Swimming behaviour of fish schools in the North Sea during acoustic surveying and pelagic trawl sampling

Swimming behaviour of schooling herring (Clupea harengus) and sprat (Sprattus sprattus), when approached by a survey vessel, was observed by aid of a true motion sonar. Observations were made both during surveying and during pelagic trawling. Horizontal swimming speed was length-dependent, but there were great variations between schools, even those with fish of similar size. The schools were usually guided in front of the approaching vessel, but often the schools avoided the path of the vessel. Vessel avoidance increased during pelagic trawling. By taking advantage of the guiding effect, and modifying the gear rigging, the capture success of the pelagic sampling trawl was increased. An attempt is made to quantify the influence of vessel-avoidance and upper blind-zone distribution on the total abundance estimate.