Fish Detection with Ship Based Echo-Sounder
Mark V. Trevorrow, DREA, Dartmouth, Nova Scotia, Canada
Traditional net-trawl and echo-sounder surveys suffer from limited sampling volume and vessel-avoidance behaviour by the fish. The effects are accentuated in shallow waters. New high-frequency (HF, 50 - 500 kHz) sidescan and multi-beam sonar technologies adapted from Naval mine-hunting activities offer a variety of potential solutions, enabling detection and possibly classification of fish over much larger distances and water volumes, and operating at ranges where the fish are un-affected by the vessel presence. Furthermore, Naval applications have fostered considerable capability for automated target recognition and classification. However, in a side-looking geometry issues such as back-scattering from boundaries (surface and seabed), boundary reflections, and acoustic refraction interfere with fish detectability. A further complication is the angular dependence of fish scattering. Thus, in a fishery survey it now becomes necessary to understand fully the acoustic propagation environment in the survey region. In general one should consider these acoustic techniques as complementary to, and not a replacement of, traditional net-sampling surveys.
The capabilities of both fixed-installation and towed sidescan sonars for fish detection in a variety of coastal and riverine environments have been experimentally demonstrated by the author and others. Some of these previous surveys are reviewed, showing examples of fish detection at ranges of order 100 to 7000 m and illustrating a variety of acoustic propagation features. Promising new approaches such as multi-beam and parametric sonars are discussed. Multi-beam sonars have the capability for scanning a wide angular aperture (typically 90 or 120 degrees) with high angular resolution (typically 128 beams) in a single ping, enabling complete coverage of the water column to horizontal ranges up to 500 m. In vessel-mounted or towed applications, the 3-dimensional spatial distribution of individual fish, or fish school size and morphology, can be directly measured. The parametric sonar concept utilizes a high-power HF transmitter and non-linear acoustical effects to generate narrow, relatively wide-band beams in the 100 to 2000 Hz region. With separate HF and LF receiver arrays, simultaneous fish detection in both the geometric and swim-bladder resonance frequency regimes should enable fish species identification.
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