Abstract
Beaked whales have mass stranded during some naval sonar exercises, but the cause is unknown. They are difficult to sight but can reliably be detected by listening for echolocation clicks produced during deep foraging dives. Listening for these clicks, we documented Blainville's beaked whales, Mesoplodon densirostris, in a naval underwater range where sonars are in regular use near Andros Island, Bahamas. An array of bottom-mounted hydrophones can detect beaked whales when they click anywhere within the range. We used two complementary methods to investigate behavioral responses of beaked whales to sonar: an opportunistic approach that monitored whale responses to multi-day naval exercises involving tactical mid-frequency sonars, and an experimental approach using playbacks of simulated sonar and control sounds to whales tagged with a device that records sound, movement, and orientation. Here we show that in both exposure conditions beaked whales stopped echolocating during deep foraging dives and moved away. During actual sonar exercises, beaked whales were primarily detected near the periphery of the range, on average 16 km away from the sonar transmissions. Once the exercise stopped, beaked whales gradually filled in the center of the range over 2–3 days. A satellite tagged whale moved outside the range during an exercise, returning over 2–3 days post-exercise. The experimental approach used tags to measure acoustic exposure and behavioral reactions of beaked whales to one controlled exposure each of simulated military sonar, killer whale calls, and band-limited noise. The beaked whales reacted to these three sound playbacks at sound pressure levels below 142 dB re 1 µPa by stopping echolocation followed by unusually long and slow ascents from their foraging dives. The combined results indicate similar disruption of foraging behavior and avoidance by beaked whales in the two different contexts, at exposures well below those used by regulators to define disturbance.
Highlights
Over the past 20 years, there has been increasing concern that noise from human activities may affect wildlife
We studied the location and duration of group clicking period (GCP) before, during, and after two naval sonar exercises in 2007 and 2008 that each involved three ships using mid-frequency active sonars (MFA) including AN/SQS-56 or AN/SQS-53C sonars whose use has been associated with beaked whale strandings [7,8]
Acoustic monitoring of actual sonar exercises indicates that some beaked whales were exposed to levels .140 dB without ceasing clicking during the sonar exposure, but the gap in distribution seen in Fig. 1B suggests that most whales stopped foraging and moved away to distances of more than 10 km, judging from the 4 km separation between hydrophones
Summary
Over the past 20 years, there has been increasing concern that noise from human activities may affect wildlife. These studies suggest that risk of injury from exposure to loud sound sources such as military sonars should be limited to tens of meters [6] It was surprising when evidence mounted that atypical mass strandings of beaked whales (family Ziphiidae) have coincided with some naval exercises involving mid-frequency active (MFA) sonars, which operate in the 3–8 kHz frequency band [7,8,9]. These strandings are the only known cases where exposure to anthropogenic underwater sound has been demonstrated to lead to the death of marine mammals [10]. Dive depths and durations were available for 33 dives from 6 individuals. Dive depth and dive duration were transformed using a Box-Cox transformation with
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