Mapping the shape, size, and density of fish schools by echo integration and a high-resolution sonar

The shape and horizontal extent of herring schools in the North Sea have been mapped by a naval sonar (SIMRAD SA 950) with a narrow horizontal beam. The density of schools surveyed was measured by the SIMRAD EK 500 echosounder. In most cases the school shape fitted a geometric category (circle, oval, square, rectangle, parabola), but a few schools showed an undefined, amorphous appearance. Fish density within schools varied by a factor of about 100, but there were clear relationships between the geometric dimensions and biomass of the schools. The variation in the dimension-to-biomass relationships was substantially reduced by classifying the schools by density.