to determine if longranges are involved. One way to avoid the prop­agation loss problem is to make measurementsat short ranges, and this is what is done in thelaboratory. This, of course, is impossible withlarge targets. Another method is to use a ref­erence target for which the target strength isknown. In this method neither the transmittingnor receiving systems have to be calibrated andthe propagation loss does not have to be measurerlbecause all echo levels are comparerl to the ref­erence level. One of the best reference targetsis a thin-walled air-filled rubber sphere, althoughfor large targets buoyancy becomes a problem.Another goorl reference target, which can beused for large targets, is a tri-plane, three mu­tually perpendicular planes, for which it can beshown that any incident ray wiII he reflected ina direction exactly opposite to the incinent rli­rection. Hence a tri-plane acts as a single Jllaneperpendicular to the incirlent rays anrl reflectsa large, calculable percentage of the incidentenergy. It is possible to measure propagationloss directly and this is fairly simple if a trans­mitting and a receiving ship are utilizen. Propa­gation loss measurements can also be mane byplacing a calibrated transponder in the vicinityof the target.

706

FISHERY IJlILLETIN: VOL. m. NO.4

Along with the problem of accurately measur­ing propagation loss, there are other problemsassociaten with target strength measurementsat sea, the most critical of these being relativemotion between the sonar beam and the fishtarget. Roll or pitch of the ship can be over­come by using a stabilized sonar beam, but driftcan cause the axis of the beam to move off target.Care must also be taken so that the target sup­port structure rloes not interfere with the mea­surements. Other problems that can arise arepoor weather, high ambient noise levels, and ex­traneous targets swimming into the beam.

Of course, there are problems associaterl withlaboratory measurements also, the chief onebeing the limitation on the size of the target.The fish must be placed at a range great enoughto insure that the incident sound energy is ap­proximately equal over the complete fish anrl toinsure that the fish is not in the near-field of thetransmitter nor the receiver in the near-fielrl ofthe fish. However, the range must not be sogreat that reflections from the bounrlaries or fishsupport interfere with the measurements.Nevertheless, by judicious choice of measure­ment range anrl by using short pulse lengths,unambiguous work can be done in a laboratorytank. In orner to obtain a true value of targetstrength the Jlulse lengths of the discrete fre­quency pulses most often utilized by fisheriessonars must he at least twice the length of thetarget in the direction of propagation, so thatan echo can be obtained from all parts of thetarget simultaneously.

A typical block diagram of the electronics re­quirer] for target strength measurements isshown in Figure 1. The transmitting systemconsists of a signal source of known frequency,a means to generate pulses, amplifiers, a trans­mitting transducer, a system to match the elec­trical imperlances of the amplifier and transduc­er, and a means to measure the outgoing signal.The receiving system consists of a receivingtransducer, amplifiers, a means to gate out un­wanted echoes, possibly a filter, and a means tomeasure the received signal. The electronicsystem is basically the same whether it is userlin the laboratory or at sea.

lf all fish were composed of the same homoge-

LOVE: MEASUREMENTS OF FISH TARGET STRENGTH

FIGURE I.-Typical target strength measurement systemelectronics.

neous material and had the same shape, it wouldtake a comprehensive measurement program todetermine the target strength of a fish at allaspects and frequencies because of the complexshape of the body. Since fish are definitely nothomogeneous and different species of fish havedifferent shapes and internal structures, theproblem of determining the target strengths forall species becomes immense. Considering thedifferences among individuals of the same spe­cies due to age, sex, condition of health, etc., it isobvious that an experimental program to pre­dict completely the target strength of any givenfish is impossible. Since the complete determi­nation of the target strength of all fish is im­possible, the most that can be hoped for is thatexperimental techniques will eventually lead toempirical results which can be generalized toapply to any species of importance.

