A detailed article-cum-press release from Navy News provides the most information currently available on three studies that will be central to addressing ongoing questions about the Navy’s assessment of the behavioral responses of whales and dolphins to Navy mid-frequency active sonar. Two of the studies took place on US Navy instrumented ranges during normal Naval sonar training exercises, and the third is a Controlled Exposure Experiment taking place this summer in the Mediterranean.  Both the Navy and outside observers (including AEI) will be looking closely at the results of these studies, since the most contentious aspect of current Navy sonar planning involves identifying the sound levels at which behavioral responses (such as fleeing or suspending foraging) become widespread enough to warrant protective measures.  Current safety guidelines only kick in when whales are within 3000 feet, far less than the range at which behavioral responses occur.  Critiques of current Navy EISs focus on the large numbers of animals predicted to change their behavior, and on a “risk function” developed by NMFS that assumes very few whales are affected at sound levels below 145dB.

The CEE study is using D-tags, which record received levels of sound while also monitoring dive patterns as a sonar signal (or simulation) is emitted nearby. Typically, very few whales are able to tagged, which has to date limited the robustness of results from CEE studies, though they provide the clearest indication of individual animal responses to human noise. There are some questions, as well, as to the absolute accuracy of the sound recordings obtained by D-tags, since the animal’s body can be between the sound and the hydrophone. The Mediterranean study is modeled after a similar project at the AUTEC range in the Bahamas in 2007, but will be taking place with a population of animals that, unlike the ones in the Bahamas, are not used to hearing sonar signals, and who therefore may be more sensitive to the sounds.

The animals at AUTEC are likely the most heavily-exposed population of beaked whales in the world. David Moretti, principal investigator for the Naval Undersea Warfare Center’s Marine Mammal Monitoring Program, noted that “the mere presence of these species on a Navy range is counterintuitive to the perception of beaked whale reactions to sonar. Given that this is an active Navy range where sonar is used, you wouldn’t anticipate this species to be present in this particular location if you believed the popular press.” However, Moretti’s broad-brush review of the background for this study is not so clearly reassuring: Though the team is still reviewing the data for this project, he noted that beaked whale monitoring during previous sonar events has shown a decline in acoustic detections of beaked whales during active sonar exercises, and increases following the end of multi-day exercises. “We believe they avoid the sonar by moving off the range, and they return after operations are finished,” said Moretti. “Once they’re off the sensors we really don’t know where they go. I can’t say definitively that the animals that leave the range are actually the same animals that come back.” The AUTEC study hopes to answer some of these questions. In 2000, of course, a pod of beaked whales at AUTEC were apparently trapped between sonar ships and shore, and became the first widely-publicized sonar-related stranding victims; some whale researchers claimed that beaked whale numbers were low for several years after this.

On the instrumented ranges (in the Bahamas and off Southern California), animals were tagged with satellite monitors that could track their position, but not dive patterns or received sound levels. In addition, overall animal density can be estimated by changes in calls recorded on permanent bottom-mounted hydrophones. Again, though, successful tagging is difficult, especially for beaked whales that remain on the surface for only brief periods between long dives. Over the course of a month at the AUTEC range in the Bahamas, 9 whales were successfully tagged. On California’s SCORE range, 8 whales were tagged in eight days of field work, and floating hydrophones were deployed outside the range to monitor animals that left the 500 square mile instrumented range. This fall, when researchers return to SCORE, they’ll be testing a new “fast and light” approach to tagging, hoping to successfully tag more beaked whales.

For more detailed coverage of Navy sonar issues, see sonar posts on AEInews.org, as well as two Acoustic Ecology Institute Special Reports: Naval Active Sonars and Navy/NRDC Fact Check.