The tails of three North Atlantic right whales break the surface in Cape Cod Bay near Provincetown, Mass., on April 10, 2008.  (Stephan Savoia/AP)

Nearly 90 years have passed since North Atlantic right whales became a protected species following their devastation by whalers, but their populations have yet to recover.

A new report by the National Academies of Sciences, Engineering, and Medicine lays out some of the reasons why. Although scientists know a combination of overfishing, pollution, climate change and human-created ocean noise is harming the whales, they have no idea which culprit is worse, and because of that, they’re not sure which one to prioritize as a target for expensive protection efforts.

The report, released Friday, focused on the impact of noise from sources such as power plants and commercial and military ships to determine how it fits in an ecosystem teeming with both natural predators and human killers. The researchers found that they can’t yet determine whether sounds are disturbing enough to wreck entire populations of whales or only just a few animals at a time.

“You can shut off noise, but if it’s only a tiny problem, it might not have a big benefit,” said Peter L. Tyack, a professor of marine biology at the University of St. Andrews in Scotland. “We don’t know what set of  cumulative impacts it’s having. We know sound disrupts the behavior, but we don’t know if sound is endangering populations.

“That shows how weak our scientific methods are,” he added. “We don’t have the science to show which are causing the problem and which we can eliminate.”

Sound is important, said Tyack, chair of the committee that conducted the study and wrote the report, because along with fishing it’s a point of stress that can be controlled. But the surveillance technology that allows scientists to determine the impact of sound is only six years old, and the first study that relied on it was published only two years ago.

The effects of climate change can’t be easily reversed, nor can removing vast amounts of toxins such as carcinogenic polychlorinated biphenyls, or PCBs. A firm understanding of how not only right whales but numerous other whale species and dolphins are affected by sound is up to 20 years away, Tyack said. But it’s not known how long right whales, a highly endangered species, can hold on.

The carcass of a rarely seen deepwater beaked whale that washed ashore on Jones Beach in Plymouth, Mass.  (New England Aquarium via AP)

Right whales are 79-ton leviathans that take 10 years to sexually mature and one year to give birth, usually in winter off the coasts of Georgia and northern Florida, and their babies take a year to wean. They are stocky, squared jawed and black, and in the 1800s and early 1900s they were hunted to near extinction, as were numerous whale species.

The whales were protected from harvesting along with others in 1930, but the threats didn’t end there. Coal-fired power plants began to proliferate, along with bigger, louder commercial and military vessels.

“Sound travels incredibly well underwater. Sound in one place has the ability to travel halfway around the world underwater,” said Jason Gedamke, director of the Ocean Acoustics Program at the National Oceanic and Atmospheric Administration’s Fisheries Office of Science & Technology. “Marine life has had millions of years to evolve underwater, and they use sound for their daily function, surviving predators and finding prey.”

But since the industrial age, a boom in ocean activity with ships, construction and industrialization has turned up the noise and altered the underwater environment. In addition to increasing the volume in water, industrialization poured carbon pollution into the air, which falls on the oceans as acids and toxins.

Researchers will have to “develop methods to assess how changes in a combination of stressors can best bring the population to a healthy, resilient state,” Tyack said, acknowledging that the task is enormous.

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