RESULTS OF MEASUREMENTS OFINDIVIDUAL FISH

Most of the early experiments conducted dur­ing the 1950's investigated a specific situation.For example, if a researcher had an echo sounderwhich operated at a given frequency, he would

measure the dorsal-aspect target strengths of anumber of fish of the commercial species foundin his geographic area, in order to obtain anaverage dorsal-aspect target strength vs. fishsize for those species. This information wasvaluable to anyone designing or using an echosounder of the same frequency to find thesespecies, but it was of minimal value to anyoneelse.

A second technique used is just an extensiollof the earlier technique. With it, different spe­cies of fish have been examined at many aspectsand/or frequencies in attempts to determine howtarget strength varies with fish size, species,aspect, and frequency. Figure 2 shows sometypical results of this technique. The resultsare from a live 21-cm black crappie which wasrotated about its dorsiventral axis and insonifiedwith frequencies of 30 kHz and 130 kHz. Itis seen that the maximum target strength occursvery near the side aspect, where the insonifiedarea is a maximum. At 130 kHz the number oflobes in the pattern is substantially greater, and

FIGum; 2.-Targ-et strength of a 21-cm black crappieversus aspect. (a):lO kHz. (b) 1:10 kHz. (From Love,1969.)

707

the maximum target strength slightly greaterthan at 30 kHz. Although these measurementshave produced some m:eful results on the gen­eral changes of target Rtrength with fiRh sizeand frequency for aRpects of special interest,little has been learned about the fine-scalechanges or about the differences among species.

The use of a third technique permits investi­gation into the nature of the echo-formationprocess either by dissection or modeling. By dis­section, researchers have discovered which partsof a fish are the major acoustic reflectors. By re­moving the swim bladders from a number ofperch, Jones and Pearce (1958) determined thatthe gas-filled swim bladder accounts for approx­imately 50ji of the dorsal and side-aspect targetstrengths of perch at L/A = 1. Hence, the swimbladder is an important contributor to targetstrength for L/A values in the interference re­gion. By systematically removing various partsof a skipjack tuna, Volberg (1963) found thatappreciable echoes could be obtained from eitherthe skeleton or a piece of flesh. Diercks andGoldsberry (1970) have indicated the possibilitythat scales may also be an important contributorto the target strength of a fish at certain fre­quencies. Unfortunately, they did not removeany scales and their hypothesis is based on con­siderations of the directivity of the scales as anarray of scatterers.

An adjunct to the determination of the partsof the fish which are acoustically important isthe determination of the acoustic impedance orreflection coefficient of these parts. The reflec­tion coefficient is defined as it was previously fortwo semi-infinite media. The impedance of thegas in the swim bladder is readily determined,and the reflection coefficient for the swim bladderis approximately -1. Determination of theacoustic impedance of fish bone or flesh is dif­ficult and care must be taken to insure validmeasurements. Shishkova (1958) measured thedensity of and speed of sound in flesh from a fewspecies of fish and determined the reflection co­efficient in fresh water to be about 0.05. Haslett(1962b) used a different technique to indirectlymeasure the reflection coefficients of flesh andbone from haddock and cod. He found the re­flection coefficient of flesh in fresh water to be

708

FISHERY BULLETIN; VOL. 69, NO.4

about 0.05, in seawater to be about 0.02, and thereflection coefficient of bone to be about 0.25.

Using these values f~)r the reflection coeffi­cients and his "standard fish dimensions," Has­lett (1962c, 1961) haR modeled fiRh bodies, back­bones, and swim bladderR. Utilizing rubberellipsoids to model the fleshy body of the fish,he found that the number of lobes obtained inpolar plots for the ellipsoids and for actual fishagreed fairly well, that is, with less than r;Ojl,error, but that the target strengths obtained forthe modelR were considerably lower than thoseobtained for the fish. Using rubber and plasticcylinders to model the backbone and copper cyl­inders to model the swim bladder, Haslett hasexamined variations in the target strength ofthese models as frequency, size, and aspect arevaried. A brief summary of Haslett's work forside aspect is shown in Figure 3. Along withhis data for the acoustic cross-sections of stickle­backs and guppies, approximations to theacoustic cross-Rections of the swim bladder, body,and backbone are given. The various curves foreach component were determined by Haslett(1965) using his reflection coefficients and theresults of hiR modeling experiments and depend

N..J

"b

FlGUlm :l.-Side-aRpect acouRtic crosR-R('ctionR determinedby HaRlett.

