By Jason Bittel
Waves lap at motionless heaps of blubber and fins and the sun bears down on chapped skin. Gulls start to, well, do what gulls do. This heartbreaking scene happened in January when nearly 100 false killer whales became stranded along a remote shore in the Florida Everglades.
Authorities tried to steer the cetaceans back out to sea, but most were too exhausted or too entangled in the mangroves to make the last-ditch effort. In the end, more than 80 of the whales died. Just a few months later, a similar tragedy played out on the coast of New Zealand, this time with hundreds of pilot whales.
When a single whale beaches itself, the cause is thought to be injury, illness or old age. But when dozens or even hundreds, of the animals come ashore at once, scientists think something more is at play. While no one can say definitively what causes mass strandings, a growing body of research seems to point to one trigger.
According to a study published last month in the Journal of Experimental Biology, noise pollution such as ship traffic and seismic testing may force marine mammals to exhaust more energy on their dives than usual. This is particularly bad news because today our oceans are noisier than ever.
The oil and gas industry searches for its next score using giant air-gun explosions beneath the surface. And when fossil fuels are found, the drills used to extract them create even more of a din. Meanwhile, the U.S. Navy sends far-reaching sonar into the sea day and night as part of routine monitoring and training exercises. Furthermore, every ship that isn’t powered by wind adds to the undersea clatter with its generators, propellers and engines. Making matters worse, sound travels much farther in water than it does in air, which means each aural insult can radiate outward for miles and miles from its source.
“For whales, dolphins and other marine life, industrial and military noise is a death of a thousand cuts,” said Michael Jasny, a marine mammal expert with the Natural Resources Defense Council. “It degrades their foraging, keeps them from finding potential mates, silences them and drives them from their homes. Human noise has emerged as a major environmental threat and there is virtually no corner of the ocean that is free of it.”
The study’s lead author, Terrie Williams, has been studying this problem for more than a dozen years as a wildlife eco-physiologist. When she started, very little was known about what was going on inside marine mammals that might be causing their mysterious, untimely deaths. That changed when wildlife veterinarian Paul Jepson published a 2003 study in Nature that found gas bubbles in the livers of stranded cetaceans. That would indicate decompression illness or the bends.
As you know, whales and dolphins breathe air at the surface and dive below for food and travel. In order to adjust between the two environments, they have what’s called a diving response or reflex, which allows the body to shift its physiological priorities from what works best in air to what works best underwater. When down below, for instance, the heart rate lowers, blood vessels constrict and blood flow slows down. So for them to fall victim to decompression is definitely odd. “It seemed impossible,” said Williams, “due to all of the biological safeguards that marine mammals have in place for diving without injury.”
However, the bubbles in stranded whales’ livers showed that the dive response doesn’t always work. Williams wondered whether that diving response was less automatic than previously thought.
Through a new technology that Williams and her team invented, the researchers were able to place a device on diving dolphins to monitor second-by-second changes in heart rate, stroking mechanics and depth changes. The scientists learned that a marine mammal’s diving response is related to both the depth to which it dives and the amount of exertion it takes to get there. This was really important, said Williams, because it showed that the movement of nitrogen and oxygen throughout the animal’s body is not set in stone. That is, a whale or a dolphin might be able to dive safely in one scenario but not in another.
The next step was to prove that an outside factor, such as noise pollution, could possibly push the animal’s physiology from its normal, safe diving state to a more rushed and risky kind of dive. This is where Williams’s most recent research comes in.
Working in a deep pool aquarium, Williams and her colleagues trained retired military dolphins to wear the cetacean equivalent of Fitbits. The dolphins were taught to navigate through an underwater obstacle course at both a regular pace and a faster, escape-like pace to simulate both kinds of dives. The animals then surfaced under a sealed hood that measured the mammals’ exhalations. In other words, Williams wanted to know “how much of the internal oxygen scuba tank is used during a dive by a dolphin, especially if it is trying to escape oceanic noise.”
Predictably, the scientists found that it cost dolphins about twice as much physiologically to perform escape dives as opposed to dives at regular speed.
Marine mammals, of course, are not all the same. Whales are built differently from dolphins and even between whale species, body shape and dive adaptations vary. (Just think about the differences between a sperm whale and a blue whale). The scientists also had to account for the fact that larger animals require more energy to start moving but need less energy to keep all that blubber cruising once they reach higher speeds.
Fortunately, the researchers were able to make use of other studies that placed accelerometers on various whale species to measure dive times and depths. Using those data, they came up with a formula that allowed them to estimate the costs of swimming fast and slow for various types of cetaceans.
As a proof of concept, Williams and company applied their findings to the Cuvier’s beaked whale, which may grow to 23 feet long and 5,500 pounds and is known for making dives of nearly two miles in depth—deeper than any other mammal. Perhaps most important, beaked whales have already been shown to be extra sensitive to noise pollution. In one 2011 study, scientists found that Blainsville’s beaked whales stopped echolocating during dives when navy sonar was present and then avoided the source of the sound for two to three days. What’s more, several other studies have shown a correlation between navy sonar exercises and beaked whale strandings.
So what happened when they crunched the numbers for Cuvier’s beaked whales? The scientists estimated that a beaked whale may have to ratchet up its metabolic rate by more than 30 percent in order to escape oceanic noise quickly—and that’s in response to a single sound event. Imagine how those energy costs might add up across repeated run-ins with acoustic pollution.
“The implications of this are enormous,” said Williams. “Have the animals expended too much of their internal scuba tank? Is there enough oxygen going to their brains when they are trying to exercise at the same time that they are diving?”
These are questions Williams hopes to answer in future experiments as she attempts to establish “that last link” between ocean noise and marine mammal strandings. But with all the evidence she and other scientists have already assembled, it raises the question—how much more do we really need to know before changing our underwater ways?
Jason Bittel writes the Species Watch column for onEarth. Reposted with permission from our media associate onEarth.