Population Level Effects of Mid-Frequency Active (MFA) sonar on Blainville’s (Mesoplodon densirostris) and Cuvier’s (Ziphius cavirostris) Beaked Whales on U.S. Navy Ranges

Background/Question/Methods

Naval exercises using Mid-Frequency Active (MFA) sonar have been implicated in several mass stranding events in which the predominant species were Blainville’s (Mesoplodon densirostris) and Cuvier’s (Ziphius cavirostris) beaked whales. Beaked whales are known to be metronomic divers.  Their dives can exceed 1,000 meters and last for over an hour and are repeated every one to two hours both day and night.  Both species produce echolocation clicks only during these foraging dives at depths below 300 meters.  Such behavior makes them ideally suited for study using passive acoustics, as detection of vocalizations can be used as a proxy for foraging and to determine the animals’ spatial and temporal distribution. Significant numbers of both species have been documented on Navy undersea ranges including the Atlantic Undersea Test and Evaluation Center (AUTEC)  in the Bahamas and the Southern California Offshore Range (SCORE)  off the coast of San Clemente Island, where MFA sonar exercises are routinely conducted.  Each facility provides a field of bottom-mounted hydrophones which have been used to monitor animal vocalizations before, during, and after sonar exercises. 

Results/Conclusions

Passive acoustic data strongly suggest M. densirostris and Z. cavirostris avoid sonar and move off the range during operations only to return upon their completion.  Sonar receive levels which cause disruption in foraging were determined along with the general spatial and temporal distribution of animals before, during, and after active operations.  The average receive level at which animals were detected foraging in the presence of sonar was 127.5 dB re µPA @ 1m with a maximum receive level of 157 dB.  Little foraging activity was observed on range during active operations, but returned to pre-exercise levels within days. The pattern in which vocalizations were detected upon cessation of exercises suggested animals had exhibited an avoidance behavior and moved off range rather than remaining on range without vocalizing (foraging). Satellite tracking tags are beginning to provide insight into the extent of movement and foraging off the range during these exercises.