LOVE: MEASUREMENTS OF FISH TARGET STRENGTH

on how the component is approximated and onthe limits of that approximation. Hence thecurves are a function of whether the swim blad­der is approximated by a spherical bubble or arigid cylinder; what the limits of the geometricand Rayleigh scattering regions are for the bodyand the backbone; and whether the body isapproximated by an ellipsoid or a plane in thegeometric region. These curves indicate thatthe swim bladder predominates over most of thegiven L/)... range, but that the body and backbonebecome significant at the higher L/>..'s. It isapparent that Haslett's measurements of theacoustic cross-sections of sticklebacks and gup­pies vary widely and do not follow any of hiscurves. This variability is not fully explainedby any of his experiments on fish or models.Obviously, the nature of the echo-formation pro­cess is quite complex if Haslett cannot explainthis variability after such comprehensive work.

In attempting to quantify fish resources theobjects of interest are usually fish schools ratherthan individual fish. When the target strengthof a school is measured, the question to be an­swered is "What is the average size and numberof fish required to produce this target strength ?"The answer will depend on the average targetstrength of the fish in the school, and any vari­ations among the individuals will be of minorimportance. If a forward-looking sonar is usedfor quantification, the minimum size and numberof fish required to produce a given targetstrength will occur at the aspect for which thetarget strength of an individual fish is a max­imum. This aspect will be near the side aspectof the fish. Thus, average values for the max­imum side-aspect target strength of an individualfish are important for quantification of fishschools with a forward-looking sonar. If a down­ward-looking sonar, or echo sounder, is used forquantification, average values for the dorsal­aspect target strength of an individual fish areimportant.

For these reasons the author has made max­imum side-aspect (Love, 1999) and dorsal-aspect(Love, 1971) target strength measurements asa function of fish size, species, and frequency.The measurements were made in the laboratory,and hollow rubber spheres were used as refer-

ence targets for calibration. It was found thatthe magnitude of the variation in target strengthfor one species was of the same order as it wasfor all species. Therefore the data for all spe­cies were combined with all other available per­tinent data and a regression line was calculatedfor each aspect using the method of least squares.

Figure 4 shows all the dorsal aspect data. Thedata were obtained using fish from 16 familiesin 8 different orders: Clupeiformes, Cyprini­formes, Gasterosteiformes, Cyprinodontiformes,Mugiliformes, Gadiformes, Beryciformes, andPerciformes. The fish ranged in length fromabout 1 cm to 1 m. Some had swim bladderswhile others did not. Insonifying frequenciesranged from 8 kHz to 1480 kHz. Note that theparameters used here are 0-/)...2 and L/)..., whichdiffer slightly from those used by Haslett. Theequation for the regression line calculated fromthese data is

and the dorsal-aspect target strength is

Tn = 19.4 log L + 0.6 log)... - 24.9 (10)

Equation (10) is for Tn at 1 m and Land>.. inmeters and is valid in the range 0.7 ~ L/)... ~ 90.

Figure 5 shows all the maximum side-aspectdata. The data were obtained using fish from13 families in 7 different orders: Cypriniformes,Gasterosteiformes, Cyprinodontiformes, Gadi­formes, Beryciformes, Perciformes, and Pleuro­nectiformes. Fish size and acoustic frequencyranges were approximately the same as those fordorsal aspect. The equation for the regressionline calculated from these data is

0-/)...2 = 0.064 (L/)...)228, (11)

and the maximum side-aspect target strength is

Ts = 22.8 log L - 2.8 log>.. - 22.9 (12)

Equation (12) is valid in the range 1 ~ L/>..~ 130, and again T s is at 1 m and Land>" are inmeters.

