Synchronous and vertically undulating swimming behaviour of Atlantic cod Gadus morhua

Predatory fish often forage in schools, but observations of actual schooling behaviour in deep-water ecosystems are scarce. In a ranching study of wild Atlantic cod Gadus morhua L. in a fjord in east Iceland, observations of peculiar synchronous and vertically undulating schooling behaviour of the fish were recorded. These fish had been conditioned to feed at the sound of a low-frequency signal. On 23 April 1996, prior to a feeding session, several of these cod followed the feeding boat which drifted slowly (0.35 m s(-1)) into the fjord. During a 24 min observational period and 500 m drift of the boat, 8 to 12 cod were monitored with an echo sounder. These fish, approximately 60 cm long, showed a striking behaviour of synchronized swimming along vertically undulating paths. Six successive undulations were observed, each 40 to 60 m long, 10 to 20 m high and lasting for 2 to 3 min. The mean angles during ascent and descent were between 50 and 60 degrees. The vertical distances between the paths of single fish decreased as the school approached the bottom and increased as it ascended up into the water column. This undulating swimming behaviour may be a foraging strategy meant to sweep part of the water column in search for the depth where prey with a vertically clumped distribution is located. Additionally, synchronized swimming, where visual contact is maintained, may facilitate the detection and capture of pelagic prey.