i - FREON P GEG Fisheries Acou5ti ~ Gcience G Technoogy E FAST:) Working Group Meeting, Ostende, Begium, 9-22 Apri, 986, Not to be cited without permission of the authors Ne pas citer sans autorisation des auteurs A methodoogy for in situ measurement of mean target strength in sma schoos Pierre Freon G François Gerotto Pôe de Recherche Océanoogique et haieutique Caraïbe ORTOM, BP 8, 97256 Fort-de-France Cédex, Martinique (French W - NTRODUCTON Different methods of TS measurement have been aready performed on singe fish (caged, tethered or wid in situ) or on number of ive fish in a cage [Johannesson and Mitson, 983; Foote, 987): Each method presents its own advantage and imitations The three main probems to sove are: () to perform the measure on fish behaving as cose as possibe from their natura: behaviour and physioogica condition, (2) to take into account the effect of the transducer beam pattern, c (3) to take account of the bias introduced by high fish density (acoustic shadowing or re-radiation) when schoo echoes are integrated During the ast decade the scientific effort was oriented toward the resoution of mosty one of these three probems at the same time, by measuring in situ individua wid fish when distributed in ow density (dua beam or spit beam) or by measuring fish in a sma cage Osen (986) intended to estimate the sound attenuation under a arge herring schoo, but as far as we know, no attempt of TS measurement as been done on wid schoo, athough this seems possibe when some particuar conditions are satisfied n this case, the above mentionned three probems are ORSTOM Fonds 5ocumenta re
r M t a overcome, EQUPMENT An EYM Simrad'sounder with a wide beam transducer (22O) has been used, connected to an Agenor [FREMER/ORSTOM) digita echointegrator and to a tape recorder (during the future experiment, a anaogic-digita converter wi be used in order to reduce the possibe biases introduced by anaogic tape recording) A wide ange camere was connected to a video tape recorder equipped with a precise revoution counter and aowing a performant sow motion and frame by frame pay-back A microphone was aso connected for comments and for checking the synchronisation between tape and video recording A 6 meter tube ended by a one meter graduated bar was used for caibrating the size of the video pictures according to the depth and to the monitor screen size (fig ) A this equipment was instaed on an instrumented raft, providing a support for the transducer and the camera more stabe and shaower than a boat in the coasta area where the experiment was carried on Some observations have been done on competey wid schoos and others on sma schoos encose2 in a arge net (purse-seine) - used as a mesocosm and providing an epparent natura behaviour of the schoo (see fig 2 and Fréon & Gerotto, this meeting) METHODOLOGY Using Johannesson E Mitson"s (983) notation, where Sv is the mean voume back-scattering strength, we get: where TS is the mean target-strength and F~ the mean density expressed in number of fish per cubic meter f some conditions are satisfied, the mean density pv of a thin fish ayer can be estimated using a sounder and a camera This can be done considering the voume V of the truncated cone deimited on the one hand by the camera fied of view and on the other hand by the upper end ower imits of the ayer Cd, and de, ' obtained from the sounder (fig 3 So we get: a! h = ds- d, j r = tg E, d, t ' R ii tg Q, diz, h -, n - (R* + r2 + Rr) 3 hf the ayer density is ikey homogeneous and presents a fairb donstant thickness (as on photo and 2), and if the mean" d-epth of this ayer is rather constant during a few seconds, then 2
some samped wiewr; aan be used for estimating the mean densit-y inside the voume V For instance on u stabe 30 second sequence the samping frequency coud be of one freme each second The frame by, frame system of the video recorder can be used, or a digitaized picture can be anaysed on a computer The species composition and the mean fish ength can be estimated either by fishing or by using the wid-eo for measuring the fish on the monitor and cacuating the' rising factor from the caibration resuts and according to the mean depth given by the sounder f the ayer thickness is too high and introduces a arge variabiity of the apparent engths measured on the screen, then ony the argest fish can be measured, considering that they are ocated in the upper part of the ayer (such a method supposes a narrow distribution of the body engths and tit anges inside the schoos) Other approaches can be deveopped using stereo camera or a second video camera (or photo camera) with a arge foca ens providing a narrow depth of fied n this ast case, a narrow interva of depth can be samped inside the ayer by measuring ony the fish presenting a good resoution The caibration of this second camera must be achieved under identica conditions to those taking pace during the experiment on the schoo (turbidity, ight intensity and direction), using the same graduated tube or better a died fish This w i provide both the precise mean depth of samping and its range f the transducer and the camera ens ure propery choosen in order to have the ange 8 of the ens widey greater than the mean ange of the transducer beam at -5 db for instance, therefore the Sv vaurs can be assumed to be representative of the mean acoustic response of the transducer for a given depth, when "the ayer is observed on the whoe screen surface Some experiments were conducted after fixing the camera and the transducer on the raft, but others were reaized with these devices fixed on the battom and oriented toward the surface According to the depth of the ayer and to the water transparency, one or the other method is suitabe f the fish directivity diagram i5 suppose'd to present a vertica axis of symetry -as usuay admitted- then the resuts must be consistent Observations from the bottom present three advantages: first the camera and the transducer are absoutey stabe and provide ess variabe data, second the pictures are perfecty contrasted ("shadow show") and third there is absoutey no infuence of the equipment on the fis,h behaviour Knowing < and pv, TS can be easiy cacuated V DSCUSSON quite simiar to the one appied when competey the main beam ndeed, fied of view can be compared to and the natura upper and ower to the top and bottom of the cage The two the two approaches are 3 4
c first the compitey natura behaviour in our experiment and se- cond a more random distribution of the fish with respect to the transducer beam pattern This methodoogica approach cannot be appied to any species and biotopes t is essentiay adapted to some coasta peagic species [or sma demersa species iving in schoos], iving in transparent water The foowing conditions must be satisf ied: () distance between the fish ayer {or schoo) and the set camera-transducer inside a 2 to 2 meter interva, ie the ayer must be cose enough to the bottom or to the surface, (4) homogeneous density of the fish ayer, without "vacuoes", and presenting a rather stabe thickness, (5) if the camera must be used from the surface, shaow ground and homogeneous sea bed coor providing a good contrast with the fish Further experiments carried on inside the encosure on the same schoo shoud provide estimations of the measument variabiity and indication on the repetitivity of the behaviour infuence on the TS For instance, TS measurement coud be done at different eves of poarization of the schoo (using artificia stress) or -at different density or thickness REFERENCES Foote, KG 987 Fish target strengths for use in echo integrator surveys J Acoust Soc Am (in press) Johannesson KA E; Mitson, R6 983 Fisheries acoustics: e practica manue for aquatic biomass estimation FAO Fish Tech Paper 240: 249 p Osen, K 986 Sound attenuation within schoos of herring CES Fish Capture Committee, CM 6:44 Sess U: 5 p FGURES G PHOTOS Fig Fied equipement for in situ measurement of T6 Photo 2 Vertica view of a of Harengub jaguana ayer Photo 2 Latera view of a of Harengua,aRuana ayer, 4
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