Computerbased detection of schools for counting, quantification of size and swimming behavior by the SIMRAD SA950 sonar

Abstract

As a help in stock assessment of pelagic, schooling fish a software for detection of schools and quantification of their size and swimming behaviour by a high resolution sonar has been developed. The system reads the echo telegrams that forms the display of the SIMRAD SA950 sonar, and organize an echo table for the colour code value of each pixel for each of the 32 beams and 512 range rings of the sonar for each ping. The settings of the sonar is written to a separate table. Detection of schools is based on identification of echo lines which are defined as a minimum number of succeeding pixels with colour code value above a predetermined threshold. The position in the beam sector and horizontal extent for each echo line detected is written to a separate echo line table. A procedure then search through this table for identification of echo blocks which are defined as adjacent echo lines that extend above a certain width. The horizontal extent, depth, and geographic position of echo blocks are written to an echo block table together with information on time and vessel navigation. If an echo block is detected in a minimum number of succeeding pings, it is assigned a school number. It is possible to store both the echo table, the echo line table, and the echo block table on disk or take printouts directly. A colour printer echogram for use with the SIMRAD SA 950 has also been developed. For this purpose the sonar is interpreted as a sounder with a 45° beam, and the maximum colour code value for each range ring in the echo table is identified and printed. The printed echogram is divided by range lines and log marks, and there are indicators for tilt angle and train of the sonar beam. The developed software can be used as a basis for automatic counting of schools, school size estimation, and quantification of swimming behavior.