Ocean acoustic waveguide remote sensing (OAWRS) is a bistatic multibeam wide area sonar system that enables unaliased monitoring of fish populations over ecosystem scales. This allows us to quantify the formation processes of vast shoals of Atlantic herring (Clupea harengus) containing hundreds of millions of individuals, imaged during the Autumn 2006 spawning season. Areal population density estimation requires calibration of the low-frequency target strength of an individual shoaling herring. This is estimated from experimental data acquired simultaneously at multiple frequencies in the 300–1200-Hz range using (1) the OAWRS system, (2) areal population density calibration with several conventional fish finding sonar systems (CFFS), (3) fish length distributions obtained from trawl samples, and (4) local low-frequency transmission loss measurements. High spatial-temporal co-registration was found between shoals imaged by OAWRS and concurrent CFFS line transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to exhibit a strong dependence on frequency in the OAWRS range, which is consistent with resonant scattering from an air-filled swimbladder. We show that a rapid transition from disordered to highly synchronized behavior occurs as the herring population density reaches a critical value, after which an organized group migration occurs.