Evidence has accumulated that a number of species of marine mammal may be affected by the operation of sonar during Navy exercises. Responses attributed to sonar exposure include mass strandings, long-term habitat avoidance, and high-speed escape. Such strong negative effects can lead to the imposition of restrictions on Navy operations and could impact local populations of marine mammals, with ecological and commercial (e.g., whale watching industry) implications. To address this problem, more information is required about the ways in which different marine mammal species respond to anthropogenic sound and in how to recognize and avoid concentrations of vulnerable species. A potentially powerful tool in this regard is passive acoustic detection. In many cases marine mammals can more reliably be detected by their vocalizations than by sightings, and systems capable of detecting these sounds have application in surveying areas prior to exercises and in real-time mitigation during exercises.

Complete descriptions of the vocalization types and rates are lacking for many whale species including some considered especially vulnerable to anthropogenic sounds. Little is known about the environmental and social contexts in which they do and do not vocalize, making it difficult to judge the overall efficacy of passive detection. Tagging studies with acoustic recording tags, such as the digital acoustic recording tag (DTAG), offer the most direct way of obtaining this information.

The objectives of this research included:

1. quantifying the probability of detecting beaked whales (family Ziphiidae) by comparing the vocalizations as recorded via the tags to hydrophones on a Navy underwater range to estimate probability of detection as a function of range and orientation of the tagged whale; 
2. developing tagging techniques and field efforts to enable observational techniques or experiments that use the tag to monitor reactions of marine mammals to exposures of man-made noise; 
3. defining exposure parameters that pose risk to beaked whales;
4. testing whether this risk extends to other signals and other species; and
5. developing, bench testing, and building a new tag design and performing field evaluation of the new tags

Blainville’s Beaked Whale

In the initial phase of the project, a quantity of second generation DTAGs were designed, produced, and characterized for field evaluation. The tags are self-contained multi-sensor recording platforms containing two hydrophone channels and a set of sensors measuring depth, orientation, position, and speed. The DTAG-2s are suitable for use with any cetacean of adequate size and have an audio bandwidth compatible with all currently known whale vocalizations. The DTAG-2s communicate with a standard personal computer via a universal serial bus interface and are straightforward to use in the field. The DTAG-2 series were the basis for the major portion of this work as noted in the objectives.

The supplemental phase of the proposed work involved development and a field evaluation of a third generation of tags (DTAG-3) on deep-diving cetaceans.

Overall, this project developed methods and tools to monitor cetaceans including species of beaked whale that mass strand during some naval sonar exercises, defined the acoustic exposures that start to pose a risk, and developed methods to study how beaked and other whales respond to sonar and other sounds.

The probability of detecting beaked whale clicks from Blainville’s beaked whales (Mesoplodon densirostris) tagged was validated on a Navy range as a function of range and aspect. Although the work did assist in developing methods to estimate the absolute density and number of beaked whales based on passive acoustic detection, a greater sample size is required before robust conclusions can be drawn from the studies of effects of sonar. Preliminary results are helping to narrow the high level of uncertainty about the possible responses of Blainville’s beaked whales to anthropogenic sound and predator calls. They also suggest that other species of odontocete (toothed whales) may respond differently than beaked whales to sonar and other sounds, which may indicate different risks for such exposures for these different species. Taken together, these results suggest that beaked whales are silent and show avoidance responses to anthropogenic sounds ranging from shipping noise, mid-frequency active sonar, and pseudorandom noise in a surprisingly narrow range. A similar but more intense response was seen in response to the killer whale (Orcinus orca) playback, which was elicited by an exposure just barely above the ambient noise. After the killer whale playback, the beaked whale had a prolonged post-dive avoidance response. This suggests that while beaked whales showed similar responses to similar exposure levels for anthropogenic sound, they have a stronger response to much lower exposure levels of sounds from a predator.

For the testing of the new DTAG-3 system, experimental tags were deployed on beaked whales and sperm whales within networks of Global Positioning System (GPS)-synchronized acoustic recording buoys. The arrival times at the buoys of vocalizations from the tagged whale can be used to estimate the position of the whale independently of the dead-reckoned whale track deduced from the GPS, speed, and orientation sensors on the tag. A comparison of the two tracking methods enabled fine-tuning of the dead-reckoning algorithm, an estimation of error magnitude, and helped identify errors in the programs. The overall design of the new tag was similar enough to the second generation DTAG design that the considerable body of software, data treatments and field experience accrued in the last eight years was immediately applicable to the new devices.

These studies identified a statistically significant response from tagged beaked whales exposed to playback of sound, and they developed and validated a method to associate acoustic exposure with a safe response. The development of the DTAG-3 was completed successfully. The new DTAG-3 contains innovative new GPS and swim-speed sensors. Additionally, recording time was greatly increased in the new tag. Ultimately, this project has helped to define how beaked whales respond to sonar, what acoustic exposures elicit responses, and has developed methods and tools that are being used to help monitor marine mammals and how they respond to sound.