<p>An orca helps herd a school of herring in the deep waters of the Andfjorden in Norway. (<a href="https://www.nationalgeographic.com/photography/article/orca-killer-whale-gallery">See more of orcas</a>.)</p>

An orca helps herd a school of herring in the deep waters of the Andfjorden in Norway. (See more of orcas.)

Photograph by Paul Nicklen, Nat Geo Image Collection

Half the World's Orcas Could Soon Disappear—Here's Why

Lingering PCB pollution poses a serious threat to the marine mammals.

They live in chatty groups, and can hunt in teams—sometimes working in tandem to create waves that dump unlucky prey off floating ice. Savvy orcas, with their splotchy two-tone flesh and rich family lives, have survived mass slaughter, being captured with nets and lassos, and being trucked and airlifted to marine theme parks.

But new research published Thursday in the journal Science suggests more than half of the world's killer whale populations could face complete collapse in 30 to 50 years, thanks to a suite of toxic chemicals the world has already banned.

Long-lived polychlorinated biphenyls, or PCBs, are organic compounds once used in capacitors, oil paints, and coolants, until they were deemed so dangerous that their manufacture was banned in the U.S. and other countries in the 1970s and 1980s. Yet today orcas across the northern hemisphere are among the most heavily contaminated animals on Earth.

Even now, PCBs are believed to be altering orca behavior, damaging their immune systems, and harming reproduction so much that researchers suspect many families of killer whales (technically dolphins) may not survive the next few decades.

"A group of chemicals we thought was no longer a threat is still present at concentrations that will continue to pose significant risk," says lead study author Jean-Pierre Desforges, with the Arctic Research Centre at Aarhus University in Denmark.

Desforges called the results "frightening"—in part because PCBs are just one of several threats facing orcas, often not even the dominate one.

PCBs Accumulate in Top Predator

While PCBs initially declined after the world stopped manufacturing them, levels in the environment have remained relatively constant in recent years. In part that's because the compounds are still found in legacy products, such as transformers, cable insulation, and some ship paints. Eighty percent of global PCB stockpiles have not yet been destroyed.

Plus, PCBs break down slowly and are drawn to the molecules of living animals, so they've worked their way into the food web. Orcas are an apex predator—they sit at the top of the food web, may eat fish, seals, sea lions, sharks, or whales, and have no natural predators. So the carcinogens build up in their blubber.

Orcas range from Brazil to the Mediterranean Sea and from the Arctic to Antarctica. Unlike many land-based predators, such as polar bears, killer whales have a hard time getting rid of PCBs. Some killer whales now carry 25 times more PCBs than amounts shown to alter fertility. Mothers even pass the pollutants along during birth or through breast milk.

"Based on the weight of evidence from a couple of decades of research, PCBs remain the number one pollutant of concern at the top of the food chain for wildlife in the northern hemisphere," says Peter Ross, one of Desforges’ co-authors and a marine mammal toxicologist at Ocean Wise, the research arm of the Vancouver Aquarium in British Columbia.

Knowing this, Desforges and colleagues compiled research on PCBs in 351 killer whales from around the world, creating the most far-reaching database of its kind anywhere. They used population-growth trends and the risks posed by specific PCB levels to forecast survival rates over a century of exposure.

They found 10 of the 19 populations they studied were already in decline, and that PCB exposure led to fewer animals over time. Especially hard-hit are killer whales living near industrialized areas around the Straits of Gibraltar and the United Kingdom, where less than 10 are thought to remain. Also at risk were populations in Japan, Hawaii, and the northeast Pacific Ocean, which tend to eat marine mammals that are themselves high in PCBs. Populations in high latitudes—around Iceland, Norway, and the poles—have minimal contamination and face far less risk.

Researchers acknowledge the study's limitations. It's based on computer modeling, and impacts to killer whales are extrapolated from studies of other animals.

"It's a great exercise, but you have to take it with a grain of salt," says James Meador, an ecotoxicologist with NOAA Fisheries' Northwest Fisheries Science Center, who was not part of the study.