Figure 6 is a nomogram solving equations(10) and (12), given the acoustic frequency, I,in kHz, and the fish length, L, in em.

709

FISHERY BULLETIN: VOL. 69, NO.4

FIGURE 4.-D a r s a I-aspectacoustic cross-section of anindividual fish.

0-/),2. 0.06"( L/>.)ua

+

o

LEGEND

o YUDANOV, GAN'KOV,AND SHATOBA (1966)

6 HASLETT (1962 d,1965,1969)

o VOLBERG (1963)

Ll DIERCKS ANDGOLDSBERRY (1970)

v HASHIMOTO ANDMANIWA (1956b)

Co SMITH \195")

t>. JONES AND PEARCE (1958)

+ LOVE (1969)

lOS ..-

102.. 6-<"-b

6

10

+

10'

710

LOVE: MEASUREMENTS OF FISII TARGET STRENGTH

10· r---,----,-----,r--,---....--,--,---T"---,-----,---,---.,

tTI),2 '0.041 ILl),)"·

•• • •.,. • l>. ~

••• •+

+ -b....+

+• •

+

0LEGEND

o YUDANOV, GAN'KOV, 8AND SHATOBA 119661 0

l> HASLETT 11962d,I965l@l

'\00 VOL BERG 119631

<)v

SHISHKOVA 119641

v HASHIMOTO ANDMANIWA 11955.19560.bl

r> SMITH \19541 <)

<l MIDTTUN AND HOFF\l9621

b. JONES AND PEARCE \19581

K McCARTNEY ANDSTUBBS (1970)

+ LOVE (1971)

10

10-1

N-<"­b

..FIGURE 5.-Maximum side­aspect acoustic cross-sec­tion of an individual fish.

<)'

10

L/),

10-5 '----'----'-_....L_-1__-1__L.._..L-__..L-_....L.._--l.__--l._-J

10-1

711

FISHER Y BULLETIN; VOL. 69, NO.4

QUANTIFICATION ANDIDENTIFICATION OF SCHOOLS

FIGURF; 6.-Nomog-ram for calculating the dorsal-aspectand maximum side-aspect target strengths of an indi­vidual fish.

Since estimates for the maximum side-aspectand dorsal-aspect target strengths of individualfish are available. the determination of thetarget strengths of fish schools at these aspectswill depend on the determination of the effectsof the number of fish in the school and theirdistribution. If the fish are widely spaced, orin a plane perpendicular to the sonar beam, sothat there is no acoustic interaction among theindividuals, the target strength of the school isequal to the average target strength of an indi­vidual plus 10 times the logarithm of the numberof fish. The probability of finding a school thatmeets these qualifications is quite small andtherefore the effects of interactions among thefish must be taken into account.

Little experimental work on the acoustic inter­actions of fish in a school has been done, althoughsome measurements of the target strengths ofgroups of fish have been made, usually withlittle concern for the distribution of the indi­viduals (e.g., Thorpe and Ogata, 1967; Shishko­va, 1960). Some theoretical work on distribu­tions of scatterers has been done, the scatterersusually being point scatterers or small bubhles(e.g., Foldy, 1945; Weston, 19()()). Weston(1967) has applied the results for bubbles to fish

'0 schools and has estimated reflection coefficientsfor regions well-helow and well-above resonance.Since there is no interference region for a bubble,he does not concern himself with interferenceeffects, and his results are of limited value in theinterference region. Boyles (1969) has dis­cussed the mathematical theory of multiple scat­tering from fish schools, but to obtain resultsin the interference region the complete spatialscattering and absorption pattern of an individ­ual fish must be known.

The identification of a Rchool of fish utilizingtarget strength information is obviously muchmore difficult than its quantification, and sinceit is not yet possible to quantify fish schools withthis information, it is surely not yet possibleto identify them.

