ONBOARD ACOUSTIC RECORDING FROM DIVING NORTHERN ELEPHANT SEALS

This study was the first phase in a long-term investigation of the imp ortance of low-frequency sound in the aquatic life of northern elephan t seals, Mirounga angustirostris. By attaching acoustic recording pack ages to the backs of six translocated juveniles, the aim was to determ ine the predominant frequencies and sound levels impinging on them, an d whether they actively vocalize underwater on their return to their r ookery at Ano Nuevo, California, from deep water in Monterey Bay. All packages contained a Sony digital audio tape recorder encased in an al uminum housing with an external hydrophone. Flow noise was minimized b y potting the hydrophone in resin to the housing and orienting it post eriorly. The diving pattern of four seals was recorded with a separate time-depth recorder or a time-depth-velocity recorder. Good acoustic records were obtained from three seals. Flow noise was positively corr elated with swim speed, but not so high as to mask most low-frequency sounds in the environment. Dominant frequencies of noise impinging on the seals were in the range 20-200 Hz. Transient signals recorded from the seals included snapping shrimp, cetacean vocalizations, boat nois e, small explosive charges, and seal swim strokes, but no seal vocaliz ations were detected. During quiet intervals at the surface between di ves, the acoustic record was dominated by respiration and signals that appeared to be heartbeats. This study demonstrates the feasibility of recording sounds from instruments attached to free-ranging seals, and in doing so, studying their behavioral and physiological response to fluctuations in ambient sounds. (C) 1996 Acoustical Society of America .