Killer Whale populations threated by pollution

Chemical pollutants in our seas could lead to the disappearance of half of the world’s killer whales in just decades.

Despite the first moves to ban polychlorinated biphenyls (PCBs) 40 years ago, the chemicals remain a deadly threat to the mammals at the top of the food chain.

A new study, involving researchers at the University of St Andrews, published in Science, shows that current concentrations could severely deplete populations of killer whales in the most heavily contaminated areas within 30 to 50 years.

Killer whales (Orcinus orca) are among the mammals with the highest level of PCBs in their fatty tissue (blubber), with values as high as 1300 milligrams per kilogram. Animals with PCB levels as low as 50 milligrams per kilogram can show signs of infertility and immunity problems.

A team of researchers from St Andrews and Aarhus University in Denmark found that the number of killer whales could rapidly decline in ten of the 19 killer whale populations. Killer whales are particularly threatened in heavily contaminated areas near Brazil, the Strait of Gibraltar, the west coast of the UK, and along the east coast of Greenland where they are affected due to high consumption of seals.

Professor Ailsa Hall, Director of the Sea Mammal Research Unit (SMRU) at the University of St Andrews, said: “In these contaminated areas, we rarely observe newborn killer whales.”

Models created by the researchers show that these threatened populations of killer whales may continue to decline or even collapse due to the persistence of PCBs.

Killer whales, whose diet includes seals and large fish such as tuna and sharks, showed high levels of PCBs which accumulate up through the food chain. While killer whales which primarily feed on small-sized fish, such as herring and mackerel, have a significantly lower PCB level and so are at a lower risk of effect.

PCBs have been used around the world since the 1930s. More than one million tonnes were produced and used in, among other things, electrical components and plastics, and have spread around the global oceans.

In 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 the large stocks of the chemicals.

PCBs only slowly decompose in the environment and are passed down from the mother to its offspring through the mother’s fat-rich milk. 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 very slowly degrade.

The research group, which includes participants from the United States, Canada, the UK, Greenland, Iceland and Denmark, reviewed all the existing literature and compared all data with their own most recent results. This provided information about PCB levels in more than 350 individual killer whales around the globe – the largest number of killer whales ever studied.

Applying models, the researchers went on to predict the effects of PCBs on the number of surviving offspring as well as on the immune system and mortality of the killer whale over a period of 100 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.

In the oceans around the Faroe Islands, Iceland, Norway, Alaska and the Antarctic, the prospects are not so gloomy. Here, killer whale populations grow and the models predict that they will continue to do so throughout the next century.