Physics applied to echo sounding for fish

PHYSICS APPLIED TO ECHO SOUNDING FOR FISH by R. W. G. HASLETT*

The problem different especially whilst

the

Account

design

must

of fish by ultrasonic

Physical

in relation

surprising This

of detection

subjects.

acoustics

to the propagation

of the

instrument

also be taken

paper

examines

recent attempts

the

needs both methods

research

application

MEDIUM

One important factor affecting the accuracy of any calculations is the degree of homogeneity of the medium. The velocity of sound in sea water, c, depends on temperature, salinity and pressure. At the surface, with a temperature of 39°F and a salinity 35 parts per thousand, the velocity is 4,800 ft/s.l A rise in temperature of 7”F, an additional salinity of 13 parts per thousand or an increased depth of 2,700 ft each cause the velocity to rise by about 1%. The velocity of sound in water has however been found to be independent of frequency up to 22.5 MC/S.’ Regions of homogeneous water usually lie in horizontal strata so that any gradients in temperature, salinity or *Kelvin

Hughes

Division

of S. Smith

and

Sons

(England)

Ltd.

a number

of widely

in the detecting

and

and the biology

remains

system,

characteristics

mechanical of fish.

of fish,

engineering.

Thus,

it is not

to be done in this field.

of physics to

I

OF THE

electrical

to put the subject on a more formal

n acoustic fish detection a short pulse of ultrasound is projected through the sea water and echoes are received from various discontinuities in the medium such as fish and the sea bed. Over the years, advanced techniques have been developed to detect echoes from single fish at great depths. This is difficult, since a fish is a small target and only a fraction of the incident sound is reflected. Information about the echoes from fish must also be displayed with sufficient resolution for them to be interpreted readily. In spite of the complexity of the situation, detailed investigation of the various effects leads to a better appreciation of how the observed echoes are formed so that more information may be obtained about the quantity and size of fish.

CHARACTERISTICS

embraces part

of sound and the scattering

of fishing

to find that considerable

echo-sounder

plays an important

the

problem

quantitative

and

describes

some

basis

pressure are likely to occur in the vertical direction. The most common cause of refraction is a temperature gradient near the surface. If a constant velocity gradient occurred in the vertical direction, a sound ray would take the form of c where c is the velocity of gsin6 sound in ftjs and 6 is the angle between the direction of propagation and the vertical (both measured at the transmitting transducer), whilst g is the velocity gradient in ft/s per foot of vertical height. For example, if there were a continuous temperature gradient as high as 1°F per 10 ft of depth (temperature and sound velocity decreasing with depth), a ray projected at 15” to the vertical would be refracted downwards into an arc of radius 27,000 ft, but if the sound were launched horizontally (6 = 90”) the radius of curvature would be 7,000 ft. On the other hand, if the water were isothermal, a ray starting at 15” would be bent upwards into an arc of a circle of radius 39,000 ft by a change in salinity of 13 parts per thousand spread uniformly over 100 ft (salinity and sound velocity increasing downwards). It is seen that these effects are relatively slight in vertical sounding (where 6 is small), but in horizontal ranging the errors would become more pronounced. In deriving a systematic method of interpretation of the echoes, these unpredictable factors may be neglected and the beam patterns of the transducers assumed to suffer no unusual effects. There is a considerable body of evidence3 that this applies with reasonable accuracy to vertical sounding in good weather, e.g. to sea-bed trawling, which is mostly undertaken in waters between 50 and 200 fathoms deep (300-I ,200 ft). an arc of a circle of radius

ULTRAsONICS/_fanuury-March

1964

11

Frequency,

is seen in Fig. 2, in which the axis of the main lobe (X m=0) is perpendicular to the face: x increases with 0. The first side lobe has a maximum Do mm21.7% and the second Do = 12.7x. These side lobes may be reduced by “tapering” either the shape of the radiating face or the power excitation over it. If the length of the transducer were increased, the width of the main lobe would decline in approximately inverse proportion. A scale of 6 has been added to Fig. 2 for l-7 3h, when the total width of the main lobe is seen to be 39”. The width between points 3 dB down compared with the maximum is 17”. Transducers whose characteristics approximate to those of a plane circular piston have also been widely used in the past. For these transducers

c/s

Fig. I. Attenuation of acoustic waves in water; its dependence on frequency. Graph for sea water after Reference 1: graph for pure water is due to Schulkin and Marsh (Reference 4). The absorption found in pure water over and above that due to viscosity is thought to result from a structural relaxation

As to the question of the attenuation of sound in sea water, this is found to increase rapidly with frequency’ (Fig. 1). Thus at a frequency of 30 kc/s (which is often used for fish detection), the total loss suffered by the sound waves in travelling to a target at a depth of 200 fathoms and back would be 5.6 dB, whilst at 300 kc/s the loss would be 80 dB. This attenuation is largely due to the viscosity of water, but there is an additional term at low frequencies which Schulkin and Marsh4 attribute to dissolved magnesium sulphate. It might be thought desirable to raise the operating frequency so that the size of the transducer required to give a certain acoustic beam angle would be reduced and the effectiveness of the fish as a reflector of sound would be increased. However, if the frequency is raised, a considerable increase of transmitted power is required and the intensity over the smaller radiating face of the transducer may become so large that cavitation of the water sets in. The resulting vacua fill with the air dissolved in the water and bubbles are formed. The onset of cavitation is characterized by a disproportionately small improvement in the size of the received echo for a given increase in transmitter power and by distortion of the acoustic waveform. The limiting intensity is about 6 W/cm2 at a frequency of 30 kc/s with a pulse length of 0.5 ms. This intensity may be increased somewhat if shorter pulses are used or if the water is flowing past the transducer.

J, is the Bessel function of the first order and N is the radius of face. The maximum of Do for the first side lobe is l2.7”<, and for the second, Do - 6.4q;. The performance of a transducer in concentrating the acoustic energy is usually given in terms of its directivity factor.5 This is the ratio of the total power actually radiated over all directions to that which would be radiated if the transducer had a uniform response in all directions, equal to that at the maximum of its main lobe. The transducer is assumed to be excited by sinusoidal waves at a certain frequency. A similar factor may be defined for a receiving transducer in which the incident sound is assumed to be of equal intensity from all directions. In the choice of optimum widths of beam for detecting fish, the angles of roll and pitch of the ship must be considered. Thus the beam cannot be made very narrow or some echoes from fish will be missed. A transducer introduced recently6 with beam angles as narrow as 13” athwart-

TRANSDUCERS

To obtain a high efficiency, a resonant transducer is used, made of a material which may be magnetostrictive, piezoelectric or electrostrictive. Examples of each type are laminated nickel, quartz and barium titanate respectively. The Q for the resonance is typically about 15. The underwater acoustic beam pattern depends largely on the size of the radiating face compared with the wavelength in water, /\, and on its shape. The acoustic pressure at angle 0 off the axis of the beam may be given as a ratio DB compared with the axial pressure, both measured at the same range in the far-field region. For an ideal rectangular radiating face, uniformly excited in phase (which is most frequently used): Do = !!!? where x = xlx sm 0 X

and I ~~ length

12

of transducer.

