94/0386

Philippe BOËTREGULATED RIVERS: RESEARCH & MANAGEMENT. VOL. 9. 121-126 (1994)

SHORT COMMUNICATION

APPLICATION OF MOBILE ACOUSTIC TECHNIQUES FISHSURVEYS IN SHALLOW WATER: THE RIVER SEINE

JEAN GUILLARDINRA. 75 Avenue de Corzem, BP 511. 74203 Thonon-les-Bains Cedex, France

PHILIPPE BOETCEMAGREF, Division Qualité des Eaux, 14 Avenue de Saint-Mandé. 75012. Paris, France

DANIEL GERDEAUXINRA, 75 Avenue de Corzem, BP 511, 74203 Thonon-les-Bains Cedex. France

and

PHILIPPE ROUXLaboratoire Ondes et Acoustique. Université Paris 7, Ecole de Physique et de Chimie Industrielle de la Ville de Paris,

10 rue Vauquelin, 75231 Paris. France

ABSTRACTMobile acoustic techniques (echo-sounding and echo-integration) for fish surveys were used in shallow waters of depthsless than 5 m. These techniques show the importance offish biomass in the side-channels and the detection of echoes inthe main channel. Diurnal spatial migrations are seen. The advantages and limitations of the method as a complement totraditional sampling methods in the river are examined.

KEY WORDS Fish surveys Acoustic techniques Shallow water Rivers

INTRODUCTION

Vertical active sonar is increasingly used for studying freshwater fish populations (Guillard, 1991), par-ticularly in lacustrine environments where the bathymetry is favourable. However, although the applic-ability of sonar in shallow waters (depth < 10 m) has been demonstrated (Gerdeaux, 1986; Gerlotto et al.,1992), only a few studies have been carried out in rivers and those that have mainly with the use of fixedand horizontal transducers (Johnston and Hopelain, 1990; Mesiar et al., 1990; Kubecka el al., 1992).

This paper briefly describes preliminary experiments with vertical mobile sonar in the River Seine anddiscusses the method, its originality and limitations, as well as the technical improvements necessary toobtain more reliable biological data in shallow waters such as rivers and off-channel areas.

METHODS

Experiments were performed using a shallow draught boat (4-61 m long x 1-73 m wide x 0-30m high)moving at a constant speed of about 6kmh~'. The acoustic equipment consisted of a Simrad EY-200echo-sounder operating at 70 kHz and using a vertical transducer fixed in front of the boat with a

CCC 0886-9375/94/020121 -06 Received 3 July 1993r 1994 by John Wiley & Sons. Ltd. Accented 11 Nnvpmher 100 ?

122 i. GUILLARD ET AL.

half-beam angle of 11° at - 3 dB; the duration of the signal was 0-3 ms. The depth resolution of the trans-ducer was 0-22 m and the acoustic cone had an average surface area of 1-9 m at 4 m depth. The systemwas calibrated using a copper sphere of -39-2 dB target strength and by the measurement of a test signal.

Electrical noise of about 0-5 ms duration at each emission prevented us from working within the first metrebelow the surface. The near-field distance of the transducer was less than 0-50 m. These limitations (i.e. thetransmission noise, draught of the boat and depth resolution of the sounder) limited our studies to the layerfrom 1-50 m depth to 0-50 m above the river bed.

A portable oscilloscope was used to inspect the data, which were stored on a digital audio tape recorderand printed on a colour printer. Data processing was carried out in the laboratory using the software HA-DAS developed by T. Lindem (Walline et al., 1992). The echo-sounder uses a time variable gain (TVG) func-tion, which is an amplifier where the gain increases with depth to compensate for the geometric spreadingloss. The amplification is (aR + 20logR) or (aR + 40log R), where a is the sound attenuation coefficientin freshwater and R is the distance in metres between the target and the transducer. According to theTVG function selected, this software allows the calculation of biomass (aR + 20 log R) or the size distri-bution of the echoes (aR + 40 log R) using the modified algorithm of Craig and Forbes (Craig andForbes, 1969; Lindem, 1983). The estimation of an average target strength for the fish population in theRiver Seine has not yet been addressed, so it is not possible to convert from echo-integration units to bio-mass. Mean densities for each 10 minutes, a distance of about 1 km, were expressed in echo-integration unitsof volts2 m~2.

A first survey was attempted downstream of Paris in April 1992; however, because of the busy boat trafficand high noise in the environment, the use of acoustic methods in such a disturbed areas was difficult andappeared to be unsuitable as a sampling method.

A second survey was conducted in July 1992 in a reach upstream of Paris (Figure 1) where traffic density ismuch lower. The mean depth is about 5 m in the main channel and 4 m in the backwaters, sandpit and deadarm, where navigation is not possible. More than 25 fish species were present, with roach, Rutilus rutilus (L.),being the most abundant (Allardi et ai, 1991).

