Abundance estimation of fish schools based on a relationship between school area and school biomass

A method for acoustic abundance estimation by horizontal guided sonar of fish schooling close to surface is presented. The basic principle is to measure the horizontal area of schools in situ, and derive the school biomass subsequently through a school area to school biomass relationship. The application of the method is exemplified through a sonar survey in the eastern North Sea-Skagerrak in September 1990. The areas of recorded schools were measured by the estimate function of a Furuno CSH-70 sonar, and selected schools were captured by purse seine to establish a school-area to school-biomass relationship. In the actual region, the biomass of herring (Clupea harengus) and mackerel (Scomber scombrus) was estimated to about 90000 and 430000 tons, respectively. The accuracy of the method is discussed and improvements are suggested.