A sonar study of the migration pattern of Norwegian spring-spawning herring (Clupea harengus L.) in July
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About 700 schools of Norwegian spring-spawning herring (Clupea harengus L.) were tracked by multi-beam sonar during transect surveys in the Norwegian Sea in July 1995 and 1996. This herring stock usually concentrates in the area west of Lofoten in July-August after its feeding migration in the Norwegian Sea, and the herring were thus expected to swim eastwards towards the coast in July. The survey area was divided into three sub-areas based on distance from the main wintering area in Vestfjorden, habitat and the age of herring. In 1995 a total of 502 schools were classified as migrating, stationary or avoiding, of which 31 % were migrating, 61 % stationary and 8 % avoiding schools. The schools were located shallow (0-60 m) during day and night, suggesting that they were feeding. A high proportion of feeding schools, with high dynamic tendencies, may thus explain the prevalence of stationary schools. Migrating schools had a mean migration speed of 0.66 m s-1, and migrated westwards in coastal areas. Stomach samples from trawl and high acoustic densities of plankton near the continental slope suggested that feeding conditions were better here than in coastal areas, and the schools probably left coastal areas because of sub-optimal feeding conditions. Herring probably track their prey by a flexible combination of predictive (genetic, learning) and reactive orientation mechanisms. By migrating westwards the herring may have improved their feeding conditions, thus extending their feeding season.
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