Rapp. P.-v. Réun. Cons. int. Explor. Mer, 189: 381-387. 1990

A wideband echo sounder: measurements on cod, saithe, herring, and mackerel from 27 to 54 kHz

E. J. Simmonds and F. Armstrong

Simmonds, E. J., and Armstrong, F. 1990. A wideband echo sounder: measurements on cod, saithe, herring, and mackerel from 27 to 54 kHz. - Rapp. P.-v. Réun. Cons, int. Explor. Mer, 189: 381-387.

The paper describes a wideband constant beamwidth echo-sounder system and pre­sents spectral responses obtained from caged aggregations of cod (Gadus morhua ), saithe (Pollachius virens), herring (Clupea harengus harengus), and mackerel (Scom­ber scombrus). The transmitter generates swept-frequency pulses with uniform- frequency spectrum output, using stored reference levels and a digital-to-analogue converter to provide amplitude and frequency stability. The transducer is a 217- element partial sphere with frequency-independent shading achieved using an auto­transformer on transmission and a series of separate amplifier stages on reception. The receiver uses a series of low-noise pre-amplifiers in an underwater housing behind the transducer with a summing point amplifier to define the beam. The received signal is processed through an eight-channel bandpass filter and a 100 kHz sampling module with a 12.5 kHz sample rate per channel. Control of the transmitter and receiver is computerized.

Measurements on caged aggregations of fish are reported. The fish were placed in a 2-m-diameter cage and observed over several days with both the wideband sounder and stereo 35-mm still cameras to determine spatial distributions and tilt angles. Data for different species are presented, showing frequency-dependent differences in the reflectivity in the range 27 to 54 kHz. These data are related to the behaviour and information obtained from the photographic system. The differences among species in spectral reflectivity are discussed and the implications for species identification considered.

E. J. Simmonds and F. Armstrong: DAFS Marine Laboratory, P. O. Box 101, Victoria Road, Aberdeen A B 98D B , Scotland.

Introduction

Using echo integration to estimate the abundance of pelagic fish stocks can provide estimates of biomass or the num bers of fish. This m ethod assumes that the average fish backscattering strength o r ta rget strength (TS) is known. Present procedures require tha t the TS does not depend significantly upon fish behaviour and tha t echoes from species of interest can easily be dis­tinguished from those of lesser im portance. The identi­fication is usually done either by visual inspection of echo traces or by partitioning of the echo-integrator data in proportion to the composition of trawl catches. The target strength is usually assumed to be constant, or perhaps dependen t on fish length but independent of fish behaviour. U nfortunately , this is probably too sim­ple a model. Fish are known to have highly variable backscattering strengths which depend upon tilt angle (N akken and Olsen, 1977; Foote , 1985). M oreover, the size of the gas-filled swim bladder and its target strength will change with the depth of the fish (Sand and H aw ­

kins, 1974; Blaxter et al. , 1979). A t norm al survey echo- sounder frequencies the acoustic wavelength is gener­ally of the same o rder as some of the dimensions o f the fish. It is therefore expected tha t there will be some frequency dependence in backscattering strength result­ing from changes in both tilt angle and swimbladder size.

By increasing the system bandwidth, it might be pos­sible to use frequency-response inform ation to help identify species ( i .e ., sw imbladder size), o r smooth out differences in backscattering strength due to fish-orien- tation changes. A w ideband echo sounder was produced to investigate these possibilities.

The main m easure o f a w ideband system is the extent to which the transducer beam shape bo th on transm is­sion and on reception remains constant and independ ­ent of frequency, thus ensuring tha t the sam pled volume is not frequency dependent. In addition, there is a need to keep the sample volume size similar to tha t used in p resent surveys so tha t da ta collected from such a sys­tem can be used directly with our present knowledge of

381

fish reflectivity. Thus a w ideband system operating around the most com monly used frequency of 38 kH z is desirable. Several approaches have been suggested and tried using non-linear acoustics for transmission and/or reception and twisted arrays for reception (Berktay et al., 1968; Berktay and Learly. 1974).

T he technique we have chosen is suggested by R odg ­ers and van B uren (1978) and uses a partial-sphere transducer which provides constant beam width on transmission and reception in a single unit. The trans­ducer is constructed from elem ents arranged in concen­tric rings on the surface of a partial sphere. The rings are am plitude shaded on transmission and reception to form the required beam shape. T he system has been designed with a multifrequency and swept-frequency transm itter and multi-channel variable-gain receiver. T he equipm ent is described in this paper along with m easurem ents on cod, saithe, herring, and mackerel.

