Sonar observations of schooling mackerel during purse seining

Abstract

The swimming behaviour and horizontal dimensions of mackerel schools have been quantified by use of a multibeam, true motion sonar during purse seining in the Northern North Sea, August 1986. The horizontal dimensions of the schools were quite variable, but on average the schools were elliptical shaped, about 1.8 times larger than wide. The average school area was proportional to the biomass of the schools. The horizontal swimming speed was on average about 3.5 bodylengths/s, but there were great variations in swimming behaviour among the schools. The schools performed strong avoidance of the vessel, and schools escaped capture in about 37 % of the purse seine sets.