Figure 3, which summarized Haslett's work,seems to indicate that no reasonable pattern oftarget strength vs. frequency can be found forany species due to the rapid fluctuations of targetstrength. Haslett's measurements were madeat three widely spaced frequencies. Measure­ments made by the author at a larger numberof more closely spaced frequencies indicate thatfor individual fish these fluctuations are not sorapid and that possibly individual fish may beidentified through the use of target strength vs.frequency (a-/L2 vs. L/"A) curves. Dorsal aspecttarget strength measurements made on six bayanchovies, Anchoa mitchiUi, one Atlantic men­haden, Bl'crool'tia tyrannus, five goldfish, Caras­sius alll'atus, and six Atlantic silversides, Menid­in menidia, revealed that all of these fish hadsimilar a-/U vs. L/"A curves (Love, 1971). Themost notable feature of these curves is a deepminimum in the neighborhood of L/"A = 10. Thisminimum is easily seen in the average curvesfor each species shown in Figure 7. Similarmeasurements on three mummichogs, Fundulushetel'Oclitus, five striped killifish, Fundulusma.ialis, six black crappies, Pomoxis ni,ql'omacu­latus. and four spotted seatrout, Cynscion nebu­losus, revealed that the a-/U vs. L/'A curve forany individual of these species bears no easilydiscernible relation to that of most, or all, ofthe other individuals of that species, or to theaverage curve for that species.

The anchovies, goldfish, and menhaden are

500

2 ,

5 .

",.

....00·

1000 :

~ ~o .

g .

~ 20 •

r IOO :

TARGET STRENGTKSOF AN

INOIVtDUAL fiSH

FOR DORSAL ASPECT,100 ~ L f ~ 14000

fOR SIDE ASPECT

150.., Lf.., 20000

• -50

•· -40 j:;•

·0 m

'c.I

+ -~ ~

>­• -20 :i

~

·500

.: 10

-50, • 5

,: 100-:

: ~-30 +. z

.~Ij

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wo~

: 1000

-10·

-20' • 200

712

LOVE: MEASUREMENTS OF FISH TARGET STREl'GTlI

SUMMARY

aiU near L/A = 10, or why the acanthopter­ygians and the other two intermediate specieshave no distinctive (T/U vs. L/A curve cannotbe answered, given the present limited knowl­edge of echo-formation by fish.

Although these differences cannot be presentlyexplained it seems probable that if there were ag-eographic area in which two species with aboutthe same size and habits predominated, and ifone species were a Clupeiform and the other aPerciform, a ship with a wide-band sonar coulddifferentiate between individuals of each specieshy examining their target strength vs. frequencycurves. This hypothetical example indicates howvery limited the present capability to identifytlsh by determining target strength is. Hope­fully, more measurements at many frequencies,with clissection and removal of various compo­nents of the fish, and more sophisticated moclel­ing techniques will explain the features of thetarget strength vs. frequency curves for indi­vir!uals of a few species. This could then lear!to the prediction of curves for other species,which in turn could greatly increase the abilityto clifferentiate between individuals of differentspecies. This information could then be appliedto the r!ifferentiation of schools of different spe­cies, althoug-h it is to be expected that the man­ner of distribution of the fish in the school willcause significant differences between the targetstrength vs. frequency curve obtained for theschool and the averag-e curve for the individualsin the school.

