A decades-old pollutant still threatens orc populations

Forty years ago, the United States banned the production of a class of organic pollutants called polychlorinated biphenyls (PCBs). International efforts to combat PCB contamination have been successful. But fat-loving compounds still linger in the environment after decades of equipment and remain in the ocean's food chain.

This persistent contamination can still cause problems for various species. Due to a unique blend of features, orcas are particularly vulnerable. A paper in Science this week calculates how serious these risks are, and the answers are sobering: while some orc populations seem to be doing well, others are likely to collapse.

The zombie pollutant

Orcas, at the upper end of the food chain, absorb contaminants from what they eat, and their fat absorbs and stores compounds compatible with fats, such as PCBs. Because they can live for 50 years or more, some people have been exposed to PCB contamination at its highest level and continue to bear this burden. And it's a burden that passes between generations: since PCBs are stored in fats, females can transfer a huge amount of their own PCB load to their young during pregnancy and breastfeeding.

The consequences for health are serious. PCBs are linked to an increased risk of cancer, a disruption of the immune system and reproductive problems. For a small calf receiving nearly half of its mother's PCB contamination, the results can be fatal. And the death of a killer whale is not the end of the cycle: a dead orc, which sinks into the bottom of the ocean where its carcass is recovered, releases these PCBs into the food chain.

All orcas do not face the same level of risk. The United States, the United Kingdom, Russia, Germany and France were the hard hitters of PCBs, producing more than one metric ton between the 1930s and the 1990s. in the fat of orcas, with populations in the most contaminated areas carrying up to 50 times the PCB concentrations of their parents more fortunate in less contaminated waters.

What they eat is also important. Some populations specialize in feeding high predators in the food chain, such as seals, sharks and tunas. As the concentration of contaminants increases at each stage of the chain, these fat seals carry much more PCB-containing waste than fish in the lower food chain favored by other killer whales. Thus even killer whales living in the same area can have very different PCB charges.

To understand the threats this poses to killer whales, ecologist Jean-Pierre Desforges and his colleagues began to calculate what could happen to orc populations with different levels of exposure. They estimated the likelihood of calves dying at different levels of maternal PCB burden and how much more orcs are likely to die from a disease caused by PCB-induced immunosuppression.

Over the next 100 years, these calculations define the fate of different groups. Some populations, such as those living in less polluted waters near Norway, should be able to grow well despite some level of PCB contamination, or if they do not, PCBs are unlikely to be the cause. But killer whales living off the coast of Greenland, Hawaii, Japan and Brazil will probably not have the same luck. The calculations predict that these populations, which represent more than half of the planet's orc populations, seem to be facing a total collapse.

Alarm, but not sure

Brendan Hickie, who studies contamination in aquatic ecosystems and did not participate in this paper, notes that these calculations are based on imperfect information, which the authors themselves point out. For example, there is no data on the survival rates of killer calves based on the contamination of their mothers. The authors therefore used mink data.

"There are always concerns about dose-response data from one species to another, and it's a very common problem in all toxicology," says Hickie. There are differences between species, but most importantly, it could go both ways: maybe mink are more sensitive than killer whales, but maybe it's the other way around.

That's not to say that calculations are useless, Hickie points out, simply uncertain: "What they do is a common practice in the world of risk assessment, it's like a doctor in a ward. urgency as best you can with the information available. "

Desforges also points out that these calculations only cover a multitude of problems encountered by killer whales. "We can not consider PCBs in isolation – in the real world, killer whales are exposed to hundreds of different chemical and non-chemical threats, such as underwater noise, ship strikes and food threats. "

To further complicate matters, Hickie pointed out that these threats can interact with one another – noise interferes with the ability of orcs to find food, which is already scarce because they compete with it. So these calculations are not a prediction of what will happen to the orcs, but rather a way to quantify the damage that PCBs are still able to cause.

Clean up our mess

For Desforges, the hope is that the growing battery of data on the lingering effects of PCBs should help prevent complacency among regulators. Older devices containing PCBs will likely continue to be used for a few more years under international conventions, he says, but "is it ambitious enough if we really want to solve this problem – if we try to protect the major predators – what more must we do?

Killer whales are currently not listed as Endangered, Threatened, or any other category of Threat Status: they are "Data Deficient". Although some populations are concerned, the entire species has not been categorized. There is one major challenge: "not knowing how many killer whales are in the world," Desforges explains. But perhaps the evidence of a threat to a significant portion of the population can "advance some conservation status."

Desforges insists that both Hickie and Desforges emphasize some reasons to hope for depressing results. And in some ways, our actions on PCBs are shaping a way forward for other pollutants; PCB bans have been instrumental in establishing regulations for other contaminants. "There are a lot of positive stories to show that the research we are really doing," Desforges explains.

The fact that we still know the damage caused by PCBs in recent decades is not very fun, says Hickie. But understanding this situation could provide a similar precedent for a better understanding of other pollutants: "We hope to learn from our mistakes and not repeat them".

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