The number of killer whales could be halved in a few decades due to pollutants in the seas, according to new research.

Steps to ban chemicals – known as polychlorinated biphenyls (PCBs) – were first made 40 years ago, but they 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, shows that current concentrations could severely deplete populations of killer whales in the most heavily contaminated areas within 30 to 50 years.

Researchers believe numbers could decline in more than half of orca populations (Audun Rikardsen/PA)

Professor Ailsa Hall, director of the Sea Mammal Research Unit, said: “In these contaminated areas, we rarely observe newborn killer whales.”

Orcas are among the mammals with the highest level of PCBs in their blubber, with values as high as 1,300 milligrams per kilogram.

Animals with levels as low as 50 mg per kg can show signs of infertility and immunity problems.

Researchers from St Andrews and Aarhus University in Denmark found the number of killer whales could rapidly decline in 10 of the 19 populations.

They 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.

In the oceans around the Faroe Islands, Iceland, Norway, Alaska and the Antarctic, the prospects are not so gloomy.

Here, populations grow and forecasts predict they will continue to do so throughout the next century.