Acoustic measurement of cardiac function on northern elephant seals

A self‐contained acoustic recording tag has obtained signatures of heartbeat from both captive and free‐ranging northern elephant seals (Mirounga angustirostris). Simultaneous electrocardiogram (ECG) and acoustic signatures obtained from the submerged captive seal confirm that seal heartbeat can be monitored acoustically. Recordings from the free‐ranging seal demonstrate shallow‐water apneic heart rates of 40 to 55 beats per minute, comparable to past observations with ECG tags. Observation of a sixfold increase in beat amplitude at the surface (110 dB re: 1 μPa) over that at depth (94 dB re: 1 μPa) may reflect increases in stroke volume at the surface. The findings suggest the feasibility of long‐term, reliable, quantitative, and noninvasive cardiac monitoring of a variety of free‐ranging marine animals using acoustic logging techniques.