A few decades ago, most countries phased out the manufacture and use of a highly toxic group of industrial chemicals called polychlorinated biphenyls, or PCBs. But the dangers of these largely banned pollutants still linger on. New research shows that PCBs remain a major threat to killer whales around the world, and could wipe out most populations in just 30 to 50 years.
From the 1930s to 1993, the world produced an estimated 1.5 million metric tons of PCBs. The chemicals were used in a wide range of applications, including paints, electronic cables and components, plastics, flame retardants, sealants, and adhesives. At the same time, there was increasing evidence showing that PCBs were extremely toxic: studies found that they could cause cancer, impair the immune system and reproduction, disrupt hormone signals and have a range of other effects on health. PCBs were, in fact, believed to be so dangerous that several countries, including the United States, banned the chemicals through the 1970s and 1980s, and many others followed suit after the 2001 Stockholm Convention. These hardy chemicals, however, have continued to persist in the environment: PCBs have been found deep in the Mariana Trench and even in the Arctic snow.
In PCB-contaminated parts of the ocean, killer whales or orcas (Orcinus orca) are struggling to survive, researchers report in the new study published in Science.
The problem is that PCBs degrade very slowly in the environment. They also tend to accumulate in fatty tissues of animals, their concentration increasing up the food chain — the higher up an animal is, the higher the amount of PCBs in its fatty tissues. Killer whales are top predators, occupying the final link in a long food chain, and have been recorded to have up to 1,300 milligrams per kilogram of PCBs in their fatty tissue, or blubber. This is over 25 times the levels that have been shown to affect fertility and immunity. Moreover, since PCB is fat-soluble, female orcas can pass on PCBs to their offspring through their fat-rich milk.
To see how PCBs affect killer whales, researchers of the recent study compiled available data on blubber PCB concentrations in 351 killer whales from populations around the world, and modeled the effects of the PCBs on the orcas’ rates of survival and reproduction and growth of populations.
“We know that PCBs deform the reproductive organs of animals such as polar bears,” study co-author Rune Dietz, a professor of bioscience at Aarhus University in Denmark, said in a statement. “It was therefore only natural to examine the impact of PCBs on the scarce populations of killer whales around the world.”
Dietz and colleagues found that in 10 out of the 19 killer whale populations they investigated, the number of whales was rapidly declining. Killer whale populations that occur in parts of the ocean least polluted by PCBs, such as those around the poles, Norway and Iceland, still have a large number of individuals and are at low risk. However, populations occurring in waters that have had historically high concentrations of PCBs, such as those around Japan, Brazil, the northeast Pacific, the Strait of Gibraltar, and the U.K., are all tending toward complete collapse in the next few decades, according to the study’s modeled scenarios.
“As the effects have been recognized for more than 50 years, it is frightening to see that the models predict a high risk of population collapse in these areas within a period of 30-40 years,” said lead author Jean-Pierre Desforges, a postdoctoral researcher at Aarhus University.
In fact, newborn killer whales have rarely been observed in these areas, added co-author Ailsa Hall, director of the Scottish Oceans Institute’s Sea Mammal Research Unit.
The killer whales’ diet matters too: whales that tend to eat large-sized prey like seals, tuna and sharks have high concentrations of PCBs in their bodies and are at the highest risk of population collapse. Those that eat mostly small fish such as herring and mackerel have lower amounts of PCBs and are at lower risk. Some killer whale populations, like those around northeast Scotland and Greenland, have been observed to have altered their diet, switching from low- to high-PCB-contaminated prey sources (such as from fish to seals), which the researchers say is likely to alter their exposure to PCBs and affect their health.
PCBs are just one among several pollutants that could be affecting killer whales. However, exposure to PCBs alone can cause killer whale populations to collapse, the researchers say. Moreover, despite countries committing to phase out PCBs more than 40 years ago, more than 80 percent of global PCB stocks are yet to be destroyed, with killer whales continuing to accumulate high levels of the chemicals in their bodies.
“This suggests that the efforts have not been effective enough to avoid the accumulation of PCBs in high trophic level species that live as long as the killer whale does,” said co-author Paul D. Jepson, an expert on orcas at the Zoological Society of London’s Institute of Zoology. “There is therefore an urgent need for further initiatives than those under the Stockholm Convention.”
Crispin Halsall, an environmental chemist at Lancaster University in the U.K., who was not involved in the study, said “the legacy of PCBs will continue to haunt us for some while to come.”
“Scientists estimate that the final resting place or ‘sink’ for PCBs is likely to be organic rich soils across the Northern Hemisphere or even ocean sediments,” Halsall wrote in the Conversation. “However, in the meantime, PCBs continue to cycle around the environment and are still present in mother’s milk. Maternal transfer from adult female to calf is the key exposure route for most marine mammals and this chemical stress (supplemented by an array of chemical pollutants other than PCBs), alongside climate change induced stress, is a major concern.”
Desforges J., et al (2018). Predicting global killer whale population collapse from PCB pollution. Science, 2018 DOI: 10.1126/science.aat1953