Methods

W i d e b a n d e c h o s o u n d e r

The transducer (Fig. 1) was constructed from 217 ele­ments arranged in eight concentric rings around a cen­tral e lem ent, on the surface o f a partial sphere. The individual transducer e lem ents were of a pre-stressed sandwich resonator type with 20-m m-diam eter alumini­um head and 10-mm steel tail mass. T he case was m a ­chined from a solid block of the nylon derivative, ny- latron GSM . T he active face of the transducer subtends an angle o f 28.3° and has a radius of curvature of 86 cm, giving a nominal beam width of 10.5°. A n underw ater housing containing transm it and receive shading com po­

Figure 1. Transducer, 217 elements on the surface of a partial sphere.

nents is situated on the back of the transducer. The constant beam width was form ed using frequency-inde- penden t am plitude shading on both transmission and reception. T he shading functions for transmission and reception are the same (Fig. 2). On transmission, the voltage shading was p roduced from an au to-transform er with nine taps, one for each ring of the transducer. On reception the same shading function, which depends upon the num ber of e lem ents and was therefo re differ­ent from ring to ring, was achieved by using a single low-noise operational amplifier p e r ring with a summing point amplifier to form the beam . Both the receive and transm it shading com ponents w ere contained in the housing on the back of the transducer, along with m er­cury reed relays which are used for transm it—receive switching. The transducer thus had a single transm it drive, and a single receive line along with transm it—re ­ceive switching and receiver pow er supplies. The trans­mitter consisted o f a m odu la to r providing V2, 1, 2, 4, and 8 millisecond pulses. For the present experim ents a 1-ms pulse was used with swept frequencies from 27 to 54 kHz. The pow er stage was a 1 kW mos-fet amplifier which opera ted up to 100 kH z (Fig. 2).

A fte r the beam forming the receive had a w ideband filter and a 50-ohm line driver to m atch the cable. The signal was then connected to eight frequency channels, each with a five-resonator filter giving 3.3 kH z b an d ­width betw een 3 dB points, followed by two gain stages with com puter control of gain to match the differences in transducer sensitivity with frequency. E ach frequency channel had a separate detec tor, and the ou tpu ts were multiplexed and sam pled at 100 kH z with 12-bit binary precision giving a 12.5 kHz sample rate per channel (80 [as interval). T he transmission ra te was set at 300 ms, giving 1000 transmissions in 5 minutes, with one minute for da ta analysis per block.

E x p e r im e n ta l rig

M easurem ents of the transducer beam shape and the spectral reflectivity of fish were carried out at the M a­rine L abora to ry ’s Loch Duich field station, on the west coast of Scotland. Fish w ere placed in a cage 2 m in d iam eter and 1 m deep. T he cage was placed betw een two aluminium frames supporting s tereo 35-mm still cam eras and low light TV cam eras below the cage. The com plete rig (Fig. 3) was suspended from a raft, the transducer 15 m below the surface in a m otorized gim- bal table and a 38 .1-m m -diam eter tungsten carbide ref­

erence ta rget positioned 10 m deeper. T he cage support fram e was 12 m from the transducer, which placed the fish at a range o f 14 m from the transducer. This a r ­rangem ent allowed the ball to be used as a reference target, with transducer position adjusted for maximum echo strength and the fish located in the region of the centre of the beam.

Photographs w ere taken th roughout the experim ents

382

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to record the fish distribution and behaviour within the cage. Results w ere collected in 6 min o r 1000 transm is­sion blocks. T he beam pa tte rns were m easured by re ­moving the cage and cam era fram es and recording the echo from the reference target only. T he transducer was steered through a range o f angular locations using the motorized gimbal table. T he m e thod of obtaining the equivalent beam angle is described in Sim monds (1984).

B e a m - a n g le m e a s u r e m e n ts

T he m easured beam angles are shown in Figure 4. This has been rep lo tted showing deviation from the theore t­ically predicted sample volum e, and for the section of the beam which passes through the experim ental fish

cage.

F ish f r e q u e n c y - r e s p o n s e m e a s u r e m e n ts

Eight experim ents were carried ou t, two each with cod, saithe, herring, and mackerel (Table 1). For all the

Equiva lent Beam Angle

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54kH z27-1 9

E.B.A. against Theory

2

54kH z

27

1

E.B.A. th rough the C age-20 ; ^ ___

_ 2 1 127 ~ 54kH z

Figure 4. Transducer equivalent beam angle (dB) from 27 to 54 kHz. measured values replotted against the theoretical value and for the part of the beam that passes through the fish cage.

experim ents the fish were rem oved from surface ho ld ­ing pens and placed in the experim ental cage, lowered to a depth of 29 m, and left for a period of days.

Table 1. Numbers of fish, mean length, standard deviation, and mean weight for eight experiments.

