Mapping the migration pattern of schooling fish by use of multi-beam sonar during conventional acoustic surveys
Original version
This report is not to be cited without prior reference to the authorsAbstract
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.