Schooling dynamics of spawning herring (Clupea harengus L.) in a bay in South-Western Norway

Abstract

The schooling dynamics of hening (Clupea harengta L.) was investigated during spawning in Raunefjord, south-westem Norway in 1994. A single school of Nonvegian spring spawning hening WSS) was tracked during daytime over a five day period in the spawning season, using multibeam sanning sonar and echosounder. Gillnet samples of the herring were coilected daily fiom the school. The school completed spawning in 3-4 bys. It remained one unit throughout the period, but when spawning was initiated, the school divided vertidy into two components, one pelagic and one derned. DiBlering individual choices in the trade-off between su~val and reproduction hr fish prior to, during and aikr spawning may have caused vertical gradients of key h r s such as food and predaton to act as dividing forces. Prior to spa*, Late matured and ripened individuals seeking d o w n d and early matured and spent individuals searching upwards probably caused the vertical school shape to be cyhdricai. As spawning proceeded, fish from the demersai component spred outwards at the bottom, causing this component to take on the shape of a carpet, whereas the pelagic unit condens.e d. into a somewhat tighter baU. Wben spa+ was completed, the two to fonn a loose flake dose to the suhce. Possible effects of timing and duration of the spa* period on acoustic survey estimates of spawning herring are discussed.