Species Number L(cm)

LSD(cm)

W

(g)

Cod 26 28.02 3.60 212.86Cod 17 23.74 4.07 132.94Saithe 26 22.63 0.77 114.69Saithe 39 26.56 1 . 4 4 168.41Herring 53 28.48 1.83 210.48Herring 96 28.42 2.16 205.64Mackerel 97 32.78 2.87 267.94Mackerel 218 28.02 2.78 212.85

D a t a an a ly s is

The sam pled data from the received echoes were sum m ed to give integrals for the reference target and for the fish for each frequency for each transmission. First the individual integrals, from the fish, for each frequency from each transmission were exam ined. For correlation betw een frequencies for 1000 transmissions:

2 (Iti—Ii)(Itj—Ij)/ X V'[(Iti- ï i)2(IIj—Ij)2]i t

where 1„ and Itj are the echo integrals from transmission t for frequencies i and j respectively, and ij and Ij are the mean values for 1000 transmissions.

384

For the correlation between adjacent transmissions for the same frequency:

where I(1_u)i is the transmission integral shifted by u transmissions from transmission t.

To exam ine the frequency-response data all the ex­perim ents were treated in a similar m anner. The small contribution produced by the cage was subtracted from the integrals. The eight frequency channels were cor­rected for different sensitivity by dividing the fish in­tegrals by the integral from the reference target and multiplying by its calculated acoustic cross-section (Fig. 5) and then dividing by the equivalent beam angle through the cage shown in Figure 4. T he resulting eight average integrals from 1000 transmissions were then divided by their geom etric mean to provide an eight- point spectrum. This p rocedure provides a spectrum for each six-minute period which was then used to define average spectra (Fig. 6).

To establish the accuracy of the individual independ ­en t sample spectra as a m ethod o f determ ining species, groups of 1000 and 100 samples were com pared with the four average spectra using a least-squares fitting p roce ­dure , weighted inversely by the s tandard deviation of each point in the average spectra. Exam ination of the cod and saithe data showed changes of spectra during the experiments. To improve the discrimination be ­tween these species, the data from the four experiments on gadoids were used to provide separate average spec­tra for unacclimatized and acclimatized fish. T hese two stages were defined as the first 24 hours and the period after 48 hours of the experim ent respectively; the four spectra are shown in Figure 7.

The least-squares fitting procedure used was (for four species):

minimum 2 ( x i ~ aji)2/Sji

w here x, is the mean sample value for frequency i, ajj is the mean value for species j frequency i, Sji is the standard deviation o f a^.

Herring

2 7 k H z5 4 k H z

*-2

S a i th e

27kHz

2 7 k H z

Mackerel

2 7 k H z

5 4 k H z

5 4 k H z

5 4 k H z

Figure 6. Average spectra ±1 standard deviation for four spe­cies: herring, saithe, cod, and mackerel.

Results

C o r re la t io n

With the exception of the cod experim ent with only 17 fish, there was no significant correlation betw een ad ­jacent frequency samples tha t would have confirmed the possibility of phase relationships between multiple targets dominating the single independent sample re­sults. In addition, the transmission-to-transmission cor­relation at the same frequency was not significant in any experiment. This allows each integral from each trans­mission to be regarded as an independent sample.

M e a s u re d ta r g e t s t r e n g th

The mackerel showed no significant change in target strength over time, and the herring showed a small decrease in target strength, com bined with a diurnal variation of about 6 dB, over the period of observation. Both these species were m easured over a period o f four days.

The saithe and cod slowly adap ted to depth over the first two days. D uring this time the target strength rose

Calculated T arget Response

54kHz

Figure 5. Frequency response (TS dB) of 38.1-mm tungsten carbide ball from 27 to 54 kHz.

25 R a p p o r t s e t P rocès-V erbaux 385

Saithe before acclimatization

27kHz 54kHz

Cod before acclimatization

27kHz

54kHz

--- 154kHz

Cod after acclimatization

27kHz

54kHz

L- 2

Figure 7. Average spectra ± 1 standard deviation for cod and saithe unacclimatized and acclimatized to depth.

steadily and m ainta ined a m ore constant value after acclimatization. These species were observed for six- day periods.

For herring and to some extent for cod and saithe, differences in the absolute level o f acoustic reflectivity were observed betw een day and night. H ow ever, no real differences w ere found in the spectra at different light levels, and the da ta for day and night were com ­bined. In all respects the absolute levels o f backscatter­ing strength followed the pattern of those reported in

Forbes et a l . . (1983).

