Wide area sonar system, such as OAWRS (ocean acoustics waveguide remote sensing), can instantaneously image large shoals of fish spanning tens of kilometers from long ranges in an ocean waveguide. An analytic and numerical model is developed for the spatial and temporal coherence of the scattered field from a large group of fish in a shallow water waveguide. The temporal coherence of the received signal depends on both the temporal coherence of the random waveguide through the medium Green’s function and the correlation time scale for fish group distribution. The spatial coherence of the received signal depends on both the spatial correlation scale of the random waveguide inhomogeneities and that for the imaged fish group distribution. The temporal and spatial correlation scale for the fish group distribution are determined from experimental data acquired in the Gulf of Maine during the 2006 Ocean Acoustics Waveguide Remote Sensing (OAWRS) Experiment. The effect of temporal and spatial coherence of the scattered field on subsequent coherent array processing and matched-filtering is discussed.