In September 2002, 14 beaked whales washed ashore in the Canary Islands bleeding from the ears. All eventually died. A post-mortem examination revealed that the whales showed signs of decompression sickness (what scuba divers call "the bends"). Decompression sickness can occur when a mammal swims to the ocean's surface too quickly, and the change in pressure produces lethal nitrogen gas bubbles that clog its blood vessels. As deep divers of the ocean, beaked whales and other marine mammals such as dolphins have evolved to easily descend and ascend every day, so evidence of acute decompression sickness indicates unusual behavior. What would make a diving whale suddenly acquire decompression sickness, something it has evolved to avoid? Many ocean scientists and marine mammal experts suspected it was a fear response, an attempt by the whales to escape something very alarming. But fear of what — what were the whales avoiding?

Over the past 40 years, cumulative research across the globe has revealed a coincidence between naval sonar testing events and acute decompression sickness in beached marine mammals. Beached or stranded whales have been noted in conjunction with naval exercises in areas as far-flung as Greece, the Bahamas, Madeira, the US, and the Philippines. Could the acoustic stimulation of a naval sonar test be enough to make a whale act against its own best interests? Many prominent organizations with a keen interest in oceans and marine life, including the Natural Resources Defense Council (NRDC), a US-based environmental group, and the National Oceanic and Atmospheric Association (NOAA), a US federal agency, are convinced that under certain conditions — particularly when sensitive species such as beaked whales are involved — the answer is yes.

Sonar technology emits sound vibrations into the water, then records the echoes of that sound off various distant objects. This method is useful for mapping the ocean floor and detecting underwater vessels, such as submarines. The sounds emitted by tactical/military sonar are particularly powerful, capable of traveling hundreds of miles through the ocean in order to track distant threats. These vibrations can strike whales with tremendous force. In fact, the impact of mid-frequency military sonar on a whale's hearing can be equivalent to that of a jet engine at takeoff on the ears of a human located only three feet away. Many marine biologists believe the intensity of this noise is disturbing to whales, leading to their fatal rapid ascent. Some other marine biologists vigorously dispute this hypothesis, pointing to behavioral experiments that suggest whales ascend at a normal pace even when exposed to simulated naval sonar.

Whatever the precise relationship between naval sonar testing and acute decompression sickness in beaked whales, the marine biology community is in firm agreement that as a whole, human-originated noise pollution in the ocean significantly disrupts marine life. In fact, the impact of naval sonar on marine mammals is just the tip of the proverbial iceberg. Geophysicists who work in the commercial oil industry, for example, frequently employ a technique called reflection seismology, which uses loud pulses of sound to assess oil pockets under the seafloor. Climatologists use much quieter but wider-reaching low-frequency sonar to broadly assess ocean temperature over time. Many scientists believe that these activities also negatively impact marine life. But by far the greatest human contributor to ocean noise is commercial shipping traffic. Since the mid-1960s, the amount of commercial vessel traffic in Earth's oceans has nearly doubled, resulting in an almost 16-fold increase in background noise intensity in places like the waters off the coast of California, an area rich with marine mammal migration. Scientists suspect that large whales that rely on long distance communication for mating, food sourcing, and navigation may no longer be able to hear what they need to among all this noise.

To what degree do we understand the full impact of our activities on marine life? Although the ocean makes up approximately 70% of Earth's surface area, when its depth is taken into account, it accounts for a much larger portion of the planet's habitat. To date, scientific studies of the impact of human activity on marine life have focused on only a tiny part of this habitat; as a result, the rules in place around the world to protect marine animals from human encroachment are mostly restricted to areas near the coastline. Clearly, we are just at the beginning of our efforts to manage our impact on the ocean's many life-forms.

Some progress is being made. In early 2010, for example, NOAA announced that it was beginning work toward creating a comprehensive "noise budget" for the oceans, based on assessment of the entire spectrum of sound pollution from human technology, from ship traffic to sonar to seismic mapping. More initiatives like this one are needed to broaden our understanding of the relationship between our scientific, defense, and corporate activities and the welfare of Earth's marine life. In order to give you context for this ongoing challenge, this Spotlight guides you through both the basics of marine mammals and the fundamental sources of acoustic pollution in their habitat.

Image: R. Wicklund/OAR/National Undersea Research Program (NURP); University of North Carolina at Wilmington.