A new study funded by the U.S. navy and the Office of Naval Research show that Beaked whales are at higher risk of developing decompression sickness since they live with extremely high levels of nitrogen in their blood and body tissues. This may explain why beaked whales seem to be especially susceptible to naval sonar. If the sonar causes the animals to surface more rapidly than they would normally do, e.g. because they are frightened by the underwater sounds, it may lead to decompression sickness which may in turn explain the strandings associated with naval sonar exercises.

Decompression sickness, commonly referred to as “the bends” among scuba divers, is a consequence of the sudden drop in pressure that occurs when you ascend rapidly from the deep. When mammals dive, nitrogen builds in our bodies. If we ascend slowly the nitrogen isn’t dangerous, but if we ascend too quickly the nitrogen forms bubbles inside the body. Tiny bubbles might not sound like anything to fuzz about, but within the body it can be lethal.

Beaked whales are believed to accumulate large amounts of nitrogen within their bodies since they make repeated dives to such great depths. They can stay submerged without breathing for long periods of time and are capable of descending down to nearly 1,500 metres. Having this inclination for decompression sickness may explain why beaked whales seem to be more vulnerable to naval sonar than other marine mammals.

“It provides more evidence that beaked whales that are being found dead in association with naval sonar activities are likely to be getting decompression sickness,” said Robin Baird, a marine biologist at Cascadia Research Collective and one of the report’s authors.

The study has focused on three species of beaked whale: Cuvier’s beaked whale (Ziphius cavirostris), Blainville’s beaked whale (Mesoplodon densirostris), and the Northern bottlenose whale (Hyperoodon ampullatus). The Northern bottlenose whale was studied off the cost of Nova Scotia, Canada while the two others were observed around Hawaii, U.S.

According to a 2006 report in the Journal of Cetacean Research and Management, 41 known cases of mass strandings of Cuvier’s beaked whales have occurred since 1960. Some of them have happened at the same time as naval sonar exercises in the area, including Greece in 1996, the Bahamas in 2000, and the Canary Islands in 2002. When the beaked whales stranded in Bahamas were autopsied, they turned out to have bleedings around their brains and ears; bleedings which may have been caused by nitrogen bubbles.

The U.S. navy has agreed to adopt certain practises to protect whales, but is resisting more stringent restrictions until more scientific evidence is at hand. The navy has budgeted 26 million US per year over the next five years to fund marine mammal research on how these animals are affected by sound.

If you wish to find out more about the beaked whale study, it is published online this week in the journal Respiratory Physiology and Neurobiology.