Dynamics of moving masses: Variability in packing density and shape among pelagic schools
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The packing density and external shape of herring, saithe and sprat schools in the North Sea and along the Norwegian coast are measured acoustically. The packing density was inversely. related to the fish size, but species and regional differences among the schools were detected. There were great variations in packing density from one school to another, and a new 'cell'-integration technique revealed even greater packing density variations within single herring schools. The schools were shaped as flattened discoides close to the surface or sea bottom and more spherical midwater, but there were great variations among the schools, and no clear species, geographical, or fish size influence on the school shape. The great variations in density, and to a certain extent in shape, are explained by the dynamics of a moving mass of polarized and syncronized individuals. Methods for adjusting the horizontal beamwidth of sonars, and egde-effects when integrating schools, are outlined.