The swimbladder is recognized as responsible for a major part of the acoustic backscattering from fish. In most fishes it has the function of a buoyancy regulator but in others its main function is rather unclear. Based on methods for exact mapping of the swimbladder shape, observations of deviations from normal appearance and shape are discussed in relation to possible effects on target strength. Evidence for both periodic variations, as from uncompensated vertical migrations, and seasonal variations, caused by the fat cycle and gonad development, are presented.

Target-strength (TS) experiments on captive, adult herring have been conducted at intervals over several years to investigate the acoustic effect of pressure and seasonal changes on fish physiology. Experiments in a large, net pen (4500 m3) were done at 18, 38, and 120 kHz frequencies with calibrated, split-beam echosounders. The main ex situ results at 38 kHz, including a vertical-excursion experiment, were combined with in situ TS data collected at 38 kHz with a probing, split-beam transducer lowered into the dense herring layers, recorded during two surveys in the wintering area of the Norwegian spring-spawning stock. Multiple-linear regression analysis was used to investigate the functional relationship between TS and the measured parameters. The mean TS of herring was found to be significantly dependent on the depth (pressure) and the gonadosomatic index. These are the additional parameters included in the new TS relationship.

There are particular difficulties in making acoustic estimates of the abundance of demersal and semi-demersal fish. One possibility which exists in any survey situation is that the fish may move from the direct path of the vessel because of the noise it is radiating. However, the problems addressed here are primarily due to the physical characteristics of the transmitted acoustic pulse from the echo-sounder and its interaction with fish close to the seabed. This paper looks at the factors controlling the detection of these fish in terms of the acoustic sampling volumes near the bottom, the discrimination theoretically possible between fish and seabed echoes and the ''depth anomaly''. The acoustic deadzone is defined and its volume is determined. Practical aspects of signal processing in this near seabed situation are then described, including seabed recognition and safeguarding fish signals from contamination by the bottom echo and from noise. Next, an echo-integrator deadzone comprising the acoustic deadzone, the backstep zone, and the partial integration zone (related to pulse length) is described and defined. Equations for calculating the effective volume or effective height of this deadzone are developed. Estimation errors due to the echo-integrator deadzone are investigated and equations derived for the necessary corrections. An example is shown of partial failure of the bottom recognition system and how the echo integrator result can be corrected to compensate.1996 International Council for the Exploration of the Sea

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Abstract:To determine whether seismic exploration affected abundance or catch rates of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), acoustic mapping and fishing trials with trawls and longlines were conducted in the central Barents Sea 7 days before, 5 days during, and 5 days after seismic shooting with air guns. Seismic shooting severely affected fish distribution, local abundance, and catch rates in the entire investigation area of 40 × 40 nautical miles.Trawl catches of cod and haddock and longline catches of haddock declined on average by about 50% (by mass) after shooting started, which agreed with the acoustic abundance estimates; longline catches of cod were reduced by 21%. Reductions in catch rates were observed 18 nautical miles from the seismic shooting area (3 × 10 nautical miles), but the most pronounced reduction occurred within the shooting area, where trawl catches of both species and longline catches of haddock were reduced by about 70% and the longline catches of cod by 45%; a relatively greater reduction was found (in catches and acoustic estimates) for large (>60 cm) than for small fish. Abundance and catch rates did not return to preshooting levels during the 5-day period after seismic shooting ended.Résumé : Pour déterminer si l'exploration sismique nuit à l'abondance de la morue (Gadus morhua) et de l'églefin (Melanogrammus aeglefinus) ainsi qu'au taux de capture de ces espèces, on a procédé à des travaux de cartographie acoustique et à des essais de pêche au chalut et à la palangre dans la partie centrale de la mer de Barents 7 jours avant, 5 jours durant et 5 jours après une série de tirs sismiques au canon pneumatique. Les tirs ont considérablement modifié la répartition du poisson, abaissé sa densité locale et gravement réduit le taux de capture dans l'ensemble de la région d'étude (soit un secteur mesurant 40 milles marins de côté). En moyenne, les prises de morue et d'églefin au chalut et les prises d'églefin à la palangre ont diminué d'environ 50% (en masse) après que la série de tirs ait commencé. Ces résultats concordent avec ceux des relevés acoustiques d'abondance. Les prises de morues à la palangre ont été réduites de 21%. Des baisses des taux de prise ont été observées jusqu'à 18 milles marins du secteur des tirs sismiques (un secteur de 3 × 10 milles marins), mais l'effet le plus prononcé a été observé à l'intérieur de ce secteur : les prises au chalut des deux espèces et les prises à la palangre de l'églefin ont diminué d'environ 70%, les prises de morue à la palangre d'environ 45%. La réduction est proportionnellement plus marquée dans le cas des poissons de grande taille (>60 cm) que celle des poissons de petite taille, tant dans les prises que dans les estimations par relevé acoustique. Cinq jours après la fin des tirs sismiques, l'abondance et les taux de capture ne s'étaient pas encore rétablis. [Traduit par la Rédaction]