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It’s unknown exactly how many killer whales are left on the planet but estimates place the number around 50,000. Of these, a small pod of eight orcas make their home in the coastal waters of the United Kingdom. Last year, the UK’s resident killer whale pod lost one its few members, when Lulu, an adult female, was found washed ashore after becoming entangled in fishing lines. And now, a newly released postmortem shows that Lulu had some of the highest levels of polychlorinated biphenyls (PCBs) ever recorded, which may have contributed to her death and has researchers worried about the rest of Lulu’s pod.

"Killer whales are incredibly intelligent, they are very nimble, socially aware animals,” explained Dr. Andrew Brownlow, head of the Scottish Marine Animal Stranding Scheme in an interview with the BBC. “It is potentially plausible that there was some effect of the PCBs that was in some way debilitating her so she wasn't strong enough or even aware enough to deal with this entanglement (in fishing line). We very rarely see entanglement in killer whales—actually this is one of the first cases we have documented.”

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PCBs are a group of manmade chemical compounds that were widely used in manufacturing during the twentieth century. Due to their stability, high boiling point and insulating capabilities, PCBs were used in products ranging from plastics and rubbers to paints and electrical equipment. Although they were progressively banned in the 1970s, PCBs don’t easily break down in nature and the compounds have remained in the air, water, and soil near factory sites and in landfills, where they can leach into waterways and finally into the open ocean. Once in the sea, the chemicals build up in the food chain, which means that top predators like killer whales are particularly susceptible to contamination.

Among the slew of PCBs’ potential health hazards are cancer, infertility, and suppression of the immune system. Dr. Brownlow explained the full implications of Lulu's "shocking levels of PCBs”:

The levels of PCB contamination in Lulu were incredibly high, surprisingly so. They were 20 times higher than the safe level that we would expect for cetaceans to be able to manage. That puts her as one of the most contaminated animals on the planet in terms of PCB burden, and does raise serious questions for the long-term survivability of this group (of UK killer whales).

A study published in the journal Nature in January of 2016 analyzed the blubber of more than 1,000 European dolphins and killer whales for traces of PCBs. The report found that PCB contamination in these animals “greatly exceed concentrations at which severe toxic effects are known to occur,” and that “despite regulations and mitigation measures to reduce PCB pollution, their biomagnification in marine food webs continues to cause severe impacts among cetacean top predators in European seas.”

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“Our research underlines the critical need for global policymakers to act quickly and decisively to tackle the lingering toxic legacy of PCBs, before it’s too late for some of our most iconic and important marine predators,” explained Robin Law, one of co-authors of the study, in an interview with The Guardian.

Even before the discovery of Lulu’s PCB levels, researchers feared that given the high presence of PCBs in European waters and the small size of the killer whale pod, Lulu’s pod was no longer viable. “The chance of survival for this group is sadly very slim,” said Dr Lauren Hartny-Mills from the Hebridean Whale and Dolphin Trust, in an interview with The Independent in February. “But it is unclear why they have not been able to produce any calves. It could be the result of human activity such as pollution which can affect reproductive success, but it may also be the result of more natural processes and the female killer whales could now be too old to reproduce.”

Lulu was estimated to be 20 years old when she died last year, well above the age of sexual maturity for female orcas which occurs before the age of ten. However, Lulu’s autopsy showed no evidence that she had ever born a calf or been reproductively active. “Lulu’s apparent infertility is an ominous finding—with no new animals being born, it is now looking increasingly likely that this small group will eventually go extinct,” said Brownlow in an interview with The Guardian. The UK pod is believed to have four adult females; however there haven’t been any calves born to the pod since it was first monitored by scientists over 20 years ago.

“There are still many PCB stockpiles in Europe, and it is absolutely essential that these toxic reserves do not reach the marine environment,” said Brownlow. Lulu’s skeleton is being stored at the National Museums Scotland, where it will be available for scientists to study.