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 distribution of Atlantic herring is typically deeper. The results suggest that the depth of measurement needs to be considered in both target strength measurements and acoustic surveys of herring. We also found higher target strengths of Pacific herring during spring than in fall. Larger swim bladder volumes may be associated with spent fish or reduced lipid content during spring. The measurements on the juvenile Pacific sand lance, which lack swim bladders, yielded a much lower target strength than Pacific herring of equivalent size. However, the observed values were appreciably higher than the TS algorithm published for sand lance Ammodytes spp. in the Atlantic Ocean.
