Wide area sonar such as ocean acoustic waveguide remote sensing
(OAWRS) has been shown to be a useful tool for instantaneously imaging
large shoals of fish distributed over continental shelf regions. Here,
we show that the population density of fish groups can be accurately
estimated by using incoherent averaging of the matched filtered
returns. Numerical Monte Carlo models are applied to simulate the
active imaging system and determine the statistics of the received
matched filtered intensity scattered off Atlantic herring shoals in
the Gulf of Maine. The model includes multiple scattering of the dense
fish groups and uses a range dependent acoustic model to simulate
modal dispersion in the fluctuating ocean environment. We illustrate
the effects of modal dispersion on the sonar imagery for various
seafloor types. We determine the conditions for when multiple
scattering is significant for Atlantic herring shoals, which depends
on the fish density, imaging frequency, and mean target strength.
Results of the model are compared to OAWRS sonar imagery collected
during the 2006 Gulf of Maine Experiment.