River entry of adult Atlantic salmon Salmo salar into the River Tornionjoki, monitored during three migration seasons (1997–1999) by horizontal split‐beam hydroacoustics, started early in June when water temperature was c. 9° C and when the discharge varied between 1700 and 2000 m3 s−1. In 1997 and 1999, migration peaked during the latter half of June, 17 days after the peak flood, at water temperatures ranging from 11· 5 to 18·2° C. Few statistically significant correlations were observed between river entry and six measured environmental factors and those that were significant were not persistent over the years. The strongest correlation ( r = −0·60) was between the number of upstream migrants and seawater level, with a time lag of 1 day in 1998. In 1998 and 1999, no clear diurnal migration pattern was observed, although in 1997 the intensity of midday migration was higher than that of the midnight migration. It is concluded that environmental factors have little effect on river entry of Atlantic salmon in a large pristine river located at high latitude.
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
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