Simulation and experimental measurement of side-aspect target strength of Atlantic salmon (Salmo salar) at high frequency

Abstract

Numerical simulations and empirical measurements of swimming Atlantic salmon (Salmo salar) were used to describe the effects of fish behavior on side-aspect target strength (TS). Simulation results were based on the numerical solution of the Helmholtz equation with the finite element method (FEM). A three-dimensional geometric model approximated the shape of the swimbladder of an Atlantic salmon. Numerical simulations were used to study the dependence of TS on the fish length, orientation, and swimming behavior. The results showed strong variation in TS, both when the side-aspect angle was changed and when the swimbladder was bent to the direction of the sonar beam. A total of 11 swimming adult Atlantic salmon 62-107 cm long were measured with a horizontally aimed echosounder (200 kHz) and video camera, and the experimental results were compared with the corresponding simulation results. The linear regression between mean TS and the logarithm of fish length (L, cm) was TS = 24.4log 10 (L) -72.9 dB. The strong variability of TS owing to the orientation and bending of the fish and large L/λ ratios reduces the usefulness of TS alone for fish size estimation or species discrimination.Résumé : Des simulations numériques et des mesures empiriques de la nage chez le saumon de l'Atlantique (Salmo salar) nous ont servi à décrire les effets du comportement des poissons sur l'intensité de la cible (TS) acoustique en aspect latéral. Les résultats des simulations se basent sur la résolution numérique de l'équation de Helmholtz par la méthode de l'élément fini (FEM). Un modèle géométrique tridimensionnel mime approximativement la forme de la vessie natatoire d'un saumon atlantique. Des simulations numériques ont servi à étudier la dépendance de TS de la longueur, de l'orientation et du comportement de nage du poisson. Les résultats indiquent une forte variation de TS, tant lorsque l'angle de l'aspect latéral est modifié que lorsque la vessie natatoire est repliée dans la direction du faisceau sonar. En tout, nous avons mesuré 11 saumons atlantiques en nage, variant en longueur de 62 à 107 cm, avec un sonar (200 kHz) dirigé horizontalement et avec une caméra vidéo et nous avons comparé les résultats expérimentaux aux résultats simulés correspondants. La régression linéaire entre le TS moyen et le logarithme de la longueur (L, cm) est TS = 24,4log 10 (L) -72,9 dB. La forte variabilité de TS reliée à l'orientation et au repliement du poisson, ainsi qu'aux rapports L/λ élevés, réduit l'utilité des TS seuls pour estimer la taille des poissons ou pour discriminer entre les espèces.[Traduit par la Rédaction] Lilja et al. 2236

Two methods to improve the accuracy of target-strength estimates for horizontal beaming

“…The TS simulations demonstrated that, just as reflecting light shows a quick flash in a particular direction, acoustic waves of sufficiently high frequency also exhibit similar behaviour. This was termed a 'flash-effect' (Lilja et al, 2004). Burwen et al (2007), by synchronizing a split-beam echo sounder with a high-resolution multibeam sonar (DIDSON; Blecher et al, 2001), confirmed earlier presumptions that, depending on the target orientation, other anatomical structures can contribute to the target backscatter and can influence the shape of the resulting echo.…”

“…Short stretches within the fish traces of tested fish displayed side-aspect detections with the strongest echoes corresponding to perpendicular swimming through the beam. The maximum target strength is known to represent the side-aspect (Love, 1969;Kubečka, 1994;Frouzová et al, 2005), although the maximum might also be located a few degrees away (Blaxter and Batty, 1990;Kang and Hwang, 2003;Lilja et al, 2004). Hence, the correction of in situ target strength for a fixed pan angle might not correspond with the right off-angle position to achieve exactly the aspect with maximum sound reflections and might cause bias.…”

“…Lilja et al (2004) used numerical simulations to show that swimbladder bending severely influences TS measurements. Bending in the direction of the sonic beam caused strong fluctuation of the backscattering amplitudes, with a strong peak of TS in a narrow angular range near the full side-aspect.…”

Hydroacoustic fish biomass assessment in man-made lakes in Tunisia: horizontal beaming importance and diel effect

“…The measured TS of a fish depends on the fish behaviour at the time the boat passes overhead, since this affects the angle by which they are detected (Gauthier and Rose 2001;Lilja et al 2004;Frouzova et al 2005;Simmonds and MacLennan 2005). Day and night difference in TS mode values were observed for herring in the Baltic Sea and these differences were attributed to changes in fish behaviour between the two photoperiods (Axenrot et al 2004).…”

Controllability method for the Helmholtz equation with higher-order discretizations

“…This property is important, for example, when studying the scattering of sonar echoes by underwater objects (see, e.g., [53]). In this section, we describe the method which we have used to compute the far-field pattern and the associated sonar cross section (SCS) of the scattered wave.…”