New kinds of flame retardants can accumulate in orcas and even be passed to their nursing calves, scientists reported Wednesday, and it's not yet clear what implications this has for the health of the animals.

The researchers, whose findings were published in Environmental Toxicology and Chemistry, measured the concentrations of chemicals that have been regulated for decades, as well as their newer replacements, in tissues from stranded whales in Norway. The team found that levels of a group of chemicals called polychlorinated biphenyls, or PCBs, remain high in wildlife, more than 40 years after being banned in the United States and elsewhere.

Because of their non-flammability, chemical stability and other properties, polychlorinated biphenyls were used in electrical equipment, plastics, dyes and many other applications, according to the U.S. Environmental Protection Agency. However, the chemicals increased cancer risk in animals and impaired the immune, reproductive, nervous and endocrine systems.

As chemicals such as PCBs are regulated, phased out or banned, other chemicals have been introduced to replace them, including those known as brominated flame retardants, or BFRs, says Katrine Borgå, a professor of biosciences at the University of Oslo in Norway and last author of the study. 

Little is known about how these chemicals build up in marine mammals, Borgå and her colleagues wrote in the study. However, top predators such as killer whales may be especially vulnerable as they accumulate contaminants from their prey.

In recent years, numerous cases have been documented of whales becoming stranded along the Norwegian coast. Borgå and her team examined chemicals in samples of blubber, muscle and other tissues collected from seven killer whales that had washed ashore, and one that had been accidentally caught in a fishing net.

The researchers found concentrations of polychlorinated biphenyls in the whales' blubber that were concerningly high, although not out of line with previous measurements from live animals.

In most cases, Borgå said, "The level exceeded thresholds of risk for negative effects in marine mammals, but we have no direct indication that the high pollutant levels we measured caused the stranding."

The researchers also found several brominated flame retardants at low levels in the blubber of all the whales. The chemicals were present in even smaller concentrations in the muscle samples, suggesting that the chemicals "associate very much with the fatty tissue," Borgå said.

This indicates that brominated flame retardants are distributed in the body in similar ways to polychlorinated biphenyls. It also means that animals that are already in poor condition may be particularly susceptible to the effects of these chemicals.

"If they don't have food available they will use that [fatty tissue] for energy, and then that can result in the remobilization of the contaminants to the bloodstream, and they can circulate around and reach target organs," Borgå said. 

One of the whales that she and her colleagues examined was a 10-day-old calf with milk in its belly. The researchers identified similar concentrations of polychlorinated biphenyls and brominated flame retardants in the newborn's tissues as in the older whales.

"Although we are well aware of maternal transfer and have studied this in other species, it was striking that the levels were so high, and that they even had the new BFRs," Borgå said.

The calf had lower levels of another group of chemicals known as perfluoroalkyl substances, or PFAS, in its tissues than the adults did. Certain PFAS chemicals are no longer manufactured in the U.S. but can still be imported in plastics, textiles and other goods, according to the EPA.

The findings suggest that perfluoroalkyl substances aren't passed as efficiently from mother to offspring as the other contaminants. Previous research has shown that polychlorinated biphenyls and other fat-soluble chemicals can be efficiently transferred to young animals through rich milk, Borgå says.

The study provides the first evidence that brominated flame retardants are passed directly from mothers to nursing calves. However, Borgå points out, the research does have a few limitations. Her team only looked at a handful of whales, did not establish why they had become stranded and didn't investigate how other kinds of pollutants or stressors might have been affecting them.

"We have established that we do find pretty much everything we look for, and some of them in very high concentrations," Borgå said. "So now we ask ourselves, how does this affect different aspects of the killer whales?"

She and her team will compare contaminant levels in killer-whale populations that dine on seals with those that prefer fish. It will also be important to investigate how other disturbances might worsen the effects of the contaminants, she says.

"With a contaminant or pollutant it is hard to treat them as a smoking gun, like, 'This is the hard evidence that it is causing the population decline or threatening the population viability,'" Borgå said.

However, when animals are already under stress because of limited food supplies or habitat destruction, they are more likely to draw on fat reserves that may be housing the contaminants, and less likely to be able to cope with their impacts.

The study, "High levels of legacy and emerging contaminants in killer whales (Orcinus orca) from Norway, 2015 to 2017," published May 19 in Environmental Toxicology and Chemistry, was authored by Clare Andvik and Katrine Borgå, University of Oslo; Eve Jourdain and Richard Karoliussen, Norwegian Orca Survey; and Jan L. Lyche, Norwegian University of Life Sciences.