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

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