But even Meador calls the results "a wake-up call" because PCBs just make other orca challenges worse.

Multiple Threats Interact

To understand how, one need only look to the Pacific Northwest and Puget Sound, a few short miles from Meador's Seattle office.

The region's critically endangered fish-eating southern resident killer whales are the most studied orcas in the world. Using photobooks and unique markings researchers can identify each individual and trace family lineage to one of three pods, known as J, K, and L.

While Desforges' study showed PCB risk to these killer whales was moderate, this population, thought to have numbered several hundred in the 19th century, is now down to just 74. The threat is deemed so great that Washington's governor this summer created an emergency task force to stave off an extinction crisis.

And killer whales have such sophisticated emotional lives the problems of these urban orcas are often quite public—and hard to watch.

This summer, J35, a 20-year-old orca nicknamed Tahlequah, lost a calf a half-hour after giving birth and proceeded to push her dead offspring around with her head for 17 days. She ultimately swam more than 1,000 miles.

While the world tracked this animal's grief ritual, scientists were monitoring another orca, three-year-old J50, which appeared to be slowly starving to death. Scientists gave her antibiotics and used an upside-down petri dish attached to a long pole to take breath samples from her blowhole. Local tribal groups crushed up salmon and tried to feed her. She finally disappeared in mid-September.

And experts this week photographed another animal, K25, that clearly had lost significant weight.

While at least three animals in this population are currently pregnant, no southern resident whale has successfully kept a calf alive in several years. From 2008 to 2014, scientists using a poop-sniffing black Labrador mix named Tucker tracked down whale fecal matter and used it to show that nearly 70 percent of all known pregnancies had failed, according to research from last year.

"We are at the lowest point now that we've been in 30 years," says Lynne Barre, NOAA's killer whale recovery coordinator.

While many factors contribute to the decline, three are key. First, unlike other orcas that eat seals or sea lions, southern residents feed almost exclusively on chinook salmon. But chinook have been in steep decline for years, and the killer whales each need hundreds of pounds of fish each day. Meanwhile, boat traffic noise is making echolocation difficult, just as the orcas need to look farther afield for food.

And when whales are hungry and working hard, they metabolize fat, releasing PCBs and other toxic chemicals from their blubber into the bloodstream. There the pollutants can damage the immune system, increasing disease risk. It can significantly reduce fertility or act as a neurotoxin, potentially disorienting whales, further complicating the hunt for food. And as starving whales get significantly smaller, the percentage of PCBs in their bodies increases, amplifying the impacts.

"All of these multiple threats are interacting," Barre says.

Other Populations at Risk

Killer whales can live as long as humans, which means some of those alive today were around during the heyday of PCB use during and after World War II. And these are slow-acting contaminants, which means adults can still see impacts from exposure as calves or while they were in utero.

That means even populations that seem healthy may actually be at risk, Ross says.

While Puget Sound's resident whale numbers are plummeting, nearby transient whales, which eat seals and sea lions, are stable —even though their PCB levels are often higher. Killer whale numbers in Canada and Alaska are actually increasing.

But since PCBs can affect almost every physiological function, "sometimes numbers don't speak with clarity," Ross says.

For example, in the late 1980s, harbor seals in Europe were staging a great comeback after major declines from PCBs and pesticides in the 1960s. But shortly after government officials declared the crisis over, more than half of the seals died after being hit with a virus. Their immune systems likely had been weakened by years of exposure, Ross says.

Deforges and Ross say banning PCBs clearly made the situation better for orcas "Without that, we probably wouldn't have killer whales today," Ross says.

But both maintain that countries need to act more quickly to clean up legacy pollutants, both at home and through the Stockholm Convention. In the meantime other threats to orcas—particularly from food scarcity, ocean noise, and the looming risks from climate change—need to be curbed quickly to pull some populations back from the brink.

"We have more than enough information to act," Ross says. "Time will tell if we do so fast enough."

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