Swimming behaviour of fish schools in the North Sea during acoustic surveying and pelagic sampling trawling
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Swimming behaviour of schooling herring, mackerel and sprat, 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 clearly length dependent, but there were great variations from one school to another, even if the school members were of similar size. Generally the schools seemed to be guided by the approaching vessel, but often the schools avoided the path of the vessel. An attempt is made to quantify the influence of vessel-avoidance and upper blind zone distribution on the total abundance estimate. By taking advantage of the guiding effect, and modifying the gear rigging, the capture success of the pelagic sampling trawl was clearly increased.