Estimating the volumes of fish schools from observations with multi-beam sonars

Abstract

Multi-beam sonar is commonly used in purse seine fishing to visually evaluate school size and biomass. However, quantitative analyses of the across-beam school dimensions may provide more accurate estimates of school volumes. These may help fishers improve their estimates of fish biomass prior to setting a purse seine set; and assist scientists to more accurately assess the distributions and abundances of pelagic schooling fishes. Fish-school volumes are evaluated using data from a simulated Simrad SX90 sonar. The accuracy of the estimates is dependent on the number of ensonifying beams, and therefore dependent on the school size and range from transducer. We present two models, derived through simulations, to correct for distortions of the target dimensions, both horizontally and vertically. The corrected school heights and widths have precisions of 8.5–10.5% vertically and 6.6–8.7% horizontally.