Intermediate range fish detection with a 12-kHz sidescan sonar

Abstract

The application of a prototype 12-kHz sidescan sonar suitable for horizontal imaging of fish distributions is described. The sonar consisted of a 40-element (20λ) array transmitting a 1600 Hz×0.2 s FM sweep with correlation processing of the received echoes. Observations are reported for two different implementations: (i) a configuration in which the sonar was towed at ∼35 m in water of depth 60–220 m in the southern Strait of Georgia, British Columbia, where the primary target was sockeye salmon, and (ii) a bottom-mounted deployment in a shallow (10–14 m) channel near Copenhagen, Denmark, where herring were present. In the Strait of Georgia, discrete fish targets were detected with up to 25-dB signal-to-reverberation ratio at ranges from 1 to 7 km. In the shallow Danish waters, herring schools were detected at ranges up to 1.2 km. In each case an acoustic propagation analysis identifies the useful detection characteristics. In the Strait of Georgia the signal is confined to a subsurface sound channel, requiring additional knowledge of vertical distributions of salmon in order to make abundance estimates. In contrast, fish detectability is nearly depth independent in the shallow Danish waters. In the Danish deployment the sonar could be swept in azimuth, providing a sequence of sector images that reveals the changing shape and movement of herring schools. These preliminary results provide an indication of the potential value and limitations of intermediate range sidescan surveys for fish stock assessment.