Resonance Structure in Echoes from Schooled Pelagic Fish

Explosive acoustic sources were used to obtain echoes from aggregations and schools of commercially important marine fish. Narrow-band spectral analysis of the echoes from these targets revealed significant structure in the frequency range from 200 Hz to 5 kHz. The targets were partially captured after the acoustic tests; three yielded northern anchovy (Engraulis mordax), one consisted of a mix of anchovy and jack mackerel (Trachurus symmetricus), and the last sample contained an aggregation of seven species of rockfish (Sebastes), whitefish (Caulolatilus princeps), and a striped seaperch (Embiotoca lateralis). The results of the direct biological sampling were combined with theoretical predictions for the resonant swimbladder response and compared with the experimentally observed resonances.