The graph

ULTRASONICS/J~~~~~~-M~~~~

of D, against

1964

x

I

-90

-40-30-20

-10 0 IO 20 30 40 B.degrees of angular rotation

60

Fig. 2. Acoustic beam pattern of a theoretical transducer with a rectangular radiating face. The scale of 0 is that for 1 3X. Overall response for two such transducers, one transmitting and the other receiving

Table 1.

BACK-SCATTERING I

Aspect

.Body

___I

Entirely convex surface Sphere __Ellipsoid

CROSS-SECTIONS OF THE FRONT

,--

a, and a2 are the two principal

I

Any

nala, y2

I

Any

aa2-q2

Broadside

rL2H2.K 2

~ I

~ “x’:‘::11,t I

a’X cos 4 sin2(kl sin $):&a 277

-sin2-

~~

ka’/2,&2 = 2ra’/23 2 x

~

I rectangular plate,

Broadside (4 = 0)

1 4 is the angular deviation from broadside aspect I is the length and a’ the radius of the cylinder k + I One side is perpendicular to the incident direction, the other side (of length b) makes an angle + with the incident direction A is the area of the plate

inclined

Large plane

and

B is overall breadth

4LZ

/

radii of curvature reflection coefficient

1 L is overall length

.rrH2B2~ =

Axis inclined Broadside (4 = 0)

and q is the amplitude

4Bz

I

REGION

Nomenclature

Cross-section (4

End-on

Short circular cylinder

SURFACES OF SIMPLE BODIES IN THE GEOMETRICAL

4xA2.( 2 x2

ships (between 3 dB points) and 8.5” in the fore-and-aft direction (4h x 6h) has been found to be satisfactory in this respect. When the face of the transducer is open to the sea, the measured beam pattern approaches reasonably closely to that calculated from theory. If a metal plate is used as an acoustic window, the beam is disturbed and the pulse is lengthened by reverberation between the plate and the tank in which the transducer is mounted. To obtain the overall beam pattern of the echo sounder (which determines the angular resolution), the DB for transmission is, of course, multiplied by D’* for reception. If these two beam patterns are similar, the side lobes of the combination become considerably less significant (see the dotted line on Fig. 2).

The amount of acoustic energy entering the water also depends on the efficiency of the transducer. An approximate simplified equivalent circuit of a polarized magnetostrictive transducer is seen in Fig. 3.’ When the transducer is transmitting, the mechano-acoustic efficiency at resonance

R2 ~~~~ and the electro-mechanical efficiency ~~-~ R, +- R, R, efficiency of rl EP== R, + R, + R, The overall transmitting the transducer is qar,,vEX. In the reverse direction, when the transducer is receiving, R’ R’S qleM :.: where ~‘Blx= ~~~- ~~ and R’, + R’, + R’, R’, i_ R’, %A

=

R’ = R7R’sf$ (being the combined resistance of R’, and aT 4 R’, in parallel). These constants may be determined by a.c. bridge measurements and are found to have somewhat different values according as the transducer is transmitting or receiving. These differences occur as a result of the change in power level.

TARGETS

Fig. 3. Approximate equivalent circuits of a magnetostrictive When transmitting, the various components are: L, and C,. Mechanical resonance R,. Mechanical losses R,. Radiation resistance L?. Electrical inductance C,. Stray electrical capacitances and capitance tune L, to resonant frequency The same terms (now dashed) then R’, is the receiving load

apply to the transducer

applied

The amount of energy reflected (or scattered) back to the transducer by a target may be expressed in terms of its back-scattering cross-section, (T. As in radar, this is the plane area intercepting an amount of energy which. if it were scattered equally in all directions from the target, would produce the echo actually observed. (This area is placed at the same position as the target, perpendicular to the incident waves, which are assumed to be plane and of uniform intensity.) The values of 0 for a number of simple bodies may be calculated8~gJ0 when their dimensions are large compared with the wavelength in water, i.e. in the “geometrical region”. Such values are given in Table I, in which y< is the amplitude coefficient of reflection at a plane interface between the target material and water.

transducer.

externally

to

when it is receiving;

H

ULTRASONICS/

January-March

1964

13

Thus.

the

target

diagram

of

becomes

very

lengths to

that

The

around that

diagram range

a

in

to

three

when

the

have

a polar

dimensions,

which

target

of energy

radiating are

the target

is many

wave-

is broadly

transducer many

related

of

similar

lobes

in

twice

as

as for

the transducer.

n varies

the size and

with

In

ideal

in (7 with

practice.

uniform range.

shape

since

over

plane

of the target

the

waves,

especially

there

at short

Having

As

has

OF

been

depends

on

amplitude

VARIOUS

seen the

in

a small

with

waves

is some

whereb”,,

of

between

a material

of

(acoustic

impedance

Z,).

and coefficients T.)ble 2,ll.“.l” ‘

of

for

material.

at ;I plane

impedance

Z,

z,

Z,

%

(dB/cm) (cm)

for

(v,,)!

The

and

water

axial

Rubber

This substances

arc

i ii

seen

with

IY

width the

I 0”

I.6

ii+;h I 0” I 05 7.7 IO” (approz.) 3.06 105 IO” 2-5 I-5

eraser)

I .60 I .55 I.46 I ,54

I 0 :z

fact

whilst

the

range

from

68 6X .T5 26 I I

the

heavy

materials

metals.

I ,9 032

with

in

Thus

reflectors.

acoustic The

presence

would ‘I< ::

increase IOO”,, many

used

to adjust

in phase from

represented

the for

fish of

flesh

bubbles, to

strength In

of

this

’ 1 There

a “pressure-release” by the

negative

of

the

swim-bladder is, however,

material, sign

in Table

poor

centre

of a pulse

PII1 Sk

I.l:NGrH

pulse

is small

compared

e.g. air, 2.

from

factor

the

conditions

and from

target

the above

equation

be applied

with

the

transduce1

intensity

at

area arc

variation

namely

directions.

of the

determines

the

the

the

and as receiver)

is the capture

to

rhr

in the water,

transmitting

ideal

applies

In

attenuation

be on the axes

~1427 gives

Since

assumed.

of

from

unit

of the

intcnhity

transmitting

to receiving

transducer.

concerns

continuous

reasonable

of many

accuracy

wavclcngths,

to the

say

IO.

