Diurnal variations in acoustic density measurements of wintering Norwegian spring spawning herring
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http://hdl.handle.net/11250/105417Utgivelsesdato
1995Metadata
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Survey data from six herring assesment cruises in the wintering areas for spring spawning
Norwegian herring were analysed with regard to variation in abundance and depth with time of
day. The data were quite consistant between cruises, showing a downwards vertical migration
at in situ dawn and an upwards migration at dusk, with a subsequent descent of parts of the
herring through the dark period. This behaviour was attributed to schooling and predator
avoidance during the day, and energy conservation and swimbladder refilling during the night.
There were significantly high average SA values during the short winter day compared to very
low values at dawn and dusk, and also quite low values at night. This variation was mainly
attributed to articulate angular distribution of the herring during dusk and dawn, and also during
the night time swim/sink/glide energy conservation exercise. The implications on assesment
surveying is discussed.