Schooling dynamics of Norwegian spring spawning herring (Clupea harengus L) in a coastal spawning area

The behaviour of Norwegian spring spawning herring (Clupea harengus L.) was studied in a spawning ground off Karmoy in southwestern Norway. A total of 47 schools were tracked from 5 to 60 minutes using multi-beam sonar and echosounder. Variables recorded included the horizontal area and vertical extent, depth, density and structure of schools and their swimming speed and direction. Herring schools were defined into five categories that differed in several respects: Immigrating schools had a large horizontal area and a more elongated shape than other schools. They swam deep and had a consistent swimming speed and direction. Searching schools were smaller and more dense and their swimming speed and direction varied. Sprawning schools settled on the bottom; they had small vertical extent and low density. Emigrating schools were smaller in horizontal area and less dense than immigrating schools and they swam higher in the water column. Feeding schools were of low density and were dynamic in their shape, density and movements. Feeding was not observed before spawning, but started shortly after spawning. Functional explanations for the observed differences are given.