HA~DU’II~‘I’lI

T. matches

which

best

the

back

not

h”i4~(7,‘,,)~

law

for

cZ’/4~(71,,),

the

is largely

overall

optimum

bandwidth

signal/noise

ratio

a pulse

bands

determined

cnvclope

by

idth

the

(between is

of the

Q

of

the

3 dB points)

I .Z T. and thi5

B

gcncrally

of

the

I‘~~rm 01‘ ;I

curve.‘” of 2.4 kc/s gives

a bandwidth

0.5 ms (I 5 wavelengths).

resolution

SUMI- OtSeveral One

a fish since

be noted which

is

reflector and

range

length.

in

may

expression

consisting

The

and

;.I fish.

(as transmitter

the target

ANI)

R.

the

incident

a difference

a rigid

to the target

to target

but it may

reflector.

echo,

it may

from

are

within

acoustic

echoes

light

the

good

of air

connection,

an air-filled

receiving

(CT/~)

a pulse

Ic.ngth 01‘

and the corresponding

between

two

C RI-3

1 Ht-

identical

targets

i\

range 14 in.

e.g.

soft,

water

little

its

contain between

whilst of

amount

buoyancy.

180’

that

diff‘er

of sound.

is a moderately

a small

air.

densities

reflectors

reflects

but tish bone of even

that

and

fish

energy,

whose

are good close

H the

the

account

both

tiaves

for

materials

densities

with

to

terms

various

the target.

transducer.

I 0 :a

air

at angle

refer

example

speaking.

hystcm,

’

rocks,

terms

which

square

Strictly

x

46 3 ,o

1;‘;

of water

factor compared

for

transducer.

results

speaking.

directivity

to a target

unit

At 30 kc/s

that

pressure

beam.

The

range

about

from

efficiency

of the transducer.

the

similar

transducer

The

Generally

efficiency

applies

that

at

Gaussian

greatly

water

factor

of the transducer

beams.

with

100 94 9’ x3 x2 81 67 66 73 24

94 9’ x4 x3 x2

I ,7

Wet fish Hcsh ,IC rubber Tap water at I5 C‘ Sea water (salinity: 35 parts per 1000. IS<‘)

in the

target

of

travelling

the

two

WATFK

100

I 0:I

the

transducer

dashed

equation the

and SURSIANC’CS

IO”

I .x1

attenuation

of

power

transducer

transducer.

41

(pencil

the

ray

equivalent

receiving

7.6

in

across

electrical

transmitting

range

off the axis

an inverse

Perspex Wet tish bone

nob%

instant~ineously

connected

is the acoustic

is the

is the relative

intensity

i-x

received

are known::’

instantaneous

the

is the directivity

D,

interface

and

VARIOUS

4.7

(RI)

T,~,.~ is the electro-mechanical

Table 2

Air Steel Brass Aluminium Granite Quart7 Clay Sandstone

the

by

the transmitting

of sound

OF

echo

U) may

variables

dissipated

resistor is

I

numerator,

REtLE(‘TIVITIES

the

cross-section

transducer

(watts)

efficiencies

A(x)usr~c~

factors,

is the power

load

receiving V’

variation

is

various

(watts)

its

depart

cross-section

target ‘I<,

%,

acoustic

all the system

accepted

acoustic

the

Z,

various

(of

7jhr, is the mechano-acoustic

reflection. acoustic

when

the

ranges.

1. the

reflectivity

coefficient

the

target

be calculated

MATERIALS

Table

EQUATION

considered

from

and KEF LE~‘TIVITII~S

ACOUSTIC

polar

the same

and

incident

GENERAL

shape.

the

of angles.

orientation. from

considered

distribution

there

for

be

scattering

complicated

long.

except

may

back

this

I tlli

PEA1

features

in the echo

transducer

is often

reception.

After

amplification \oon

as possible.

levels

of

Ggnal

may

acting

the

SOL’N\rl)f-K

sounder used

may

for

both

receiving

amplifier

transmitter

and

than

reception

be mentioned. transmission

of the

transmission though

to are

b(‘IIO

even

be more

electronic

Echoes

the

of the

transmission is

IN

the of

must

pulse.

the

is achieved

in the

minimum

The by

full

be regained

difference

170 dB.

and the

detectable

change-over means

as

power

of

from a fast-

switch. usually

displayed

as ;I permanent

record

011

Tlmc

marks

at

intervals

of

lmln

A

single

fish

aft

Ilnitlol

edge

of

sea-bed

echo/’

Fig. 4. A typical chart from a recording scale-expander showing numerous fish echoes within 8 ft of the sea-bed echo. The initial edge of the sea-bed echo is at the foot of the chart. The fish were cod and haddock in a depth of 100 fathoms

a roll of paper by a pen moving with constant velocity. The signals are amplified and pass via the pen through the paper, which is chemically treated. The resulting electrolysis gives an intensity modulated record. The pulse is transmitted in synchronism with the motion of the pen across the paper, which is the time-base representing depth, e.g. 150 ms is equivalent to 60 fathoms. The paper is driven slowly lengthways so that consecutive scans of the pen are closely adjacent or, indeed, may overlap to some extent. This gives a valuable feature known as “trace-to-trace correlation” whereby the observation of a faint echo can be made more readily. It may be shown that a regularly recurring echo may be observed against a random noise background even when the echo has a signal/noise ratio less than one. The improvement in detectability is about 2.4 dB each time the number of correlated echoes is increased twofold.r6 Cathode ray tubes are also used for displaying the echoes during sea-bed trawling,*’ when only the echoes from fish within about 8 ft of the sea bed are required; 8 ft is the height of the mouth of the trawl. The range scale can be greatly expanded if the triggering of the time-base is delayed until an instant corresponding to 8 ft short of the sea bed. When this is done, the length of the time-base need be only 3.3 ms, i.e. the time taken for sound to travel 8 ft and back. A c.r.0. scale-expander such as this displays the highfrequency envelopes of the echoes from fish. Moreover, fish echoes may be resolved when in close contact with the seabed echo, owing to a difference between the rates of rise of these two echoes.17 This gives a resolution better than the theoretical CT/~. In rough weather, there is some loss of trace-to-trace correlation owing to vertical motion of the trawler. In an equipment introduced recently, this difficulty has been overcome by locking the displays to the sea-bed echo.” The fish echoes are recorded on a magnetic drum and replayed after the arrival of the sea-bed echo, which is used to trigger the time-bases. Since the range of interest is only a small fraction of the total range, the signals can be replayed many times in the waiting period, to give a virtually continuous and steady picture on the cathode ray tube. At the same time, a high-speed recording scale-expander has been introduced,6 in which the rotary movement of the pen is triggered by the sea-bed echo at intervals of 0.6 sec.