RESULTSThe echograms show a clear difference between fish density in the main channel and in the two backwaters(Figure 2). A noise level of 220mV, compared with the largest echo amplitude of 1-5V and TVG(aR + 20 log R) starting at 1 m was used for echo-integration. The noise level was chosen to eliminaterecords of bubbles, zooplankton and to take account only of the fish. The signal was not filtered andsurface noise, bubbles and zooplankton aggregations, which were lower than the noise level used for echo-integration could be seen on the echograms. The analysis showed difference in fish density between the areas(table I).

Differences in fish densities between night and day were also observed in the backwaters, as pointed out byGuillard (1991). During daylight, fish were schooling near the bottom (Figure 3); some fish may have movedout of the area of near to the banks.

The main channel and the sandpit were surveyed with the 40 log R TVG starting at 1 m. With respect to thehypothesis of the Craig and Forbes' modified algorithm (the number of single fish echoes must be higherthan 1000), only an estimate of the size distribution of the data collected in the sandpit was applicable(1162 single fish echoes). A multimodal distribution offish size was observed when the results were dividedinto 2 dB classes (Figure 4).

To assess the influence of the TVG in shallow waters a survey was carried out without using the TVGfunction. In this instance the results of echo-integration were lower (table I).

DISCUSSIONDespite the shallowness of the water bodies studied, use of the amplification function is essential andrecording must start as near to the surface as possible. In such shallow waters (about 4 m deep) with an

FISH SURVEYS 123

La Tombe

Figure 1. Map of the sampling area, about 100 km upsiream of Paris. The rectangular area delimits the sampling zone: the sandpit is onthe right of the map

irregular bottom, moA of echo-integration software are not easy to use because it is impossible to processlayers < 1 m deep, which is often insufficient when the whole sampled water height is about the same size.

Technical improvements might allow the collection of more reliable data, but as the acoustic method inshallow waters is a poorly studied subject, they have not been tested. For example, the length of the acoustic

J. GUILLARD ET AL

DEAD-ARM CHANNEL SANDPIT

Figure 2. Night echogram showing the difference between the main channel and backwaters. The signal is not filtered and the noise leveland bubbles, which are not taken into account in the echo-integration, can be seen

emission could be reduced to improve the depth resolution, and to decrease the area above the bottom whichcannot be examined. The resolution would be improved by the use of a higher frequency, but this willincrease the noise level. Considering that a large amplification gain is not necessary in such shallowwater, the transmission noise of the transducer may be attenuated, which will reduce the blind area belowthe surface. Finally, a larger half-beam angle transducer, of about 20". will increase the size of the acousticcone and. hence, the volume of water sampled. The use of a less noisy motor will reduce the avoidance offishnear the surface.

Nevertheless, these experiments confirm the potential of mobile, vertical and active sonar in this kind ofenvironment, even if the depth is less than 5 m. Echograms and echo-integration can be obtained, whichconfirm the presence offish in the main channel and in backwaters, and the major role played by off-channel

Table I. Mean fish densities in echo-integration units (V" m ~) in three areas

Time

With a TVG function30 June 1992

2230 h1 July 1992

0500 h1430 h

Without TVG function30 June 1992

2200 h1 Jul; 1992

2200 h2300 h2400 h

Main channel

0062

00410046

003700110017

Sandpit

1-90

1 170-38

015

012Oil0089

Dead-arm

1-50

150-33

FISH SURVEYS 125

0 m

2 m

FISH SCHOOLS

Figure 3. Daylight echogram in the sandpit, with the fish schooling near the bottom. The signal is not filtered and the noise level andbubbles, which are not taken into account in the echo-integration, can be seen

Classes of 2 dB

Figure 4. Distribution offish target strength in the sandpit, from the modified algorithm of Craig and Forbes (1969). in 2dB size classes

areas. As it is difficult to obtain reliable fish samples in large lowland rivers with classical fishing methods(Dahm, 1987) the use of acoustic techniques may greatly improve our knowledge offish populations insuch environments. It is important to encourage the use of different techniques in environmental studiesto improve the techniques and reduce their limitations.

126 J. CUILLARD ET AL.

REFERENCES

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2I0-2Ï9.Dahm, E. 1987. 'Bibliography of existing literature on selectivity of inland water fishing gear published by European authors'. EIFAC

Occasional Paper No. 18 European Inland Fisheries Advisory Commission of the Food and Agriculture Organization. 46 pp.Gerdeaux, D. 1986. 'Ecologie due gardon (Rutilus rulilus L.) et sandre (Lucioperca lucioperca L.) dans le lac de Créteil de 1977 à 1982.

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Mesiar, D. C, Eggers, D. M. and Gaudet, D. M. 1990. 'Development of techniques for the application of hydroacoustics to countingmigratory fish in large rivers', Rapp. P-r. Réun. Cons. Int. Explor. Mer, 189, 223-232.

Walline, P. D., Pisanty, S. and Lindem, T. 1992. 'Acoustic assessment of the number of pelagic fish in lake Kinneret, Israel', llydrobio-logia, 231,153-163.