Observing Atlantic herring by parametric sonar.

Abstract

Atlantic herring (Clupea harengus) has been observed in situ by the Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar, with nearly horizontal transmit and receive transducer arrays mounted flush on the hull of R/V G. O. Sars, at N71.4 E16.3. The primary frequencies are in the band 15–21 kHz, with nonlinearly generated difference frequencies in the band 0.5–6 kHz. The beamwidths are 6–10 deg depending on frequency and range, but with exceedingly low sidelobes. The observation of herring in schools and layers was accomplished with the vessel both at rest and sailing at its ordinary survey speed of 10 knots. The observations with the parametric sonar were confirmed by concurrent, synchronized observations with the Simrad EK60/38-kHz scientific echo sounder and by trawling with a pelagic net. The herring length varied from 28.0–36.5 cm. Present work suggests that parametric sonar will become a powerful new tool in marine ecosystem studies, enabling the numerical density of schooling and shoaling fish to be determined, and the size of swimbladder-bearing fish to be estimated by detection of swimbladder resonance. [Work partly supported by Norwegian Research Council Grant no. 184705.]