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-be am sonar and echosounder. Variables recorded included the horizontal a rea and vertical extent, depth, density and structure of schools and t heir swimming speed and direction. Herring schools were defined into f ive categories that differed in several respects: Immigrating schools had a large horizontal area and a more elongated shape than other scho ols. They swam deep and had a consistent swimming speed and direction. Searching schools were smaller and more dense and their swimming spee d and direction varied. Sprawning schools settled on the bottom; they had small vertical extent and low density. Emigrating schools were sma ller in horizontal area and less dense than immigrating schools and th ey swam higher in the water column. Feeding schools were of low densit y and were dynamic in their shape, density and movements. Feeding was not observed before spawning, but started shortly after spawning. Func tional explanations for the observed differences are given.