Tracking herring schools with a high resolution sonar. Variations in horizontal area and relative echo intensity

Fourteen herring schools off northern Norway were tracked for about 1 hour each by the 95 kHz Simrad SA950 sonar onboard RN "G.O. Sars" in May 1994. The sonar was connected to a work station that contained software for reading the echo telegrams of the sonar, printing of an echogram, automatic detection and measurement of schools, and logging of the sonar data. The horizontal area and relative echo intensity of the schools were recorded ping by ping as well as swimming depth and distance and bearing to the vessel. The position, speed and heading of the vessel were also recorded. Inter-and intra-school events as interpreted from the sonar display were recorded in a separate protocol during the school tracking. The recorded horizontal area and relative echo intensity of the schools varied considerably. Linear models with school area or relative echo intensity as dependent variables, and with range, tilt, speed and swimming angle relative to the sonar beam as continuous effects, did not explain more than 15% and 30% of the observed variations for most schools, respectively. There was a negative correlation between relative echo intensity and range for all schools. Inter-and intra-school events occurred at average rates of about 14 minutes, and inter-school events such as split and joint influenced school size. The sound absorption and the degree to which the sonar beam insonifies the schools in the vertical plane are proposed as the major sources of variation for recorded horizontal area and relative echo intensity of the schools.