From rest, the pen experiences an acceleration of 4OOg, moves across the paper under its own momentum at almost constant velocity and is braked by eddy currents to complete exactly one revolution. The time taken to cross the paper (6 in wide) is 10 ms. A typical chart is shown in Fig. 4.6 This chart was obtained with a transmitter power of 8 kW and a narrow acoustic beam (4X / 6h). When the beam from the ship passes over a fish, the apparent range of the fish changes so that a crescent shaped trace is recorded.ls~ls As can be seen in Fig. 5A, the fish appears to move along the line YZ relative to the ship, T, and the range of the fish has a minimum, TF, when it is on the vertical beneath the transducer, TS. (The trace is really half a hyperbola on curvilinear coordinates.) If we ignore side lobes, the acoustic beam approximates to an elliptical cone, TCD. Hence, the time taken for the fish to pass through the beam and thus the number of consecutive echoes received from a fish, are both proportional to its depth and inversely to the speed of the trawler. During sea-bed trawling, as can be seen from Fig. 4, separate echoes from single fish are observed for the great majority of the time. These displays allow detailed examination of the amplitudes and characters of echoes from fish.

GEOMETRY OF FISH, MOUTH OF TRAWL AND ACOUSTIC BEAM

Certain aspects of the geometry are worth noting. In the fore-and-aft direction of the ship (Fig. 5A), fish above the head line (the top of the mouth of the trawl) will not be caught. The trawl is, of course, some distance behind the ship, but the projected position of the head line H is relevant when deciding which fish are likely to be caught. Not all the fish which appear to be between the head line and the sea bed by reason of their ranges will pass into the trawl, since the range annulus observed on the echo sounder corresponds to the thick spherical cap XHUVSW whereas the true range of interest is the parallel-sided slab NHN’DSC. Thus fish in the volume XHN,UHN’ are actually above the head line and will not be caught, whilst the echoes from fish in the volume WSC,VSD will not be observed, since they are obscured by the sea-bed echo; they may nevertheless pass

L L 0

v

,”

N’

-

7 J'

,

G

P

J

Y

D

on ship

Transducer Sea

L

TAB.

Volume ships

bed T<‘D.

Fish Projected

position

of

head-line

the

net.

These

crescent-shaped forward fish

however.

the

between

OPQR,

to

would

The

echoes

In

from

causes

have

sounding,

of the waves. On

on

propeller.

line

beam

dropped

;I direct Fig.

noise,

of this

so

\,ic,ca IYI’.YN.

fore-and-aft

transducer

onto

path

received

and

which

this

on a typical

which

the

2 kc:s

and

beam

A further with

and

the sea bed. transducer. as

I7

A fish

the arc EFG.

echoes

represent

sample

increases

reference

level

i.e. a power

level

contribution

to

This

the

it is reflected

Apart

from

square

tho

\11:ASL’REErlt:N-IS

The

peak

aspect

OF

levels

as in the and from

SIGNAl.

of the last

LE\‘EL.S

echoes

section

a typical

noibc

character.

A small.

in a variety past

and

01

b,hich

the hull

of

by the motion

the side of the hull.

which

of the sea. when

constant

towing

the largest amount

the trawl noise

(usually

at

from

the

is received

by

comes

of noise

and

(the

indirect

the ship

continues its

is

a 3Of>

’ w.

IO-’

attenuation.

beam

propeller

of water noise)

direction bandwidth

0.8 ILV across

from

in

3 dB

noise) down

back

intensity

to

to the falls

off

of the depth.

bandwidth.

same

is generated

travels

between

overall

noise

in

signals

at 30 kc/s

26

fore-and-aft

is about

where the

the

lebel

various

operating was the

the

reference the

of 2.1

noise

arc very

against or the

in

instrument.

range.

of

angle

this

the

as the

levels

echo-sounder

athwartships

3h). With

inversely

the

transducer

(?A

\,arics

is taken

shows

of the vessel.

of the ship’s (tlow

4 knots).

and

beam

volume

a narrower

rolling

by the flow

trawler

of trawl

acoustic

Moreover. the

if the tish echoes

generally

against

mouth

the

111 ath\vx-

in

from

6.‘;

ment

restriction

noise

particularly

of about

or

along

However

by motion

the state

a deep-sea

;I speed

mouth

JKLM.

to be detected of

the

(J’K’L’M’)

to

observed

and

to

the vessel and the transducer depends

the

of

K

in time.

cxatnplc.

cavitation

the trawl

volume

accuracy

owing

by acoustic

transducer. the

line.

against

envelope

random

for

that

and

depth.

60 ft.)

be anywhere

echoes.

fish

which.

is about

directly

appreciably

on the

or wider

the head

stabilization

vertical

ways.

the

of the

them

observed

be the

narrower

sea-bed

;I cignal

although

may

The

move

corresponds

to

be said

is made

require

pivcs

TF

sample.

background

of

tows

over

than

are

mouth

are above

the

do not passes

narrower

annulus

it ought

it may

beam

the

of the

positions

trawl

Depending

than

trawl

at range

Thus,

serious.

because

the

as JKLM)

range

;I statistical close

trawl.

caught

of the

of which

fish ship

bram

chapc

sea hcd

5B) is not as satisfactory.

that

the the

be either

are

whereas

observed

if

may

observed

parts

that

when

acoustic

points

assumes

of the

fish

width

too

the crests

the true

(Fig.

diagrammatically

more

(The the

and

time

beam

(shown that

if one

of arrival

acoustic

show

athwartships

ship

the

ship,

covered

Perpendicular

to the sea bed.

even

behind

not

since

crescent-shaped

of the

in relation

arc

direction

traces.

motion

The situation

time

uncertainties

in the fore-and-aft

h!

Ckomctr) rclatmg po\,fish,

direction

Volume

tllc

however.

covered

B

5. of

direction

of trawl

-rs.

into

K’

L’

A

Fig. tion

___ Depth.

fathoms

I ROM

received

from

I-ISII

A I’ St:A

on the

a 3Ocm

same

codling

sea bed are also indicated

equip-

in dorsal in Fig.