S p e c ie s d is c r im in a t io n

The results for the four species from all eight experi­m ents are shown in Table 2. These show tha t it is pos­sible to distinguish herring from m ackerel and both of these from both gadoid species to be tte r than 9 5 % confidence using 1000 independent samples. H owever, discrimination betw een saithe and cod is only at the 9 0 % level. By reducing the num ber of samples to 100 the discrimination rate drops to 90 % for herring, m ack­erel, and the gadoids, with the differentiation between

Table 2. Percentage species recognition rates for four species categories using average spectra from all experiments: (a) for 1000 samples; (b) for 100 samples; and (c) for 100 samples with six categories, with cod and saithe separated into “acclimatized and unacclimatized states” .

Species Actual speciesrecognized

Herring Saithe Cod Mackerel

(a) 1000 samples

Herring 96.2 4.1 0.1 0.5Saithe 3.8 91.8 10.5 2.1Cod 0.1 4.1 89.4 1.5Mackerel 0.0 0.0 0.0 95.9

(b) 100 samples

Herring 92.4 12.8 3.4 0.6Saithe 7.6 69.4 19.8 2.2Cod 0.0 17.7 76.8 1.6Mackerel 0.0 0.0 0.1 95.7

(c) 100 samples with six categories

Herring 89.3 9.4 2.5 0.0Saithe 10.6 72.9 21.3 0.2Cod 0.1 17.6 76.1 1.0Mackerel 0.0 0.0 0.1 96.1

cod and saithe dropping to 7 2 % . To improve the dis­crimination between the gadoids, the saithe and cod spectra were replaced by four new average spectra, two for each species: the first for the initial 24 h of da ta (unacclimatized) and the second for all da ta after 48 h (acclimatized). This increase in possible categories has only a very slight effect; it improves the separation of cod and saithe da ta to 74 % and m akes no significant difference to the discrimination of the gadoids from herring and mackerel.

ConclusionsWe have dem onstra ted tha t it is possible to use acoustic m ethods to identify caged herring and mackerel and the gadoid species cod and saithe with 9 0 % confidence, using only 100 independent samples from an eight-chan­nel 27 to 54 kH z swept-frequency sounder. This confi­dence level exceeds 95 % if 1000 samples are taken. The discrimination is independent o f day o r night, and in the case of the gadoids, state of acclimatization. T he repea t­ability betw een experim ents and over time, despite sub­stantial changes in backscattering strength, indicates a high probability tha t fish in the wild will also exhibit distinct and repeatab le spectra. These may not, how­ever, correspond to the spectra shown here. T he re ­qu irem ent for only 1000 samples, equivalent to a 10-m- high by 45-m-wide shoal observed from a vessel trav ­elling at 10 knots, for 90 % discrimination m eans tha t this could be of value as a m ethod of species identifica­tion w here fish shoals of different species occur within a small area.

Saithe after aclimatization

386

AcknowledgementsWe would like to thank D. N. M aeLennan for providing the frequency response of the tungsten carbide ball, S .T . Forbes for his assistance in developing the wide­band system, and P. J. Copland, I. B. Petrie, and T. V. Taylor for assistance with the experim ental work and construction of the system.

ReferencesBerktay, H .O . , Dunn, J .R . . and Grazey, B .K. 1968. Con­

stant beamwidth transducers for use in sonars with very wide frequency bandwidths. Appl. Acoust., 1: 81-99 .

Berktay, H. O., and Learly, D. J. 1974. Far field performance of parametric transducers. J. acoust. Soc. Am., 55(3): 539-546.

Blaxter, J. H. S., Denton, E. J., and Gray, J. A. B. 1979. The herring swimbladder as gas reservoir for the acoustics-Iat- eralis system. J. mar. Biol. Ass. UK, 59: 1-10 .

Foote, K. G. 1985. Rather-high-frequency sound scattering by swimbladdered fish. J. acoust. Soc. Am ., 78(2): 688—700.

Forbes, S. T., Simmonds, E. 1., and Edwards, J. I. 1982. Target strength of gadoids. ICES/FAO Symposium on Fisheries Acoustics, Bergen, Norway, 2 1 -2 4 June 1982.

Nakken, O., and Olsen, K. 1977. Target strength measure­ments of fish. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: 52 -69 .

Rodgers, P. H ., and van Buren, A. L. 1978. A new approach to constant beamwidth transducers. J. acoust. Soc. Am ., 64(1): 38-43 .

Sand. O., and Hawkins, A. D. 1974. Measurements of swim­bladder volume and pressure in cod. Nor. J. Zool., 22: 31-34 .

Simmonds, E .J . 1984. A comparison between measured and theoretical equivalent beam angles for seven similar trans­ducers. J. Sound Vib., 97(1): 117-128.

Simmonds, E .J . , and Copland. P. J. 1986. A wide band con­stant beam width echo sounder for fish abundance estima­tion. In Proceedings of the Institute of Acoustics, Salford, 1986, pp. 173-181.

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