Mapping the migration pattern of schooling fish by use of multi-beam sonar during conventional acoustic surveys

Abstract

Information on migration pattern and swimming behaviour of pelagic fish schools in the vicinity of steaming survey vessels has been collected by a multi-beam sonar system connected to an external device for data collection during three conventional acoustic surveys. Two cruises were conducted on herring in the North Sea in July 1991 and 1992, and the third one on capelin in the Barents Sea in January 1992. A method for graphic presentation of swimming tracks of recorded schools was established for analyzis of the behavioural pattern of the schools. Based on such swimming tracks, schools were categorized by pre-defined criteria as migrating, vessel avoiding or undetermined. Estimates of the vessel avoidance frequency, swimming speed, the migration direction, and the migration speed and depth of the schools, have been obtained. During both North Sea surveys, the majority of observed herring schools were migrating south. The average migration of the capelin was northwards. Mean swimming speeds of herring schools were in accordance, both with theoretical considerations as well as results of earlier sonar studies conducted under similar conditions in the North Sea. The validity of estimated swimming speeds of capelin was more uncertain, as the swimming capacity of that species is not well known.