6

for attenuation (BC) of the sea-bed echo, the slope of the straight line AB is found to be approximately 3. Measurements have also been made of the signal strengths returned by an air-filled spherical metal float lowered on a line to various depths. 2o Since u for the sphere is known (Table I), this affords an absolute experimental check on the validity of the general acoustic equation for vertical sounding, if all the variables of the echo sounder are also measured. The signal strength predicted by calculation for this target agrees with the measured echo.3 Comparisons between the signals received from fish and from spherical targets suspended at the same depth in calm seas enable the back-scattering cross-sections of the fish to be determined. Cushing et alzO gave their results in terms of “target strength” T (Fig. 7) compared with the echo received from a perfectly reflecting sphere of radius 2 m. These may be related to u (in cm2) since:

2

IOr 105-

5L 2

S-

IO-

i-

5-

to*-

2-

5-

I“E 22

5--

% 2_ ‘0 lo-‘5 2 5-

2N6 IOX< ” z E 6

2-

? 2 $

0 2 IO-Z-

x ”

zF i

520-

5-

a

LL

2IO’-

On the other hand, Hashimoto2’, 22 has given values of the “reflection loss” (PI) at lm, where

5 _ 2-

,9, = 10 log[4T(p):]

IO5-

*I

r’i

252-

IO-b5-

I

10

Shishkova’s resultsz3 for horse mackerel are in terms of the radius (R,) of the equivalent sphere so that u = rRp2 if the unit of length is the centimetre. This type of fish has no swim bladder and in Fig. 7 these readings are generally lower than the others, which are for fish with swim bladders. Measurements at sea, however, are difficult to make, owing to the motions of ship and target, and alternative methods have been used, giving higher accuracy.

0

I’@

I-

0’ /

0 50 2-

(dB)

I

+/

J,

2

I

34

6

I

8

I

IO

1-u

15 20

30 40

60

Lenqlh of flrh,/h

MEASUREMENTS ON FISH IN A TANK

Some Jones water. taken. planes swim reduce

very accurate measurements have been made by and Pearce2” on perch suspended in a tank of fresh The fish were killed just before the readings were The resulting back-scattering polar diagrams in two are seen in Fig. 8. The effect of taking away the bladder from a fish of this length (I9 cm) was to the amplitude of the signal by about 50%.

Fig. 7. The acoustic back-scattering cross-sections of full-size fish at 30 kc/s compared with those scaled from miniature fish at I.48 MC/S using a scaling factor of 5O:l Full size X 5 cm 0 Gushing, sea water, dorsal aspect ml_Shishkova, sea water, dorsal aspect rj Hashimoto, sea water, dorsal aspect Jones and Pearce, fresh water, dorsal aspect Scale model, h = 0.1 cm ---Haslett, fresh water, maximum in side aspect ____ -Haslett, fresh water, dorsal aspect

SCALE-MODEL

for a transmitter power, W, of 800 W. Thus a fish of this size can just be detected at a depth of 120 fathoms whilst the sea-bed echo might be observed at a depth of 1,600 fathoms. The ratio of fish echo to sea-bed echo is --60 dB at 25 fathoms, i.e. 1000 : 1 down in amplitude, at this frequency (30 kc/s) and pulse length (0.5 ms). A large cod of length 85 cm would give a signal level in general some 11 dB higher than this codling. When the range is increased, the signal from the fish declines more rapidly than that from the sea bed. The effect of attenuation on the curve DF in Fig. 6 may be eliminated by raising point F by an amount EF corresponding to the attenuation at range F. The slope of the straight line DE is then approximately 4 as is to be expected for a small target such as a fish. It is interesting to note that with due allowance made

These problems have also been investigated by a scale-model techniques which gives precise control over the experimental conditions and larger equivalent ranges than the method described in the last section. When operating at I.48 MC/S, the scaling factor is 50 : 1 compared with the full-size frequency of 30 kc/s. Small fish of suitable sizes are available (sticklebacks and guppies) which have similar shapes and structures to those of the cod and herring families. These small fish also have analogous swimbladders. The acoustic back-scattering cross-sections of these scalemodel fish in dorsal and side aspects are also shown in Fig. 7.” It is noted that these graphs show regular undulations which are attributed to interference between the echoes received from various parts of a fish. A typical back-scattering polar diagram of a scale-model fish (of length 30.4h) is shown in Fig. 9, whilst Fig. 10 indicates how the number of lobes may be expected to vary with size of fish.g

TECHNIQUE

OF MEASUREMENT OF CROSS-SECTION

uL.TRASONI(‘S/Janu(IrY-Murch

1964

I7

Fig.

8. Acoustic

back-scattering

(4 X) at 30 kc/s terms target

of

(after

relative

strength

Jones vgnal

SHlm

bladder

full

B.

Swim

bladder

emptied

c‘.

Swim

bladder

full

II.

Swim

bladder

emptied

of air.

reHecting

of length

co-ordinate cross-section sphere

XI

cm

is given (0)

in and

of radius

Zm

plant

horizontal

vertwal

of Bir.

of a perch radial

a perfectly

horizontal

of air.

The

back-scattering

with

ofair.

diagram>

Pearce).

voltage.

(7’) compared

A.

polar

and

plunc

plane

vcrtwal

plant

120;*400 Side

candL Ie-/ Ventral

Fig.

9.

(Below).

(stickleback) A.

Horizontal

B.

Vertical

Radial acoustic

Back-scattering

of

length

3.04

cm

polar (30.4

dqyamc

of

ti

scale-model

li\h

X).

plant plant

sc~lc

represents

crowsection

A

amplitude of

I .73

In IW

mllllvolts cm’

at

I.48

20

mV

MC

c.

