Ex Situ Target Strength Measurements of Pacific Herring and Pacific Sand Lance

Abstract

Abstract Near-dorsal aspect target strengths (TS) of individual live juvenile and adult Pacific herring Clupea pallasi and juvenile Pacific sand lance Ammodytes hexapterus were measured ex situ with 120-kHz dual-beam acoustics. An experimental frame was designed to support a transducer, fish cage, standard calibration target, and two video cameras. Pacific herring were measured at a variety of depths while they swam freely in an acoustically transparent cylinder that was secured 3 m below the transducer. Pacific sand lance were measured while tethered at a 4-m depth. We found that both depth and season affected the target strength of Pacific herring. The TS of individual Pacific herring declined as they were lowered in the water column. The decline was in accord with that expected from the effects of Boyle's Law on the swim bladder volume. This impact of depth may explain the apparent discrepancy in published target strength algorithms for Atlantic herring C. harengus and Pacific herring. The depth distri...