Abstract
A large shoal of fish occurring in the vicinity of the 220-m isobath was observed using a standard 38-kHz fisheries echosounder and a 1.5- to 10-kHz low-frequency fish sonar (LFFS) for several days in July 2000. The fish behaved like herring, exhibiting a rapid rise to the sea surface at dawn, formation into schools, and a rapid descent to the sea floor. Schools remained at depth through the day and gradually rose to the sea surface at dusk and then rapidly descended and spread out into a diffuse scattering layer at 75- to 125-m depth. Shifts in resonance frequencies during migration, release of gas bubbles during migration, and strong avoidance of the vessel when maneuvering, all suggest the fish were most likely round herring, Etrumeus teres, which are common at these depths in the NMFS historical trawl survey data. An examination is made of some of the scattering characteristics of the schools and layers of these fish and comparisons of the 38-kHz data to scattering at 500-Hz bands from 1.5 to 5 kHz and 1-kHz bands from 5 to 10 kHz. [Work supported by ONR.]
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