When the sea goes silent

New research suggests a possible cause of dolphin and whale strandings: severe to profound hearing loss

When the sea goes silent

45 dolphins beached themselves in shallow water off the coast of Australia | Reuters

When weakened by disease, starvation or injury, dolphins succumb to an instinctual fear of drowning. Seized with panic, they swim to shallower and shallower water to keep breathing, and often wind up stranded on a beach, where the sun, sand and wind quickly end their lives.

Now, thanks to new research from the University of Southern Florida (USF), scientists have discovered one of the elusive root contributors to whale and dolphin strandings—deafness.

“Whales and dolphins are acoustic animals. They use sound to feed, they use sound to breed, they use sound to fulfill every biologically important goal of their existence,” says Michael Jasney, an ocean-noise expert with the National Resources Defense Council, an international environmental group. “If you take away their ability to hear, you take away their link to the world.”

About 1,200 to 1,600 whales and dolphins are stranded on the U.S. coast annually. Less than 200, or about 17 per cent, survive until they can be saved, and rescuers face a critical decision when trying to keep them alive. They have to quickly determine whether to have the animal euthanized, pushed back to sea, or brought to a lab for rehabilitation. David Mann, a biological oceanographer at USF and lead author of the study that put forward the new research, hopes to make that decision easier.

Between 2004 and 2009, Mann and his team attached electrodes to the heads of 33 stranded whales and dolphins to analyze their brain waves and test aural responses, a technique borrowed from human infant hearing tests. Nearly a third of the animals were found to have severe to profound hearing loss, meaning their communication and echolocation abilities were intensely handicapped. This suggests large numbers of strandings could stem from hearing disorders.

It’s a timely finding. Most marine mammals suffer from diseases, genetic disorders and geriatric problems similar to those that cause hearing loss in humans, but over the last 100 years they’ve also been subjected to increasing doses of high-decibel noises. “The level of low frequency sound is essentially doubling every decade and has been since the 1960s,” says Jasney, adding that noise from supertankers, oil and gas exploration and military operations is like a pea-soup fog to acoustic animals. “This cacophony of noise has practically obliterated the communication range of some species.”

Mann says the dolphins he studied were from areas with little industrial shipping or seismic exploration, and that their hearing loss was most likely a result of natural causes like old age. However, he says his research is still invaluable when it comes to rehabilitating whales and dolphins across the globe. “The deaf ones, they don’t look any different,” he says, adding that dolphins with severe hearing loss will almost certainly die in the wild, making identifying deafness crucial for rescuers who need to know whether animals can survive outside of captivity.

Jasney acknowledges the importance of Mann’s research, but says ocean noise, regardless of whether it’s contributing to deafness, is causing problems for marine life. “Sound travels much farther in water then it does in air,” he says. Endangered fin whales have fallen silent across vast swaths of ocean because they can’t communicate over air guns used in seismic exploration. The calving rates of northern right whales have plummeted compared to their southern-hemisphere cousins, whose waters are host to less large-scale shipping. Several species of whales are now vocalizing at much louder levels—trying to shout over the din—and military sonar, which can operate at 240 decibels—100 dB louder than jet engines—is linked to mass strandings worldwide.

“Sonar somehow causes hemorrhaging, producing nitrogen bubbles in the liver and other organs, and causes other pathologies that you tend to see in human and land animals that are suffering from severe decompression sickness,” says Jasney. But scientists still know very little about marine mammals’ hearing, and much more research is needed to understand the connection between ocean noise and hearing loss.

“There’s a lot of speculation because there’s no baseline data,” says Katie Moore, manager of the International Fund for Animal Welfare’s Marine Mammal Rescue and Research Team. She also monitors hearing in stranded dolphins, and is heading a project to make hearing tests a diagnostic tool for rehabilitation. “Until we better understand what’s normal, we’ll never understand what impact ocean noise is having on them. But we’re getting there.”

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