One of the UK's last killer whales was contaminated with "shocking" levels of a toxic chemical, scientists say.
The animal, called Lulu, was found dead on the Isle of Tiree in Scotland last year after becoming entangled in fishing lines.
But tests now reveal her body contained among the highest levels of polychlorinated biphenyls, or PCBs, ever recorded.
The chemicals were banned from the 1970s but are still in the environment.
Researchers now fear that other animals in Lulu's pod also have similarly high levels of contamination. The group, which is found off the west coast of Scotland, is thought to consist of just eight animals.
Dr Andrew Brownlow, head of the Scottish Marine Animal Stranding Scheme and veterinary pathologist at Scotland's Rural College (SRUC), told BBC News that Lulu had "shocking levels of PCBs".
He said: "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)."
PCBs were used widely in industry during the last century.
The manmade chemicals are extremely stable, resistant to extreme temperatures and pressures, and have insulating properties. Because of this they were used in everything from plastics to paints and electrical equipment.
But after concerns about the toxicity to humans and animals was raised, a series of bans were put in place around the world from the 1970s onwards.
However the chemicals take a long time to break down and have lingered in the environment, particularly in landfill sites where they can leach into waterways and on into the sea.
They then build up in the marine food chain, which means top predators such as killer whales are particularly affected. Dolphins, porpoises are also susceptible.
Levels of PCBs are measured in milligrams per kilogram of lipids (fatty acids) in an animal's body.
Dr Brownlow said: "The threshold where we think that there is some form of physiological effect caused by PCBs is around 20-40mg/kg stored within the tissues.
"Lulu had a level of PCBs of 957mg/kg - and this has put her as one of the most contaminated individuals we have ever looked at."
Scientists believe Lulu's age, estimated to be at least 20, may be one reason that the levels of PCBs were so high, because they had built up over the years.
The chemicals have a range of effects. There is evidence that they can impair the immune system. They also affect reproduction, preventing killer whales from bearing young.
"That's certainly what we found in the case of Lulu," explained Dr Brownlow.
"Having examined her ovaries, there was no evidence that she had ever been reproductively active or had ever had a calf."
The chemicals can also affect the brain. Scientists believe the contamination could have been implicated in Lulu's death.
"Killer whales are incredibly intelligent, they are very nimble, socially aware animals. [Lulu] would have spent most of her probably very long life existing around the waters of the West Coast.
"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."
PCBs are a global problem, but a recent study revealed that European waters are a hotspot because of the level of the chemicals once used.
It is estimated that there is a million tonnes of PCB-contaminated material waiting to be disposed in Europe.
But getting rid of them is expensive and difficult - they need to be incinerated at more than 1,000C to be destroyed.
Prof Ian Boyd, chief scientific adviser at the Department of Environment, Food and Rural Affairs (Defra), said that the issue was very concerning but also complicated.
He said: "The records show PCBs have been declining in concentration in the marine environment, so the regulation we have in place is working.
"It's just they take a very long time to disappear. Overall I think we are going in the right direction, but it is going to take many more years to get to a point where they are going to disappear entirely."
He added: "Lulu was fairly old, so she will have accumulated [PCBs] over her lifetime and that's the reason she had such high levels. It's a legacy she'd carried from her early years, probably."
But some scientists think more should be done. Dr Paul Jepson from the Institute of Zoology at the Zoological Society of London said PCBs were not an intractable problem in Europe.
"PCB levels in the United States have slowly declined in humans and other biota such as fish for many years now, and the overall PCB mitigation is generally considered to be successful in the US.
"This is partly related to numerous US Environmental Protection Agency (EPA) Superfund sites, which the EPA is actively working to decontaminate. We urgently need a similar approach in Europe".
Killer whales - and other cetaceans - around the world are being hit by PCB contamination. But in the UK, with just eight remaining resident killer whales, the future looks very bleak.
Scientists have not seen any calves born in the 25 years they have been studying these animals, and it looks almost certain that they will eventually vanish from the UK's waters.
Lulu's skeleton is now stored at the National Museums Scotland collection centre, which has one of the largest whale collections in the world.
Principal curator of vertebrates' Dr Andrew Kitchener said the whales remains would be available for scientists to study.
"What these collections are here for is so we can use them for the benefit of living animals in the wild today and in the future. They do have a value for living populations."
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