DEVELOPMENT AND USE OF SONAR MAPPING FOR PELAGIC STOCK ASSESSMENT IN THE CALIFORNIA CURRENT AREA l

A method for pelagic fish stock aBBessment is presented which utilizes a fixed sonar beam for mapping fish schools. Samples of the two major acoustic properties of fish schools are presented, i.e., acousti­ cally derived horizontal dimensions (representative of school volume) and target strengths (which may be representative of school compaction). Sampling biases and sources ofsampling variability in the measurement of these properties are discussed. The results of two experiments, conducted to determine the weight ofa fish school as a function ofits acoustic characteristics, are presented. In the first experiment, an acoustically transparent trap was used to recreate an aggregation of fish and in the second, commercial fishing boats were chartered to capture whole schools. An automated sonar data acquisition and proceBBing system is described and test results presented. The results of paired automated surveys of the Los Angeles (southern California) Bight are presented and discussed. The paper reports development of the sonar-fish school mapping method first documented by P. E. Smith in 1970. Field investigations, conducted in cooperation with the Navy and the California Department of Fish and Game, indicate a median school size of 30 m diameter, a mean fish density of 15 kg of fish biomass per square meter of horizontal school area, and a biomass estimate of 1.23 to 2.30 x lOS metric tons for pelagic schooled targets in the Los Angeles Bight.