Underwater sounds generated by Thunnus albacares and Thunnus thynnus were recorded and studied to explore the possibility of passive-acoustical detection. Possible tuna sounds were recorded at the Monterey Bay Aquarium, Monterey, California, and Maricultura del Norte in Ensensada, Baja California, Mexico. At both locations, the most prevalent sounds seemingly associated with tuna were low-frequency pulses varying from 20 to 130 Hz, lasting about 0.1 s, and usually single and apparently unanswered. A behavior similar to coughing was coincident with these sounds: the animal' s mouth opened wide with its jaw bones extended and its abdomen expanded, then contracted abruptly. On one occasion in Mexico, this behavior and associated signal were simultaneously recorded. Because these measurements were made in noisy environments, this study should be repeated under more controlled conditions before tuna vocalizations can be claimed with certainty. Nevertheless, the center frequencies of these sounds appear to vary with respect to the resonant frequencies of the tuna's swim bladder, suggesting a passive-acoustical proxy for measuring the size of tuna. Matched filter and phase-difference techniques were explored as means for automating the detection and bearing-estimation processes. © 2003 Elsevier Science B.V. All rights reserved.
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