Multibeam and single‐beam sonar observations of Atlantic herring in the Gulf of Maine

In September 2006, several schools of Atlantic herring were simultaneously imaged with a Reson 7125 SeaBat multibeam sonar (400 kHz) and a Simrad EK60 Scientific echo sounder (38 kHz, 120 kHz, and 200 kHz) on Georges Bank in the Gulf of Maine. One school was imaged five separate times over the span of one hour. These two sonar systems, along with pelagic trawl catch data collected on a separate ship, provided a synoptic view of the fish school as it changed depth and size, fragmented, and eventually dispersed. Of particular interest is the combination of data from the two sonars. The multifrequency, split‐beam EK60 provided estimates of the fish school density directly under the ship, but was not capable of accurately extrapolating these measurements to the entire fish school, which was largely out of the EK60 field of view. The multibeam sonar was generally able to image the entire fish school and provided useful information on school volume, fragmentation, and general school morphology, but was more difficult to use for quantifiable measurements, due to the unknown angle of ensonfication of the fish in the outer beams.