1s equivalent Scaling

factor

to

nn

50:l

GENERAL TRENDS OF SCATTERING LENGTH

CROSS-SECTIONS

OF FISH WITH

OF FISH AND WAVELENGTH

Agreement between the scale-model and full-size results is reasonably good (Fig. 7). Moreover, the scale model is able to show the trends in more detail; for example, at higher frequencies than 30 kc/s when the fish is many wavelengths long, the general trend of the cross-section in side aspect (apart from interference effects) is to be proportional to the fourth power of length and inversely proportional to the square of the wavelength. Hashimoto also found this general trend22 which suggests that, in side aspect, the fish acts as a plane area (see Table 1). Again, for the same sizes of fish in dorsal aspect, the general trend of cross-section accords with the cube of the length of the fish and is inversely proportional to the wavelength. This trend has also been confirmed by Hashimoto22 at sea. Thus it appears that the fish acts as a cylinder in which the ratio radius/length is constant, i.e. the shape of the fish is constant. At the other end of the scale, when the size of the fish is small compared with the wavelength (in the Rayleigh scattering region), the cross-section is proportional to the square of the volume and inversely proportional to the fourth power of the wavelength. In the intermediate region, the fish acts rather like an ellipsoid in that the general trend corresponds to the square of length and is largely independent of wavelength. The polar diagrams of fish are, however, found to be quite different from those of cylinders in that the energy is more widely distributed for fish.g This difference is attributed to the presence of other structures within the fish which contribute to the echo. The general trend indicates the shape of the structure from which the predominating component of the echo comes. The frequency responses of fish of various sizes can also be determined from the scale-model results (Fig. 11). These seem to be generally in agreement with the ratios between the signals received from fish of different sizes measured at sea on three echo sounders operating simultaneously at

Fig. 10. The numbers of lobes in the back-scattering polar diagrams of fish of various lengths expressed in terms of the incident wavelength, X

i

6 4

ik-’

3 2

I

20

30

4050

100 Frequency,

200

400

1,000

kc/s

r’ r’

’

’ “““’

’

’ “1111’

Fig. 1I. Approximate frequency responses of fish of lengths 90 cm, 45 cm and 22.5 cm determined by scale-model measurements in the geometrical region. At high frequencies there are many maxima, so the general trend of the maxima is shown as a dotted line on each graph. These results are compared with those published by Hashimoto andManiwa for a fish of length 15 cm (n) and by Shishkova for fish of lengths 45 cm (x) and 22.5 cm (+)

results22 also agree separate frequencies. 9,25 Hashimoto’s with these frequency responses, but Shishkova’s readingsz3 are rather low.

VARIATION

OF SEA-BED ECHO WITH

RANGE

The scale-model method has also been used to investigate the effect of type of sea bed on the law of variation of the sea-bed echo with range. The results for three targets are seen in Fig. 12. The very small target consisted of a cork disc of diameter 1 mm (y = 1000/o), which was placed perpendicular to the axis of the beam at each range. Its scale acoustic crosssection (from Table 1) was approximately 4nA2/h2 “- 8 x 10m2cm2 which corresponds to a fish of length about

ULTRASONICS/January-March

I964

19

40 cm at 30 kc/s (from Fig. 7). In Fig. 12, this target is seen to obey the inverse fourth power law of range plus the attenuation of water. The effect of attenuation is relatively small, raising the graphs at a range of 150 cm by a facto1 I .21/l only. The “large specular target” was a sheet of aluminium 30 30cm and 0.161 cm thick. As 0.161 cm equals 0.43 of the wavelength in aluminium, this target will have a total reflectivity of 90%; (see Fig. 3 of Reference I I). An “image” of the transmitting transducer would be expected to appear in the plate so that the echo intensity would fall off inversely as the square of range (after the attenuation of the water had been eliminated. This is indeed found to be so in Fig. 12. The “large rough target” consisted of ;I similar plate covered by a layer of brass turnings. The main dimensions of these particles (measured under a travelling microscope) varied between 0.12 mm and 2.5 mm (0.12h and 2.5h). These correspond to fish of lengths from 0.6 to 12.5 cm at 30 kc/s. The pulse length was 25,~s, equivalent to I .25 ms at 30 kc/s and the beam angle was 9” between zeros. The signal power received from this target was seen to be

Thus again there is close agreement between full-size and scale-model results. It must be inferred that the sea beds met whilst trawling are diffuse reflectors. A confirmation of this is seen in the observed sea-bed echo which is found to be many times longer than the transmitted pulse. In fact, the sea bed is known to consist of shingle, corrugated sand and clay.

MEAN

STRUCTURE

OF

A

FISH

AND

APPROXIMATIONS

To enable calculations to be made of the acoustic propertics of a fish, a mean structure”” (see in Fig. 13) has been determined for the cod and herring families in which only those parts which are acoustically significant have been included. In this diagram all dimensions are related to the overall length of the fish (L) and the positions of the sectional views are measured from the reference point (the nose). The overall length of the body excluding fins is L, (0,93L), whilst the maximum height. H. is 0.195L and maximum

L= I.00

I-

_________(

20-

Very 5

J

>

.

&2 20 >

.

small

target

[xlo-~)

0 & -

coo5

‘:i

Large target

speculai (x10)

\

20

Fig. echo N.B. shotin

50 Range of

17.

100 I50 zoo target. cm

Scale-model

amplitudes The in

ordinate brackets

I-16.

measurements. with must for

range

for

be multiplied each

Varlatlons

13. .A scale drawing

calculations

of

dimensions

are

the

showing

acoustic

-0~36---i

the

awrage

echoes

_eivcn as fractions

from of

the

dimension\

different ~Y~~;III

of

parts

can

length

of

the

lish

011 n 111~11

be based. lish

(I

.2ll

I

01

targets.

I’

C’ertcbral

by the constant

s

Swim

c;

Gut

various

”

graph

inversely proportional to the cube of range. just as had been discovered previously at sea (Fig. 6). The ratio of the echoes from the sea bed and from a fish of length 30 cm at 25 fathoms is 60 dB in Fig. 6. whereas in Fig. I2 the ratio between the echoes from the rough target and a fish of length equivalent to 40 cm at the corresponding range (100 cm) is 50 dB. Three items account for the difference: the ratio of the values of the scale cross-sections of fish of different equivalent lengths. 40 cm and 30 cm, is 8 ](y” 7 dB: the difference (4dB) between the 1.5 10.’ scale-model beamwidth (9”) and the effective bcamwidth at sea (14” and limited by pulse length): and the lower reflectivity of the sea bed compared with brass ( 2 dB; set Table I ).

coIumt>

hladdel

I3

MaxImum

breadth

Ii

Maximum

hclght

oi of

hod). body.

Ia\

lit]\

1~511Iin\

breadth, B, is 0.1 121.. The fins are ignored as they do not reflect appreciable amounts of sound since they are 40 thin as to be acoustically transparent. Apart from the body consisting of fish t&h. other important structures are the swim bladder and the vcrtebrill column or backbone. The swim bladder” is ;I cigar-shaped air-tilled envelope situated .just under the backbone. The (rut consists mainly of tissue of much the same density as the &Y flesh and is not expected to contain much gas. An approximation to the swim bladder is ;I cylinder ot length 0.24L and radius 0.0245L. having a reflectivity. ‘I<. of loo”,,. Similarly, the core of the backbone may he taken as a cylinder of length 0.651. and of mean radiub 0.006L. ‘/< 24”,, in sea water. The back-scattering polar diagrams of ellipsoids with the

Table3.

