REACTIONS OF HEWING SCHOOLS TO THE SOUND FIELD OF A SURVEY VESSEL

The underwater sound of R/V ''Johan Hjort'' as a function of aspect an gle has been mapped by cruising along parallel transects at various di stances from the recording unit. At frequencies over 125 Hz there was a minimum in front of the vessel, and lobes of higher intensity on bot h sides. About 20% of the herring schools recorded during a survey in the North Sea seemed to react to the survey vessel. The distance at wh ich the schools reacted varied from about 25 m and up to about 1000 m in front of the vessel. Within a sector of 20 degrees on each side of the path of the vessel, the fraction of schools reacting increased to about 50%. Most of the schools that reacted seem to be herded in front of the vessel. This reaction pattern may indicate either that the sch ools reacted to the higher sound intensity to the side of the vessel o r that the pattern occurred because of a ''pursuit effect''. Despite t he apparent herding, more than 40% of the schools in front of the vess el were not recorded by the echo sounder. It is therefore argued that horizontal avoidance close to the vessel may have caused an underestim ation of the biomass of herring of about 20%.