The packing density and external shape of herring ( Clupea harengus ), saithe ( Pollachius virens ), and sprat ( Sprattus sprattus ) schools in the North Sea and along the Norwegian coast were 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 discoids close to the surface or sea bottom, and were more spherical in midwater. There were, however, great variations in shape among the schools, which could not be related to species, geographical regions, or fish size. The great variations in density, and to a certain extent in shape are explained by the dynamics of a moving mass of polarized and synchronized individuals. The size of the schools varied by a factor in the order of 10 000, and there were regional differences in average school size.
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Ole Arve Misund (1993) studied this question.