Half the world’s killer whales – including the remaining few in British coastal waters – could be wiped out by a chemical first banned more than 40 years, warns new research.
Almost half a century after the first moves were taken to ban the use of PCBs, the chemical pollutants remain a deadly threat to animals at the top of the food chain.
The findings, published in the journal Science, show that current concentrations of PCBs cold lead to the disappearance of half of the world’s killer whales from the most heavily contaminated areas within just 30 to 50 years.
Killer whales, also known as orcas, form the last link in a long food chain and are among the mammals with the highest level of PCBs, or polychlorinated biphenyls, in their tissue.
Researchers have measured values as high as 1,300 milligrams per kilo in the fatty tissue, or blubber, of killer whales.
For comparison, a large number of studies show that animals with PCB levels as low as 50 milligrams per kilo of tissue may show signs of infertility and severe impacts on the immune system.
They found that the number of killer whales is rapidly declining in 10 of the 19 populations investigated.
The researchers said that killer whales are particularly threatened in heavily contaminated areas such as the waters near Brazil, the Strait of Gibraltar and around the UK.
Around the British Isles, the researchers estimate that the remaining population counts less than 10 killer whales.
Also along the east coast of Greenland, killer whales are effected due to the high consumption of sea mammals such as seals.
The killer whale is one of the most widespread mammals on Earth and is found in all of the world’s oceans.
But today, only the populations living in the least polluted areas consist of a large number of individuals.
Overfishing and man-made noise may also affect the health of the animals, but researchers say that PCBs particularly can have a dramatic effect on their reproduction and immune system.
Killer whales whose diet includes seals and large fish such as tuna and sharks critically accumulate PCBs and other pollutants stored at successive levels of the food chain.
But those that primarily feed on small-sized fish, such as herring and mackerel, have a significantly lower content of PCBs and so are at lower risk.
PCBs have been used around the world since the 1930s.
More than one million tonnes of PCBs were produced and used in items such as electrical components and plastics.
Together with DDT and other organic pesticides, PCBs have spread around the global oceans.
Through the 1970s and 1980s, PCBs were banned in several countries and in 2004, through the Stockholm Convention, more than 90 countries committed themselves to phase out and dispose of large stocks of PCBs.
But PCBs are only slowly decomposed in the environment.
Moreover, PCBs are passed down from the mother orca to its offspring through the mother’s fat-rich milk.
Researchers say that this means that the hazardous substances remain in the bodies of the animals, instead of being released into the environment where they eventually deposit or degrade.
Study co-author Professor Rune Dietz, of Aarhus University in Denmark, said: “We know that PCBs deform the reproductive organs of animals such as polar bears.
“It was therefore only natural to examine the impact of PCBs on the scarce populations of killer whales around the world.”
The research group, which also includes scientists from Britain, the United States, Canada, Greenland and Iceland, reviewed all the existing literature and compared all data with their own most recent results.
That provided information about PCB levels in more than 350 individual killer whales around the globe – the largest number ever studied.
Applying models, the researchers predicted the effects of PCBs on the number of offspring as well as on the immune system and mortality of the killer whale over a period of 100 years.
Dr Jean-Pierre Desforges, of Aarhus University, said: “The findings are surprising.
“We see that over half of the studied killer whales populations around the globe are severely affected by PCBs.”
He said the effects result in fewer and fewer animals over time in killer whale populations, with the situation worst in the oceans around Brazil, the Strait of Gibraltar, the north east Pacific and around the UK.
Dr Desforges said the models show that killer whale numbers in those areas have virtually been halved during the half century where PCBs have been present.
Professor Ailsa Hall, who together with Bernie McConnell developed the models used by Sea Mammal Research Unit in Scotland, said: “In these areas, we rarely observe newborn killer whales.”
Dr Desforges added: “As the effects have been recognised for more than 50 years, it is frightening to see that the models predict a high risk of population collapse in these areas within a period of 30 to 40 years.”
A female killer whale may live for 60 to 70 years, and although the world took its first steps to phase out PCBs more than 40 years ago, killer whales still have high levels of PCBs in their bodies.
Study co-author Dr Paul Jepson, of the Zoological Society of London, added: “This suggests that the efforts have not been effective enough to avoid the accumulation of PCBs in high trophic level species that live as long as the killer whale does.
“There is therefore an urgent need for further initiatives than those under the Stockholm Convention.”
But the researchers said that in the seas around the Faroe Islands, Iceland, Norway, Alaska and the Antarctic, the prospects are not so gloomy as killer whale populations are growing.
And the models predict that they will continue to do so throughout the next century.
