Variation in acoustically measured abundance from repeated surveys of herring

A large concentration of wintering herring was surveys continuously with acoustics over 37 hours in a fjord in Northern Norway. 16 surveys of the same area were conducted. Total abundance and statistics were computed for each survey. Distribution graphs showd a clear diel variation, with deep daytime layers and more shallow night time distribution. Acoustically measured abundance at night was 50% of the daytime values. The results are discussed in terms of behaviour induced diel variation in acoustic backscattering, and potential influence on acoustic abundance measurements