ACOUSTIC

CROSS-SECTIONS OFTHE

FRONT

SURFACESOF

VARIOUS PARTS OF AFISH(IN

THEGEOMETRICALREGION)

L=overalllengthof fish, cm Approximate shape

Part

~

Formula

Dimensions

)

aside cm2

o--

cm2

I Swim

bladder

27ra’2< d

Cylinder

/ Backbone (core)

Cylinder

x27ra’2,1i 2

x Body

Ellipsoid

Plane

j

area

I

Dorsal rB2L12.R 2 4H2 Side aspect 4aA2$ x2

0

a’ = 0.006L I = 0.65L <,= 24to26%

I.0

OF CROSS-SECTIONSOF

PARTSOF

1

I .o x 10-3L3/X

4.7 x IO-“L2 (in fresh water) 8 x 10-5L2 (in sea water)

sea 2.2 x lO-dLh/X2 (in fresh water) 3.7 x lO-GL4/X2 (in sea water)

Table 4 EXAMPLES

OF CALCULATIONS OF ACOUSTIC CROSS-SECTIONS OFVARIOUS PARTSOF A FISH

I ,48MC/S in fresh water. h Length

of fish

E.wample ~~ --, cm h

Aspect

Part

0.1 cm

~ Calculated

Measured

~,fbr t/jfarr.

jUyfi+ok

s 3 cm2

I I

1 I.0

10 1 Side

Swim bladder Backbone Plane body

8.9 x IO-“*, 2 x IO-’ I.0 x IO-’ 2.2 x IO-2 ~

2

i I.0 I

10

gL$-bilazder

8.9 x lo-%* I.0 x 10-s 4.4 x IO-3

Dorsal

~Ellipsoidal 3

4

5

2.4

2.4

6.0

6.0

24

Side

examples

K IO-’

~ 24 ~Dorsal

60

60

i Side

/ Dorsal

Swim bladder Backbone Plane body

1.2* I.4 x IO-’ 7.3 Y IO-’

I .7

Swim bladder, Back bone Ellipsoidal body

I .2* I.4 Y IO-’ 2.5 < lO-L

0.6

Swim bladder-~-G ~ 2.2 Backbone Plane body ~ 28* Swim bladder Backbone Ellipsoidal bodv

32

* ~ ?2 I ~ I.6 ; 10-z ~

8

A FISH * Predominant

Some

I.3

body

6

CALCULATION

I

7.5 > lo-hLz (in sea water)

fresh

A 2 L,H/2

same dimensions (L,, H and B) have been compared with those of models of the body of a fish in various materials.2’ The magnitudes and general distributions of lobes were found to be similar so that calculations for the body tissue may be based on an ellipsoid of these dimensions and of the same acoustic reflectivity. This approximation will be satisfactory for all target aspects except broadside. It is clear, however, from Fig. 7 that the fish is acting as a plane reflecting area in broadside aspect at the higher frequencies and for the longer fish, since under these conditions the general trend of the graph accords with the fourth power of fish length. Although there are flat areas on each side of a fish which might give larger signals than the corresponding ellipsoid, experiments with models of the fleshy body alone show only a square law for fish lengths between 5X and 30/\ (Fig. 9 of Reference 27). The regular arrangement of the upper spines of the backbone in a plane lattice must also give a contribution in side aspect. The projected area of the fish in side aspect is approximately L, H/2. A table of formulae for the acoustic cross-sections of various parts of a fish may now be drawn up (Table 3). Over 99%) of the incident energy passes through the tissue so that the values of ‘& for the swim bladder and bone immersed in water can be taken, although these structures are really surrounded by fish flesh.

x 10-3L3/X (mean)

4.4 x lO-3L* (in fresh water)

~ T( = l’yit($

only 2

8.9 % 10-XL31h

8.9 x 10-3L3/h

I

L, = 0.93L H = 0.195L B = 0,112L 9 = 4zayh$in

Side TH~L,~.R, 2 4BS I

Alternative body (for long fish at high frequencies)

a’ = 0.0245L

I !$iy$

contribution

of calculated values of the acoustic cross-

sections of various parts are seen in Table 4, which are compared with the measured values for the whole fish from Fig. 7. The largest contribution (marked with an asterisk) will, of course, take charge. In examples 1, 2, 3. 4 and 6 in Table 4, the biggest signal is from the swim bladder, whilst in example 5, the echo from plane side of the fish predominates. In view of the approximations which have been made, the agreement is satisfactory.

The formulae of Table 3 give the signals reflected by the front surface of each of the structures and can only be regarded as reliable when the parts are wholly in the geometrical region, i.e. when the parts are much bigger than the wavelength. Although the echoes reflected by the rear surfaces of these structures have been ignored, this may not lead to serious errors, since the echo from the back surface

ul.-rRAsoNlcs/Junuary-March

1964

21

of the swim bladder is insignificant and also the “highlight” on the rear surface of the body is largely screened by the bladder in dorsal and side aspects. As smaller and smaller fish are considered, first the radius of the backbone and then the radius of the swim bladder will approach the Rayleigh scattering region. The fleshy body of fish will be the last to enter this region,

SUMMARY

OF CONCLUSlONS

I. In vertical echo-sounding for fish detection, variations of propagation of sound in the sea water do not significantly affect the results in good weather. 2. The acoustic beam patterns of the transducers. and their efficiency of transformation from electrical energy to sound energy, or rice IVYSU,may be determined readily. 3. By means of a general acoustic equation relating the variables of the system, the strength of the echo from a target of known acoustic back-scattering cross-section may be predicted with reasonable accuracy. 4. The acoustic back-scattering cross-sections of the front surfaces of a number of simple bodies may be calculated if the materials of which they are composed are known. 5. Recent types of display introduced for use with echo sounders allow much more detailed examination of the magnitudes and characters of the echoes from fish than hitherto. 6. These echoes have been systematically investigated by using a scale model. This has given information of the acoustic cross-sections and back-scattering polar diagrams of fish and also of their frequency responses, and enables the size of a fish to be determined from the strength of its echo.

