TILT ANGLE DISTRIBUTION AND SWIMMING SPEED OF OVERWINTERING NORWEGIANSPRING SPAWNING HERRING

When plankton production in the feeding areas decreases in the fall, a dult Norwegian spring-spawning herring migrate into two fjords in nort hern Norway. In these wintering areas the herring occupy deeper water. Lacking the ability to refill the swimbladder they are constantly neg atively buoyant. This leads to different adaptive behaviour during the day and al night, behaviour which is reflected in swimming angle. Spl it-beam tracking methods and still-frame photography have been used to study the herring behaviour inside the dense wintering schools. Negat ive buoyancy seems to be controlled through constant swimming at speed s between 0.25-0.42 ms(-1) because these are sufficiently high to gene rate lift when the pectoral fins are used as spoilers. During the day, when the layers aggregate, the average swimming angle is close to hor izontal while positive average swimming angles of up to 40 degrees wer e recorded at night. A bimodal distribution of tilt angles, with one p ositive and one negative component, indicating a ''rise and glide'' sw imming strategy was also observed at night. Vertically undulating spli t beam tracks confirmed this particular type of swimming behaviour. As adult herring are directional targets at the echo-sounder frequency u sed for acoustic assessment of the stock, the possible impact of the o bserved lilt angles on average acoustic target strength is discussed. (C) 1996 International Council for the Exploration of the Sea