Extinction cross-section of Norwegian spring-spawning herring

• Published in: ICES Journal of Marine Science
• Main Author: Foote, K. G.
• Format: Text
• Language: English
• Published: Oxford University Press 1999
• Subjects: Articles; Bergen; Ofotfjorden
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/56/5/606; https://doi.org/10.1006/jmsc.1999.0479

Acoustic abundance estimation of dense, vertically extended aggregations of fish requires knowledge of the extinction cross-section. An established method based on a comparison of the area backscattering coefficients of fish and underlying flat bottom has been exercised for herring in Ofotfjorden in January 1994, but for the first time with the powerful combination of the SIMRAD EK500 scientific echosounder and Bergen echo integrator. This has enabled the flatness of the bottom to be controlled to within a single meter over 0.1-nautical-mile (nmi) intervals of echo integration. This control enhances the quality of the new data over those collected in three previous years. The estimate for the ratio of the extinction cross-section to the backscattering cross-section is accordingly revised. The new value for the ratio is 2.41, with standard deviation of 0.33. This has been used in acoustic abundance surveys of herring since winter 1993–1994. The variability in measurement results remains an object of investigation. This is viewed in the light of the optical literature, which suggests that the extinction cross-section of finite but complicated physical scatterers may be as sensitive to details of orientation as is the backscattering cross-section. The possibility of exploiting this property is considered.