Some of the more important results of mea­surements of fish target strength to date are:(1) it has been determined that practically everycase of interest to the fishing inclustry is in theregion of interference effects, (2) the major con­tributors to the target strength of a fish in thisregion have been determined and their acousticimpedances measured, (3) the variations of tar­get strength with aspect for an indiviclual fishhave been examinecl, (1) estimates of the dorsal­aspect and maximum side-aspect target streng,thof an individual fish have been made, (!l) there

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malacopteryg-ians, the more primitive teleosts;the crappies and seatrout are acanthopteryg-ians,the more advanced teleosts; and the mummi­chogs, killifish, and silversides belong to inter­mediate orders which have characteristics ofboth groups (Berg, 1917; Bertin and Arambourg,1958). In general, the malacopterygians havephysostomous swim bladders, osseous bone tis­sue, intermuscular bones, comparatively manyvertebrae, fins without spines, and cycloid scales.In g-eneral, the acanthopterygians have physo­clistous swim bladders, osteoid bone tissue, nointermuscular bones, comparatively few verte­brae, fins with spines, and ctenoid scales. Con­sidering that the swim bladder, bones, andpossibly scales of a tlsh contrihute to its acousticcross-section, it is obvious that the malacopter­ygians and the acanthopterygians have signifi­cant structural differences in components whichhave been shown to be acoustically important.Why the malacopterygians and one intermediatespecies display the characteristic minimum in

N~

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FIGI1RE 7.--Avl'rag-1' Jnl'asurl'll dorsal-aspl'ct acollstil'cross-sections for different species of fish. (From Love,

1971.)

713

are indications that the identification of individ­ual fish based on target strength vs. frequencycurves is possible for limited cases.

The two goals of fish target strength measure­ments, namely quantification and identificationof fish schools, have not yet been attained. Thequantification of fish schools and the identifica- .tion of many individual fish should be hopefullyaccomplished in the next few years. The iden­tification of schools will require the informationon quantification of schools and identification ofindividuals, and therefore will probably not beaccomplished for some time.

LITERATURE CITED

BERG, L. S.1947. Classification of fishes, both recent and fossi!.

J. W. Edwards, Ann Arbor, Mich. 517 p.BERTIN, L., AND C. ARAMBOURG.

1958. Supre-ordre des Teleosteens. In P.-P. Grasse(editor), Traite de Zoologie, Agnathes et Poissons13: 2204-2500. Masson et Cie., Paris.

BOYLES, C. A. .1969. Preliminary report on the mathematical the­

ory of the multiple scattering of an acoustic pulsefrom a random collection of volume scattererswith application to scattering from fish schools.Tracor, Inc. Doc. RL/69-074 U, Rockville, Md.,83 p.

CUSHING, D. R., F. R. H. JONES, R. B. MITSON, G. H.ELLIS, AND G. PEARCE.

1963. Measurements of the target strength of fish.J. Br. Inst. Radio Eng. 25, p. 299-303.

DIERCKS, K. J., AND T. G. GOLDSBERRY.1970. Target strength of a single fish. J. Acoust.

Soc. Am. 48(1, Part 2): 415-416.FOLDY, L. L.

1945. The multiple scattering of waves. I. Generaltheory and isotropic scattering by randomly dis­tributed scatterers. Phys. Rev. (Second Series)67: 107-119.

HASHIMOTO, T., AND Y. MANIWA.1955. Ultrasonic reflection loss of fish shoal and

characteristics of the reflected wave. Tech. Rep.Fishing Boat No.6, p. 113-139. [English trans!.:Ministry of Agriculture, Fisheries and Food, Fish­eries Laboratory, Lowestoft, England (1957)].

1956a. Study on reflection loss of ultrasonic waveon fish-body by millimeter wave. Tech. Rep. Fish­ing Boat No.8, p. 113-118. [English trans!.:Ministry of Agriculture, Fisheries and Food, Fish­eries Laboratory Lowestoft, England (1957)].

714

FISHERY BULLETIN: VOL. 69, NO.4

1956b. Study on reflection loss of ultrasound of mil­limeter wave on fish-body, (2). Tech. Rep. Fish­ing Boat No.9, p. 165-173. [English trans!.: Min­istry of Agriculture, Fisheries and Food, Fish­eries Laboratory, Lowestoft, England (1957) J.

HASLETT, R. W. G.1962a. Measurement of the dimensions of fish to

facilitate calculations of echo-strength in acousticfish detection. J. Cons. 27: 261-269.

