- As the top predators in the ocean, killer whales suffer from the magnifying level of pollutants that build up in the marine food web.
- Scientists found that North Atlantic orcas feeding on marine mammals carry significantly higher levels of pollutants than orcas that eat fish.
- Levels of polychlorinated biphenyls in the orcas’ blubber are ten times higher than the toxic threshold for these dangerous household chemicals.
With their shiny black tops and pure white undersides, the killer whales, or orcas, are the sleekest hunters in the ocean. But being apex predators comes with a great cost. Their bodies absorb the chemical pollutants that build up in the long chain of prey leading to their meals. Now, a recent study in Environmental Science & Technology has shown that what orcas choose to eat could affect their survival more than scientists thought.
Every year over the past decade, a team of international marine biologists went to the North Atlantic Ocean to collect samples of killer whales’ blubber—the fat layer beneath their skin. The study, which covered an area spanning the Canadian Arctic, Eastern Canada, Greenland, Iceland, and Norway, was the most comprehensive of its kind. The team’s analysis of 162 North Atlantic killer whale (Orcinus orca) samples showed a startling level of various chemical pollutants, despite their remote ranges.
“These killer whales are pretty much isolated,” said Anaïs Remili, a marine biologist at McGill University in Montreal, Canada, and lead author of the study. “They’re super elusive, and we don’t really know where they are. They’re far away from any human settlement.”
To collect the blubber samples, Remili and her colleagues shot darts fired from an air gun into the orcas’ skin layer. “You have to approach the whales slowly to make sure that you don’t scare them,” she said. Each sample was about 2 to 3 centimeters (1 inch) long.
The scientists found a wide array of persistent organic pollutants, or POPs, in the orcas’ tissues. These chemicals come from discarded household items like paint, plastics, coolants, and pesticides that ultimately drain into the sea.
One category of pollutants was polychlorinated biphenyls (PCBs), chemicals used as coolants or lubricants in electrical components. “They are very stable chemicals that do not degrade in the environment,” Remili said.
These toxins stick strongly to the body fat in orcas. Not only are they impossible to get rid of, but mothers easily pass them down to their calves through their milk.
While killer whales exist in all oceans, each group feeds on different prey. Fish carry some of these pollutants, but marine mammals—higher up in the ocean’s food web—build up the toxins at far higher concentrations.
The team measured about 100 parts per million of PCBs in the Western North Atlantic population of orcas. These hunters feed mainly on toothed whales and pinnipeds, such as sea lions, rather than fish.
That diet makes all the difference, Remili told Mongabay: “These (numbers) were extremely high, especially for killer whales in this pristine area.” Indeed, these levels of PCBs are ten times higher than the threshold that scientists had estimated as dangerous to the immune systems and fertility of orcas.
“We don’t know yet what is the driver behind these food preferences,” said Remili. “But it would be so much better if they ate something else.” The concentration of PCBs in Eastern North Atlantic orcas, which mostly feed on herring, is about 10 parts per million—an order of magnitude lower than their Western counterparts.
“It’s been 50 years since the United States and Canada banned PCBs,” Remili said. And yet, “we still measure crazy high contaminants in these individuals. That’s just heartbreaking.”
“The reality is that [the contaminants] will be with us forever,” said Juan José Alava, a marine ecotoxicologist at the University of British Columbia in Vancouver, Canada, who was not involved in the study. When killer whales die and sink to the bottom of the ocean, the pollutants will stay in the ecosystem as their bodies decompose, he noted.
Still, Alava believes there is hope. “We can put in place legislation right now to control, prohibit, and eliminate new kinds of persistent organic pollutants,” he told Mongabay.
In 2004, the leaders of more than 150 nations participated in the United Nations Stockholm Convention, which requires all countries to eliminate PCBs by 2025.
“Unfortunately, we don’t speak orcas, but we can take action to conserve them for future generations so they can enjoy how beautiful they are,” said Alava.
Remili, A., Dietz, R., Sonne, C., Samarra, F. I. P., Letcher, R., Rikardsen, A. H., … McKinney, M. A. (2023). Varying diet composition causes striking differences in legacy and emerging contaminant concentrations in killer whales across the North Atlantic. Environmental Science & Technology, 57 (42), 16109-16120. https://doi.org/10.1021/acs.est.3c05516
Kristel Tjandra (@KristelTjandra) is a graduate student in the Science Communication M.S. Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.
Correction: Due to an error in the final stage of production, the concluding quote was misattributed to Anaïs Remili.