Detection of migratory herring in a shallow channel using 12- and 100-kHz sidescan sonars

Abstract

A hydroacoustic monitoring program for migratory herring was conducted in Drogden Channel, near Copenhagen, Denmark, beginning in June, 1996. Using installations of 100-kHz sidescan sonar oriented across the channel, herring schools were observed at ranges up to 500 m in water depths of 10–14 m. In October 1997, this effort was supplemented with a 12-kHz sidescan mounted on a motorized tripod allowing azimuthal sector scanning. This 12-kHz sidescan was able to detect herring schools up to a 1200-m range. Two examples are examined to assess the limits of herring school detectability and the feasibility of abundance estimation. In both cases, herring schools are detectable by their transience and up to 20 dB signal excess relative to the nominally stationary background reverberation, dominated by seabed backscattering. Attempts are made to extract quantitative abundance estimates using methods based on school size and density, and through quantitative echo-integration. Acoustic ray tracing was found to be useful in interpreting the results, and in particular modeling boundary-reflection focusing as a potential source of positive bias in herring abundance estimates.