Bias Correction of Rockfish School Cross Section Widths from Digitized Echo Sounder Data

Abstract

The image of a fish school on an echogram represents a cross section through the actual three-dimensional school. Measurement of the size of this cross section, specifically the cross section width (or horizontal distance at a given depth), can be useful for further analysis of biomass or for species recognition. However, the measurement of fish school cross section widths from an echogram can be severely biased when the school dimensions are close to the dimensions of the transducer beam pattern. This is a particular problem for species that form small schools, especially in deep water. Using characteristics of the acoustic beam and measurements of both single fish and of school cross section widths, we developed a bias correction method. Corrected cross section widths of rockfish (Sebastes spp.) schools in our study area ranged from 70 m down to the dimensions measured for single fish and averaged less than 50% of the uncorrected measurements.