Abstract:

Two microprocessor-based acoustic recording tags, deployed on northern elephant seals migrating from California to Alaska, were recovered in August 1997 after over 4 months at sea. The tags' hard disks contained measurements of pressure, temperature, and ambient noise, as well as acoustic signatures of swim speed, swim stroke rate, respiration, and cardiac function, obtained as one subject (``Trane'') swam across the northeastern Pacific and the other (``Rocky'') north along the West Coast. The subjects dove an average of 58 (Trane) and 81 (Rocky) dives per day during the 26 days that logger batteries supported data acquisition; maximum dive depths reached 780 m (Trane) and 770 m (Rocky). To function over prolonged periods under these demanding conditions, the tags were designed with robust enclosures and software- and hardware-based power conservation techniques. The hydrophones were potted in rear-facing conical resin housings, which were found empirically to reduce flow noise over 100--200 Hz by 10--15 dB compared with semicylindrical and tapered cylindrical hydrophone housings. The results suggest that hydrophone tagging methods offer a comprehensive and reliable means of sampling acoustic stimuli and associated behavior for free-ranging marine animals over long periods at sea. [Work supported by ONR and MBARI.]