Determining the extinction cross section of aggregating fish

Abstract

When fish are aggregated over a flat bottom, and fish and bottom echoes can be distinguished, it is possible to determine the fish extinction cross section by a simple application of the echo integration method. The theory for this is developed. Measurements at 38 kHz are presented for aggregations of the same 1983‐year class of herring over flat‐bottomed fjord areas in 1988, 1990, and 1991. The ratio of extinction and backscattering cross sections is found to lie in the approximate range from 1.2–2.3, depending on fish size and time of day.