Diel variation in acoustic density measurements of overwintering herring (Clupea harengus L.)

Abstract

Survey data from nine herring (Clupea harengus L.) assessment cruises in the wintering areas of spring-spawning Norwegian herring were analysed with regard to variation in vertical distribution and acoustically measured estimates of abundance with time of day. The main features of the vertical distribution were a downward vertical migration at dawn and an upward migration at dusk, although some of the herring remained at depth, independent of time. In a non-feeding overwintering situation this behaviour was attributed to schooling and predator avoidance during the day, and energy conservation and possible swimbladder refilling during the night. There were generally higher average acoustic area backscattering coefficient (sA) values during the short winter day compared to low values at night, and a minimum at dusk. The diel variation in sA values is attributed to variation in angular distribution of individual herring, from daytime schooling to dusk and dawn vertical migration to swim/sink/glide behaviour during the night, and also to survey ship avoidance by shallow layers of herring at night. These implications as they relate to assessment surveying are discussed.