Low-frequency acoustic imaging of Pacific salmon on the high seas

Near-surface fish are visible in high-resolution acoustic images of the sea surface obtained from the Gulf of Alaska in late February 1992. Images were made using time domain beamforming of data taken using explosive sources and a horizontal-line-array (HLA) receiver. images are of high resolution to a range of 4 km. Targets in the images are determined to be Pacific salmon based on the historical absence of other large fish and a match of a peak in their scattering spectra near 500 Hz with predictions of swimbladder resonance. Scattering levels indicate that the salmon occur singly or in small groups of two to four individuals. Counts of 114 salmon/km(2) are comparable with historical estimates of 160 salmon/km(2). Salmon were seen in daytime images, but at night, only a few weak targets were observed. At night the salmon are hypothesized to have migrated very close to the sea surface where they became acoustic dipoles and were acoustically invisible to the HLA. The spatial arrangement of salmon in the images suggests that at small spatial scales (2-200 m), they are uniformly distributed whereas at larger spatial scales (400-2000 m), they are aggregated.