Intermediate range fish detection with a 12-kHz sidescan sonar

The application of a prototype 12-kHz sidescan sonar suitable for horizonta l imaging of fish distributions is described. The sonar consisted of a 40-e lement (20 lambda) array transmitting a 1600 Hz x 0.2 s FM sweep with corre lation processing of the received echoes. Observations are reported for two different implementations: (i) a configuration in which the sonar was towe d at similar to 35 m in water of depth 60-220 m in the southern Strait of G eorgia, British Columbia, where the primary target was sockeye salmon, and (ii) a bottom-mounted deployment in a shallow (10-14 m) channel near Copenh agen, Denmark, where herring were present. Tn the Strait of Georgia, discre te fish targets were detected with up to 25-dB signal-to-reverberation rati o at ranges from 1 to 7 km. In the shallow Danish waters, herring schools w ere detected at ranges up to 1.2 km. In each case an acoustic propagation a nalysis identifies the useful detection characteristics. In the Strait of G eorgia the signal is confined to a subsurface sound channel, requiring addi tional knowledge of vertical distributions of salmon in order to make abund ance 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 ch anging shape and movement of herring schools. These preliminary results pro vide an indication of the potential value and limitations of intermediate r ange sidescan surveys for fish stock assessment. (C) 1999 Acoustical Societ y of America. [S0001-4966(99)00811-5].