The impoverishment of marine and terrestrial ecosystems seems to proceed without respite, plunging us into the sixth mass extinction in the planet's history. In some areas of land, thanks to rewilding, wildlife is returning to reclaim its ancient spaces, but beneath the ocean's surface the situation is desperate. Joining the long list of endangered species are orcas (Orcinus orca): according to a new study, at least half the populations of these magnificent marine mammals are destined to be wiped out by the chemical pollutants present in the oceans.

Why orcas are dying

The study Predicting global killer whale population collapse from PCB pollution, published in Science, reveals that polychlorinated biphenyls, known by the acronym PCB, are poisoning orcas. These are chemical substances produced through industrial processes, synthesised by Monsanto from the beginning of the last century. PCBs, explain the researchers led by Jean-Pierre Desforges of the Department of Bioscience at the Arctic research centre of Aarhus University, accumulate through the various links of the food chain, and orcas, as apex predators, are among the most contaminated animals on the planet.

Young orcas at risk

Orca calves are particularly threatened, as pollutants become concentrated in the mother's milk and thus end up in the diet of newborns. PCBs can also cause severe damage to reproductive organs, cause cancer and compromise the immune system of orcas. The researchers, who analysed 351 specimens, found concentrations of these substances inside the cetaceans up to one hundred times above safe levels.

Where orcas will disappear

The research has highlighted that some orca populations are at greater risk than others; those swimming off the coasts of industrialised countries could disappear in as little as 30–50 years. In particular, the survival of the last orca population in the United Kingdom is hanging by a thread: a recent analysis of a carcass revealed the presence in British waters of one of the highest PCB levels ever recorded. The pods (the family unit in which orcas organise themselves) that inhabit the waters of Gibraltar, Japan, Brazil and the north-east Pacific are also destined to collapse. Orca populations in Norway, Iceland, Canada and the Faroe Islands are, by contrast, far less contaminated, by virtue of their greater distance from the most polluted waters.

The orca apocalypse

"It's like the orca apocalypse," said one of the study's authors, Paul Jepson of the Zoological Society of London. "Even in an uncontaminated environment these animals reproduce very slowly." Orcas take an average of twenty years to reach sexual maturity and eighteen months to wean their young.

Why the oceans are full of PCBs

Until a few decades ago, before they were found to be highly toxic and carcinogenic, PCBs were widely used in the manufacture of, among other things, electrical components, plastics, paints, refrigerants and carbon copy paper. In the United States their production was banned in 1979, yet in many parts of the world PCBs are still in use and will not be completely phased out until 2025. As well as still being employed, PCBs released into the sea in the past continue to damage marine life, as these substances do not break down easily and are highly persistent. The Stockholm Convention on Persistent Organic Pollutants, which entered into force in 2004 to address the issue, has so far failed to fulfil its purpose. "I think the Stockholm Convention is failing," said Jepson. "All we have done in Europe is ban PCBs and then hope they would disappear on their own."

The future of orcas

The future of orcas — whose true intelligence and emotional depth remain unknown to us — looks bleak. Nevertheless, a hope for the species lies in those populations living in less polluted waters, which, one day, if we ever manage to clean up the oceans, could be used to repopulate the waters from which they have disappeared and in which their mysterious and captivating vocalisations might once again resound.