Abstract
Measurements of target strength were made of pelagic fish schools in the Los Angeles Bight where the dominant schooling species is the northern anchovy, Engraulis mordax. Data were collected using a hull‐mounted, side‐looking 30‐kHz sonar system. Among the parameters studied were peak target strength, integrated target strength, target extent, and variation of peak target strength with aspect. Three schools were interrogated, while the vessel was hove to, with varying pulse durations: 2, 10, 50, and 170 ms. Results indicate that peak target strength varies directly with the pulse duration when the pulse length is less than the target extent, whereas integrated target strength appears to be independent of pulse duration. School peak target strength and target extent, for 10‐ms pulse duration, were measured while the vessel circled a school through 360°. Variations in target strength of 20 dB and in target extent of 30 m were observed. The minimum target strength corresponded with the maximum target extent, which may possibly be explained by the school's geometry and variations of individual fish target strength with yaw aspect.
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