Backscatter measurements of Atlantic herring in the Northwest Atlantic

Abstract

Annual fisheries acoustic surveys have recently been implemented in the Gulf of Maine and Georges Bank regions to estimate the abundance and biomass of Atlantic herring (Clupea harengus). Research was devoted to the verification of backscatter measurements from Atlantic herring using multifrequency echo-integration, omni-directional sonar, pelagic trawling, and underwater video operations. Results indicated that individual backscatter measurements during the day were significantly higher compared to the night and twilight periods. The diurnal behavior patterns of Atlantic herring contributed to this variability because of their vertical migration to near bottom during day and to surface waters during night. Observations showed that herring shoaled in stationary schools above offshore banks during the night to feed, and tend to swim more actively during twilight and day periods. An increase in the backscatter from herring during the day may be explained by their more horizontal orientation when actively swimming. Fisheries acoustic surveys conducted during day and night must incorporate systematic adjustments to account for the diurnal bias in acoustical estimates.