Sonar observations of schooling herring: School dimensions, swimming behaviour, and avoidance of vessel and purse seine

Abstract

Horizontal dimensions and swimming and avoidance behaviour of herring schools were quantified by means of a multi-beam, true-motion sonar. The schools were usually elliptical with an average length to width ratio of about 1.5:1. A relationship between the school biomass and the average school area was established. Individual schools maintained a fairly consistent swimming pattern, but there were great variations between schools. In about 70% of the cases the schools avoided the vessel horizontally, and vertical avoidance was observed when the vessel passed over the schools. Avoidance behaviour was most apparent with spawning, migrating schools, and was consistent with the generalized emission pattern of vessel-generated sound. The herring were usually easy to capture in winter darkness, but they escaped capture in 36% of the sets in summer daylight. Limitations of multi-beam sonar for the study of fish schools are discussed.