The difference-frequency band of the Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar, nominally 1–6 kHz, is being used to observe Atlantic herring. Representative TOPAS echograms of herring layers and schools observed in situ in December 2008 and November 2009 are presented. These agree well with echograms of volume backscattering strength derived simultaneously with the narrowband Simrad EK60/18- and 38-kHz scientific echo sounder, also giving insight into herring avoidance behavior in relation to survey vessel passage. Progress in rendering the TOPAS echograms quantitative is described.

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In July 2002, two commercial vessels were used to study the distribution of Northeast Atlantic mackerel (Scomber scombrus L.) during their feeding migration in the eastern part of the Norwegian Sea between 62°N and 70°N. Pelagic trawling and school tracking with SIMRAD 24–36 kHz sonar demonstrated that the stock was distributed throughout the study area. Information about time, geographic position, size, depth, speed, and direction was stored for each school during tracking. This study reports analyses of data from 63 schools that were tracked for 30 s or longer. All schools were recorded at depths of less than 100 m, and the majority (65%) were found between the surface and 40 m. The direction of migration (north 0° ± 22.5°, northeast 45° ± 22.5°, etc.) was non-random, with east and west as dominant swimming directions. School size and migration speed varied from 1 to 7000 tonnes and 0 to 6 m s−1, respectively. Methodological improvements are discussed.

The Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar operates over the frequency band 15–21 kHz, with nonlinearly generated difference-frequency radiation in the band 0.5–6 kHz. The TOPAS transducer mounted on R/V G. O. Sars is flush with the hull in the near-horizontal plane. The sonar has been calibrated by the standard-target method using a 280-mm diam sphere of aluminum alloy 6082 T6 [K. G. Foote et al., J. Acoust. Soc. Am., 121, 1482–1490 (2007)]. The target was suspended beneath the vessel at each of three ranges, successively 100, 200, and 300 m. Because of conditions in Soerfolla fjord on Dec. 10, 2008, the target sphere was moving slowly relative to the vessel. Its instantaneous position was determined by geometrical considerations through synchronous observation with the Simrad EK60/38-kHz scientific echo sounder, with split-beam transducer mounted approximate to the TOPAS transducer. Data were collected for a number of parameter settings for each of three signal types: continuous wave, chirp, and Ricker pulse. Measurements are compared with predictions based on laboratory measurements of the frequency-dependent sensitivities of the parametric transmitter and conventional linear receiver, using a range-compensation function based on theoretical nearfield modeling. [Work partly supported by Norwegian Research Council Grant No. 184705.]

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