7. As to echoes from fish and from the sea bed, the scale-model results agree with the full-size measurements at sea. 8. The determination of an average structure 01’ a lish has enabled approximations to be made to its various parts. so that the acoustic cross-sections of the front surfaces of these parts may be calculated. The computed values are in satisfactory agreement with the measured values for the whole fish. 9. By means of investigations of this type. it is hoped to elucidate the principles underlying the formation of ultrasonic echoes from fish. A great deal more research is needed. particularly concerning interference between echoes received from the front and back surfaces of a structure translucent to sound. 10. The geometrical relations between the positions of the fish, the acoustic beam and the mouth of the trawl must be considered when estimating the catch. A description of a new echo-sounding equipment for the detailed observation of fish echoes will appear shortly.“”

A(‘KNOWLEDGEMENl-S

The author is indebted to the Directors, Kelvin Hughes Division of S. Smith and Sons (England) Ltd. for permission to publish this paper. Acknowledgement is due to the following for permission to reproduce illustrations: Journnl qf the British Institution qf Radio Engineers for Fig, 4 and 6; British Journal o/’ Applied PhJwics for Fig. 7, 9, IO and I1 ; Journul of' Experk mental BiologJv for Fig. 8; Journal du Conseil /nternationul pour I’explor mer for Fig. 13: lnterscience Publishing Co. Ltd. for Fig. 3; and N.D.R.C., Washington, for Fig. I.

REFERENCES

of the retlectivities of solids small specimens,” Procdit7g.s of the Physicnl Society of Lmrlorr. 79. Pt. 3, No. 509, 5.59 (1962). 12. KAYE, G. W. C.. and LARY, T. H., “Tablch of physical and chemical constants,” Longmans (11th edn.. 1957). 30. 64. 13. HOI,~;MAN. C. D., “Handbook of chemistry and physics,” Chemical Rubber Publishing Co., Cleveland, Ohio (40th edn., 1958). 2105, 2499. 14. JONES, F. R. H.. and MARSHALL, N. B.. “The structure and function of the teleostean Biologirnl Rrviebls of’ III<, swim-bladder.” Cumhriclge Philosophical Societ_v. ZN, 16 (1953 ). 1.5.RIDENOI’R. L. N., “Radar system enginecring,” M.1.T. Radiation Laboratory Series. McGraw-Hill. New York (1st edn.. 1947). 34. 16. GRIFFITHS. J. W. R., and MOK~;AN, I. G., “The chemical recorder and its use in detecting pulse signals in noise.” J. .Sockt.v of Intrunw~t T~~chmdog~~.X. No. 2. 62 (1YZh). 17. HOPKIN. P. R., “Cathode ray tube displays for fish detection on trawlers.” J. Briti.rh Irntitutiot~ of Rdio D?girrwr.~. _?5. No. I, 73 ( 1963). IX. Cuwlnc;, D. H.,“The uses ofccho sounding for fishermen.” H.M.S.O.. London (19631. 19. RICHARDSON. I. D.. C~SHIXC;. D. H.. HAI~UFN JONtS, F. R., BLYFRTOPI. R. J. H.. and BLA~XER, R. W., “Echo sounding experiments in the Barents Sea.” Fishitrg lnvestigntions, Lordm. Series 2, 22. No. Y (1959). 20. CUSHINC;, 0. H., JONES, F. R. I-I.. MITSON. R. B.. ELLIS. G. H., and PFARCF, G.,. “Measurements of the target qtrcngths ot

I. DwswN 6, N.D.R.C., Washington.“l’hysics of Sound in the Sea.” Summary Technical Report, (1946) 16. 105. 2. RICHARDSON, E. G.. “Ultrasonic physics,” Elsevier, 172 (1952). 3. HASLETT, R. W. G., “The quantitative evaluation of echo-sounder signals from fish,” J. Briti.vh Institution of Radio Etzpinerr.s, 22. No. I. 33 (1961). 4. SCHL:LKIN, M., and MARSH. H. W.,“Absorption of sound in sea water,” J. British Imtitution of Radio Etzginerr.s. 25, No. 6,493 (1963). 5. HORI-ON, J. W., “Fundamentals of sonar.” United States Naval Institute, Annapolis, Maryland (19.57), 185. 6. Haswrr, R. W. G.. “A high-speed echosounder recorder having seabed lock.” J. British Itrstitrrtiorr of Radio .Ggimerv. -14. No. 6. 441 (1962). 7. FISCXFK, F. A.. “Fundamentals of ciectroacoustvzs.” Interscience. London (19.55). 120. 143. 8. HASL&I I, R. W. G., “The back-scattering of acoustic waves in water by an obstacle: I. Design of a scale model and investigation of its validity.” Procrrdiirqv of the Ph~.simI Socirt,v of Londm. 7Y. Pt. 3. No. 509. 542 (1962). Y. HAsLur, R. W. G., “Determination of the acoustic back-scattering patterns and crosssections of fish,” British Jorrrr~al of Applictl Phg.G.s, 13, 349 (I 962). 10. MENTZER, J. R., “Scattering and diffraction of radio waves.” Pergamon, London (1955). 31. 76, 104, 131. I I. HASL.FTT, R. W. G., “The back-scattering of acoustic waves in water by an obstacle.--

‘2

I ILTRASONICS

I Jmuury-Mtrrch

II. Determination

using

I964

fish,” J. Britivh In stiture oI Radio 1Jlrgitw’cr.r. 25. 4. 299 (19631. of tishing boat 21. ~~A&HIMOIO, T.. “Report Ministry of Agriculture and laboratory.” Forestry. Tokyo. Japan f 195.7). 22. HAS~{IMO~O. T.. and MANI\%A. Y.. “Stud? of the retlcction loss of ultrasonic wave on tish body by millimctrc wave,” Technical Report No. 8, Fishing Boat Laboratory, Ministry of Agriculture and Forestry, Tokyo, Japan (March. lY56). 113. 23. SHISHKOV, E. V.. “Study of acoustical characteristics of fish.” F.A.O. World Fishing Gear Congress. London (May. lY63). Paper 74. 23. Jo\t.s. F. R. H.. and Pi AK(b. G.. “tcho sounding evpcrlments with perch to determine the proportion of the echo returned by the swm bladder.” J. ~,..rpl’ri,frort[ll Bio/qTJ,. 3.c. 2. 437 (1Y581. 2.5. (‘IISHII*C;, D. II.. and RI( ,,AI/ .-lpplied Ph.vsic\. 13. 61 I (1962). 28. HOI’I\IN, P. R.. and HAst.eII. R. W. G.. “A neu ultrasonic fish detection cquipmcnt for trawlers” (to bc publi\hcd).