Tilt angle distribution and swimming speed of overwintering Norwegian spring spawning herring

• Published in: ICES Journal of Marine Science
• Main Authors: Huse, Ingvar, Ona, Egil
• Format: Text
• Language: English
• Published: Oxford University Press 1996
• Subjects: Articles; Norway; Northern Norway
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/53/5/863; https://doi.org/10.1006/jmsc.1996.9999

When plankton production in the feeding areas decreases in the fall, adult Norwegian spring-spawning herring migrate into two fjords in northern Norway. In these wintering areas the herring occupy deeper water. Lacking the ability to refill the swimbladder they are constantly negatively buoyant. This leads to different adaptive behaviour during the day and at night, behaviour which is reflected in swimming angle. Split-beam tracking methods and still-frame photography have been used to study the herring behaviour inside the dense wintering schools. Negative buoyancy seems to be controlled through constant swimming at speeds between 0.25–0.42 ms−1 because these are sufficiently high to generate lift when the pectoral fins are used as spoilers. During the day, when the layers aggregate, the average swimming angle is close to horizontal while positive average swimming angles of up to 40° were recorded at night. A bimodal distribution of tilt angles, with one positive and one negative component, indicating a “rise and glide” swimming strategy was also observed at night. Vertically undulating split beam tracks confirmed this particular type of swimming behaviour. As adult herring are directional targets at the echo-sounder frequency used for acoustic assessment of the stock, the possible impact of the observed tilt angles on average acoustic target strength is discussed.