The study noted many possible reasons for decline, including hunting, entanglement in fishing nets, noise disturbance from large ships or oil and gas operations, ingestion of plastic debris and ocean ecosystem changes.
The most likely, the study concluded, are drops in deep water oxygen levels, along with Navy sonar.
As the ocean’s low oxygen zones have expanded because of warming water, the fish and small squid that beaked whales feed on may have diminished, the study stated. Meanwhile jumbo squid, which are resistant to low oxygen conditions, have expanded their ranges, and may compete with beaked whales for prey, the study stated.
Sonar can spook whales, Moore said, driving the animals away from feeding areas. In extreme instances, it can disrupt their diving behavior in a way that leaves the whales with a cetacean version of “the bends.”
Researchers have observed strandings of beaked whales following sonar testing, Moore said, and behavioral studies in the Bahamas showed that the whales flee from sonar noise, and occasionally die from its physiological effects.
Robert Gisiner, program manager for the Navy Living Marine Resources Research Program, said the Navy acknowledges that sonar poses risks to marine mammals, but said only it has been linked to strandings in only about 40 cases. The Navy has funded research by Barlow and other researchers, he said, and follows protective measures approved by the National Marine Fisheries Service, the parent agency of the Southwest Fisheries Science Center.
The study also noted, paradoxically, that some Navy sonar ranges show high densities of beaked whales. But the authors said it’s unclear if the whales are truly thriving there, or whether the areas offer high feeding opportunities but also high risks - sort of like an underwater inner city.
Gisiner, however, said the finding “suggests that even frequent sonar use in some areas does not necessarily lead to reduced beaked whale populations.”
Moore acknowledged that there’s more to be learned, and said isotopes studies may help pinpoint changes in the whales’ diets, and future marine transects will flesh out their population trends.
Gisiner said the Navy will be part of those efforts.
“The Navy is a world leader in funding marine mammal research, and will remain proactive in funding studies that shed light on how beaked whales and other marine mammals may be affected by sonar and other underwater sound,” he said.
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