Diurnal variations in acoustic density measurements of wintering Norwegian spring spawning herring

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