Persistent Chemical Threat May Wipe Out Half the Orca Population

orca pcb 1000

Orcas are under long-term threat from persistent PCB chemicals (Photo: wildestanimal/Shutterstock)

A new report published in the online journal Science Magazine predicts that half of the world's orca population may be wiped out over the next 30 to 50 years, due to the lingering presence in the oceans of a chemical known as PCB.

The highly toxic PCBs (polychlorinated biphenyls) were mass produced in the first half of the 20th century, used in the manufacture of plastics, paints, electrical equipment and sealants. They were banned as early as 1972 by Japan, with the US following suit in 1978 and the UK in 1981. Since 2004, following the 2001 Stockholm Convention on Persistent Organic Pollutants, more than 90 countries have agreed to phase out the manufacture and use of PCBs, according to a report in Science Daily.

The problem with PCBs is that they are very stable chemical compounds, difficult to dispose of and take many years to decompose. As a result, they linger in the environment and have been leeching – much like other chemicals such as insecticides – into the world's water supplies, where they eventually find their way into the oceans.

PCBs are highly soluble in fat, of which orcas possess plenty in the form of blubber. The PCBs are not only toxic to the animal itself, but are also passed on through the fat-rich milk produced by the mother as she nurses her calves. Furthermore, PCBs are known to have a deleterious effect on an animal's ability to reproduce.

'We know that PCBs deform the reproductive organs of animals such as polar bears,' said Professor Rune Dietz of Aarhus University, Denmark, co-author of the research paper. 'It was therefore only natural to examine the impact of PCBs on the scarce populations of killer whales around the world.' Research has shown that PCB levels of 50 milligrams per kilo of tissue may lead to infertility and immune system deficiency. Samples taken from the blubber of orcas during the recent study has recorded levels as high as 1300mg/kg.

orca calf pcb

Female orcas can live up to 70 years of age and reproduce with a single calf every five years (Photo: Martina Sacco/Shutterstock)

The situation is reported as being worst in heavily contaminated waters surrounding the UK, Straits of Gibraltar and Brazil, according to the paper. The population of orcas around the UK is thought to be less than ten individuals, with no calves having been sighted for over 20 years. The diet of the orcas is also thought to play a part in how they are affected by PCBs. Orca populations that feed on fatty seals and large fish such as tuna appear to be in much more danger than populations that feed almost exclusively on smaller fish.

The research into the plight of orcas affected by PCBs is not new. A 2007 study by the American Chemical Society reported that orcas 'may face risks until at least 2063.' The new research has studied PCB levels in 350 individuals around the globe – the largest number of orcas ever studied – thanks to participants from the United States, Canada, England, Greenland, Iceland and Denmark.

'The findings are surprising,' said lead investigator Jean-Pierre Desforges, also from Aarhus University. 'We see that over half of the studied killer whales populations around the globe are severely affected by PCBs.' Ten out of the nineteen orca populations that were studied have been shown to be in 'rapid decline' and, in part due to the longevity of orcas (females can live for up to 70 years) and their very slow reproductive rate (one calf every five years, on average), it is thought that the species 'may disappear entirely' from some of these areas within the next few decades.

'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,' said Paul Jepson of the Zoological Society of London, also a co-author of the latest paper. 'There is therefore an urgent need for further initiatives than those under the Stockholm Convention,' he said, 'but PCBs are so difficult to get rid of that we'll be dealing with the legacy for a long time.'





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