Broadband classification and statistics of long-range, mid-frequency sonar measurements of aggregations of fish

Scattering from fish can constitute a significant portion of the high-amplitude echoes in the case of a horizontal-looking sonar system operating at mid-frequencies (1–10 kHz). In littoral environments, reverberation from fish with resonant gas-filled swimbladders can dominate bottom and surface reverberation and add spatio-temporal variability to an already complex acoustic record. Measurements of sparsely distributed, spatially compact fish aggregations have been conducted in the Gulf of Maine using a long-range, broadband sonar with continuous coverage over the frequency band of 1.5–5 kHz. Concurrent downward-looking, multi-frequency echosounder measurements (18, 38, and 120 kHz), and net samples of fish are used in conjunction with physics-based acoustic models to classify and statistically characterize the long-range fish echoes. A significant number of echoes, which are at least 15 dB above background levels, were observed in the long-range data and classified as due to mixed assemblages of swimbladder-bearing fish. These aggregations of fish produce highly non-Rayleigh distributions of echo magnitudes. The probability density functions of the echoes are accurately predicted by a computationally efficient, physics-based model that accounts for beam-pattern and waveguide effects as well as the scattering response of aggregations of fish. [Work supported by the U.S. Office of Naval Research.]