Forty years ago, the US banned production of a class of organic pollutants called polychlorinated biphenyls, or PCBs. International efforts to deal with PCB contamination have had substantial success. But the fat-loving compounds are still hanging around, leaching into the environment from decades-old equipment, and lingering in the ocean food chain.

That lingering contamination can still cause problems for a range of species. Because of a unique blend of characteristics, however, orcas are particularly at risk. A paper in Science this week calculates just how bad those risks are, and the results are sobering: while some populations of orcas seem to be doing just fine, others are at risk of collapse.

The zombie pollutant

Orcas, at the very high end of the food chain, absorb contaminants from what they eat, with their blubber soaking up and storing fat-compatible compounds like PCBs. Because they can live for 50 years or more, some individuals were exposed to PCB contamination back when it was at its highest and are still carrying that burden around. And it's a burden that gets passed between generations: because PCBs are stored in fat, females can transfer a huge amount of their own PCB load to their young during pregnancy and nursing.

The health consequences of this are serious. PCBs are linked to increased cancer risk, immune-system disruption, and reproductive problems. For a tiny calf receiving nearly half its mother's PCB contamination, the results can be fatal. And the death of an orca isn't the end of the cycle: a dead orca calf, sinking to the ocean bed where its carcass is scavenged, releases those PCBs back into the food web.

Not all orcas face the same level of risk. The US, UK, Russia, Germany, and France were the heavy PCB hitters, churning out more than a million metric tonnes of it between the 1930s and 1990s. That geographical spread still shows up in orca blubber, with populations in the most contaminated zones carrying as much as 50 times the PCB levels of their luckier relatives in less-contaminated waters.

What they eat matters, too. Some populations specialize in feeding on predators high in the food web, like seals, sharks, and tuna. Because the concentration of contaminants increases with each step up the chain, those blubbery seals carry considerably more PCB heft than the lower-food-chain fish favored by other orca populations. So even orcas that live in the same region can have vastly different PCB burdens.

To understand the threat this poses to orcas, ecologist Jean-Pierre Desforges and his colleagues set about calculating what might happen to populations of orcas with varying levels of exposure. They estimated how likely calves were to die at different levels of maternal PCB burden and how much more likely orcas were to die from disease due to PCB-caused immune suppression.

Running over the course of the next hundred years, these calculations spell out the fates of different groups. Some populations, like those in the less-contaminated waters near Norway, should manage to grow healthily despite some level of PCB contamination—or if they don't, the PCBs likely won’t be the cause. But the orcas living off the coasts of Greenland, Hawaii, Japan, and Brazil possibly won't be so lucky. The calculations predict that those populations, making up more than half of the world's orca populations, seem set for total collapse.

Alarm, but not certainty

Brendan Hickie, who studies contamination in aquatic ecosystems and wasn't involved with this paper, notes that these calculations are based on some imperfect information—something the authors themselves point out. For instance, there isn't data on survival rates for orca calves based on their mothers' contamination. So the authors used data from mink.

"There are always concerns about taking dose-response data from one species to another, and that's a very common issue in all of toxicology," says Hickie. There are differences between species, but importantly, it could go either way: maybe mink are more sensitive than orcas, but maybe it's the other way around.

This doesn't mean the calculations are useless, Hickie emphasizes, just uncertain: "What they're doing is standard practice in the world of risk assessment. It's like a doctor at an emergency ward in a hospital: you've got to do the best you can with the information that's available."

These calculations are also only looking at one of a multitude of problems orcas face, Desforges points out. "We can’t consider PCBs in isolation. In the real world, orcas are exposed to hundreds of different chemicals, and non-chemical threats like underwater noise, ship strikes, and food threats.”

To complicate things even more, Hickie highlighted that these threats can interact with one another—noise hampers orcas' ability to find food, which is already scarce because they're competing for it with fishing industries, and the resulting weight loss releases PCBs from their blubber. So these calculations aren't a prediction of what will happen to orcas so much as a way to put numbers on the havoc that PCBs are still capable of wreaking.

Cleaning up our messes

For Desforges, the hope is that the growing pile of data on the lingering effects of PCB should help to prevent complacency among regulators. Old PCB-containing equipment will likely still be in use for a few more years under international conventions, he says, but "is this ambitious enough? If we're serious about addressing this problem—if we're trying to protect top predators—what more do we need to do?"

Orcas are currently not listed as endangered, threatened, or any other threat status category: they're "data deficient." While there are concerns about some populations, the species as a whole hasn't been categorized. There's a major challenge here of "not even knowing how many killer whales there are in the world," says Desforges. But maybe evidence of a threat to a significant portion of the population can "drive the conservation status forward a little bit."

Both Hickie and Desforges point to some reasons for hope in the face of depressing findings—there are orca populations that are thriving, Desforges emphasizes. And in some ways, our actions on PCBs are forging a path for other pollutants to follow; the PCB bans were instrumental in helping to bring about regulations for other contaminants. "There's a lot of positive stories to show that research we do actually matters," Desforges says.

The fact that we're still figuring out the damage done by PCBs decades down the line is "not a lot of fun," says Hickie. But understanding this situation could provide a similar precedent for a better understanding of other pollutants: "hopefully we learn from our mistakes and don't repeat them."

Science, 2018. DOI: 10.1126/science.aat1953 (About DOIs).