Schooling dynamics of Northeast Atlantic mackerel (Scomber scombrus) in the Norwegian Sea using multibeam sonar

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Pelagic fish in the Norwegian Sea perform seasonal migrations from overwintering, via spawning, to feeding grounds. Northeast Atlantic mackerel (Scomber scombrus) are highly migratory, fast-swimming, and an obligate schooling fish. Mackerel schooling dynamics in nature is unknown because they lack a swimbladder, resulting in a weak acoustic signature, and therefore are difficult to detect in the summer when in loose schools. However, high frequency omnidirectional SONAR is capable of detecting mackerel in the acoustic echosounder blind zone close to the surface. Mackerel schooling dynamics were studied in relation to temperature, zooplankton abundance and density of conspecifics in four geographically separate regions of the Norwegian Sea during summer. The thermocline depth had a profound influence on the depth distribution of schools. Mackerel were found shallower than 40 m depth where the temperature was at least 6° C. The fish generally swam north except for in the SW region, coinciding well with prevailing current directions. Fish were significantly larger in the north than in the south, and plankton abundance was higher in the west than in the east. The observed school dynamics in relation to abiotic and biotic factors were explained in terms of mackerel ecology during the summer feeding migration.