EXXON VALDEZ Oil Spill Restoration Project Final Report. Ex-situ Target Strength Measurements of Pacific Herring 'Clupea Pallasi' and Pacific Sand Lance 'Ammodytes Hexapterus.' Restoration Project 01468

摘要

Near-dorsal aspect target strengths (TS) ofindividual, live, juvenile and aidult 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. Herring were measured at a variety of depths while they swam keely in an acoustically transparent mesh cylinder secured 3 m below the transducer. Sand lance were measured while tethered at 4-m depth. The TS of individual herring declined as they were lowered in the water column. The decline was in accordance with that expected from Boyle's Law. We examined the impact of depth and show that the result at 40-m depth is equivalent to that from the equation of Thorne (1977a), which is commonly used for Pacific herring, and at 150 m is equivalent to the relationship recommended by Foote (1987) for Atlantic herring. Both results are reasonable for the respective vertical distributions of the two species. Thus, we explain the apparent discrepancy in published algorithms for Atlantic and Pacific herring TS by their different nighttime over wintering depth preferences and Boyle's Law. Since the vertical distribution of herring varies, we recommend that TS estimates be referenced to the depth of measurement. This suggestion may be applicable to all fish with swimbladders. The measurements on the juvenile Pacific sand lance, which lack swimbladders, yielded a TS= 201ogL(cm)-80.0. This is appreciably higher than the algorithm published for Atlantic sandeel of the same genus, TS= 201ogL(cm)-93.7, but is in reasonable accord with other information about fish without swimbladders.