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In situ swimming speed and swimming behaviour of fish feeding on the krill Meganyctiphanes norvegica

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In situ swimming speed and swimming behaviour of dielly migrating planktivorous fish were studied at a 120-m-deep location. Acoustic target tracking was performed using a hull-mounted transducer and submersible transducers located on the sea bottom and free hanging in the water column. The original data displayed a relationship between distance to transducer and swimming speed. A simplistic smoother applied during post-processing, appeared to break this relationship. Target tracking thus provided robust results on in situ swimming behaviour throughout the water column. Swimming speeds of deep-living fish, mainly Norway pout (Trisopterus esmarkii) and whiting (Merlangius merlangus), were highest during the day (speeds centred around 14–16 cm·s–1) and decreased somewhat by night (modes around 10–11 cm·s–1). Fish in the upper 10–30 m swam somewhat faster (speeds ranging from 16 to 24 cm·s–1). Fish in the upper layer at night were mainly Atlantic herring (Clupea harengus), sprat (Sprattus sprattus), and whiting. We ascribe the reduction of swimming speed in deep-living fish at night to a switch from visual feeding during daytime to nonvisual feeding by night. We suggest that shallow-living fish could forage visually even by night. Most tracks were fairly short, but some long tracks unveiled elaborate swimming paths as well as cyclic swimming behaviour.

Nous avons étudié la vitesse de nage in situ et le comportement de nage chez des poissons planctonophages à migration journalière à site d'une profondeur de 120 m. Nous avons fait un suivi acoustique des cibles à l'aide d'un transducteur monté dans la coque et des transducteurs submersibles sur le fond de la mer et en suspension dans la colonne d'eau. Les données originales montrent une relation entre la distance du transducteur et la vitesse de nage. Une méthode simpliste de lissage durant le post-traitement des données semble faire disparaître cette relation. Le suivi fournit donc des résultats robustes in situ du comportement de nage dans toute la colonne d'eau. Les vitesses de nage des poissons des profondeurs, surtout des tacauds norvégiens (Trisopterus esmarkii) et des merlans (Merlangius merlangus), sont maximales durant le jour (vitesses concentrées autour de 14–16 cm·s–1) et diminuent un peu la nuit (modes autour de 10–11 cm·s–1). Les poissons dans les 10–30 m supérieurs nagent un peu plus vite (vitesses allant de 16 à 24 cm·s–1). Les poissons des couches supérieures la nuit sont surtout des harengs atlantiques (Clupea harengus), des sprats (Sprattus sprattus) et des merlans. Nous expliquons la réduction de la vitesse de nage des poissons des profondeurs la nuit au passage d'une alimentation visuelle durant le jour à une alimentation non visuelle la nuit. La plupart des tracés sont relativement courts, mais quelques tracés plus longs révèlent l'existence de trajets élaborés de nage, ainsi que de comportements cycliques de nage.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: August 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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