1962b. The back-scattering of acoustic waves inwater by an obstacle II: Determination of thereflectivities of solids using small specimens.Proc. Phys. Soc. 79: 559-571.

1962c. Determination of the acoustic scatter pat­terns and cross-sections of fish models and ellip­soids. Br. J. App!. Phys. 13: 611-620.

1962d. Determination of the acoustic back-scatter­ing patterns and cross sections of fish. Br. J.App!. Phys. 13: 349-357.

1964. The acoustic back-scattering cross sectionsof short cylinders. Br. J. App!. Phys. 15: 1085­1094.

1965. Acoustic backscattering cross sections of fishat three frequencies and their representation on auniversal graph. Br. J. App!. Phys. 16: 1143-1150.

1969. The target strengths of fish. J. Sound Vib.9: 181-191.

JONES, F. R. H., AND G. PEARCE.

1958. Acoustic reflexion experiments with perch(Pe1'ca fluviatilis Linn.) to determine the pro­portion of the echo returned by the swimbladder.J. Exp. Bioi. 35: 437-450.

LOVE, R. H.

1969. Maximum side-aspect target strength of anindividual fish. J. Acoust. Soc. Am. 46: 746-752.

1971. Dorsal aspect target strength of an individ­ual fish. J. Acoust. Soc. Am. 49: 816-823.

MCCARTNEY, B. S., AND A. R. STUBBS.

1970. Measurements of the target strength of fishin dorsal aspect, including swimbladder resonance.In G. B. l<'arquhar (editor), Proceedings of theInternational Symposium on Biological SoundScattering in the Ocean, p. 182-214. Maury Cen­ter for Ocean Science Report 005, Preliminary ed.

MIDTTUN, L., AND 1. HOFF.

1962. Measurements of the reflection of sound byfish. Rep. Norw. Fish. Mar. Invest. 13 (3), 18 p.

SHISHKOVA, E. V.1958. An investigation of the acoustic properties

of fish. Tr. Vses. Nauchn.-issled. Inst. Morsk.Rybn. Khoz. Okeanogr. 36: 259-269. [Englishtrans!.: Associated Technical Services, Inc., EastOrange, N.J. (1960).J

LOVE: MI':ASlJREMENTS OF FISH TARGET STRENGTH

1960. Sound reflecting capacity of pelagic and ben­thic fish. Rybn. Khoz. :l6 (10): 56-6:l. [Englishtrans\.: in Translations on USSR fishing industryand marine resources No. 12, Clearinghouse Fed.Sci. Tech. Inf., JPRS 44,174 (1968).J

1964. Study of acoustical characteristics of fish.In Modern Fishing Gear of the World 2, p. 404­409. Fishing News (Books) Ltd., London.

SMITII, P. F.191>4. Further measurements of the sound scat­

tering properties of several marine organisms.Deep-Sea Res. 2: 71-79.

THOItPf;, H. A., AND C. T. OGATA.

1967. Hydroacoustic reflections from captive an­chovies. Lockheed Calif. Co. Rep. 20989, San Di­ego, 45 p.

VOLBERG, H. W.196:3. Target strength measurements of fish.

Straza Industries Rep. R-I0l, El Cajon, Calif.,146 p.

WESTON, D. E.1966. Acoustic interaction effects in arrays of small

spheres. J. Acoust. Soc. Am. :39: 316-322.1967. Sound propagation in the presence of bladder

fish. In V. M. Albers (editor) Underwater acou­stics 2 (Ch. ,,): 55-88. Plenum Press, New York.

YUDANOV, K. I., A. A. GAN'KOV, AND O. E. SHATOBA.

1966. Reflection of sound by commercial fish of theNorthern Basin. Rybn. Khoz. 42 (12): 57-60.[English trans\.: Clearinghouse for Fed. Sci.Tech. Inform. Document TT-67-61940.]

715