The future of more than half of the world’s killer whale population is under threat, in part, because of man-made chemicals produced throughout the mid-1900s leaching into the ocean, according to a new study.

Killer whales (Orcinus orca) are toothed whales belonging to the oceanic dolphin family, of which it is the largest member. The most recent estimate places the global population at a minimum of about 50,000 animals. In the northeastern Pacific (from California to the western Aleutian Islands and the Bering Sea), the population is estimated at around 2,500 killer whales.

A new study published in the journal Science on September 28 is painting a very gloomy picture of the future of more than half of the Orca population, and it is because of PCBs, a class of chemicals banned in several countries in the 1970s and 1980s.

Killer whales are considered apex predators, and as such, are at the higher levels of the food chain. This means they are particularly at risk of poisoning from bioaccumulation of toxins, including polychlorinated biphenyls (PCBs). This has raised concerns about the health consequences of current PCB exposures.

“A significant finding is that a single chemical class — PCBs — continues to threaten wildlife decades after their production and sale [has] ceased,” Peter Ross, co-author of the study and vice president of research at the non-profit organization Ocean Wise, told ABC News. “This, together with the global nature of the threat, underscores the vulnerability of killer whales.”

Ross also spoke with Global News and cited the British Columbia killer whale population as being “ground zero” for research since the 1970s, adding, “U.S. scientists can study killer whales in our waters and we know more about ours than any other in the world.”

The study on killer whales

The study used globally available data on PCB concentrations in killer whale tissues. The researchers found that the combined PCB effects on reproduction and immune function among 10 out of the 19 whale populations studied have the potential to put them at high risk of population collapse.

The study notes that this population collapse will likely be seen in those groups that feed at high trophic levels and are the closest to industrialized areas. And this includes not just the pods usually seen along the California coast and up to Alaska.

In the Pacific Northwest, wild salmon stocks, a main resident killer whale food source, have declined dramatically in recent years. In the Puget Sound region, only 75 whales remain with few births over the last few years. On the west coast of Alaska and the Aleutian Islands, seal and sea lion populations have also substantially declined.

What are PCBs and why are they dangerous?

PCBs are an old chemical and from the early 1920s, until their ban in the U.S. in 1979, an estimated 1.5 billion pounds of PCBs were made for things such as microscope oils, electrical insulators, capacitors, and electric appliances such as television sets or refrigerators. PCBs were also sprayed on dirt roads to keep the dust.

The widespread use of this chemical compound allowed it to enter the air, water, and soil during manufacture and use. Wastes from the manufacturing process that contained PCBs were often placed in dumpsites or landfills. And it wasn’t until questions were raised about the unintended impacts of PCBs on human and environmental health that an effort to ban them took place.

PCBs have been shown to cause cancer in animals as well as a number of serious non-cancer health effects in animals, including effects on the immune system, reproductive system, nervous system, endocrine system, and other health effects, according to the US Environmental Protection Agency (EPA).

Even so, it wasn’t until 2004, through the Stockholm Convention, that more than 90 countries have committed themselves to the phase-out and disposal of the large stocks of PCBs.

Marie Noel, a marine mammal toxicologist with the Ocean Wise Pollution Research program, did not work on the study but does look at contaminants in marine life. In talking about the persistence of PCBs in the environment, she said, “Now we have some of the highest PCB levels in marine mammals.”