What is killing killer whales?

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Killer whales, the ocean’s fiercest predators, are easily recognisable by their black and white markings.

But their future seems less clearly defined.

Marine experts are concerned about an invisible threat to the animals that has been building in our seas since World War II.

That was when industries began extensively using chemical flame retardants, such as PCBs.

These chemicals were later found to harm human health and the environment, and governments around the world banned their use in the 1970s.

But their legacy lives on in the world’s seas and oceans, say biologists, posing a modern threat to animals such as killer whales, also known as orcas.

Small population

Ingrid Visser grew up watching killer whales, the largest members of the dolphin family, from the shores of her native New Zealand. She has dedicated her life to knowing more about the animals.

The island nation’s orca population is made up of fewer than 200 individuals and as such is listed as threatened.

“They hunt in New Zealand waters in the shallows for the rays and in deeper waters for the sharks,” says Dr Visser.

“[These] orca are unique as they are the only population that has so far been recognised to specialise in hunting for rays and sharks.”

But according to Dr Visser’s studies, this diet could be the reason the population is not growing.

As large mammals, killer whales consume a large amount of prey.

But this position at the top of the food chain, as “apex predators”, makes them particularly vulnerable to changes in their prey.

That is because orca feed on fish that in turn eat polluted prey or absorb pollution from the water. So the orca ingest all of the pollution in the chain, in a process called “bioaccumulation”.

Dr Visser says her studies of the bodies of stranded orca and the sharks and rays they feed on have confirmed this fear.

“Their prey is definitely polluted – we are seeing spikes in the same chemicals as are seen in the orca,” she says.

New Zealand orca are not the only ones living with pollution, according to Alex Rogers, Professor in Conservation Biology at the University of Oxford, UK.

“Studies have identified high levels of flame retardant chemicals in orca particularly from the Northern Hemisphere, for example from the north Pacific, particularly off Canada and the Arctic,” he says.

“These chemicals have also been found at high concentrations in orca from the Southern Hemisphere.”

PCBs (polychlorinated biphenyls) were banned globally from the 1970s.

In recent years, the European Union has also banned the use of PBDEs (polybrominated diphenyl ethers) in foam for furniture and electrical devices due to their potential toxicity.

“The two main groups of flame retardant chemicals, PCBs and PBDEs have a range of effects on animals including interference with thyroid function and vitamin A metabolism, negative effects on neurological and reproductive development and impacts on immune function,” says Prof Rogers.

Persistent threat

But despite actions to limit use of these chemicals, also referred to as persistent organic pollutants (POPs), marine experts suggest the damage has already been done.

“PCBs are not water soluble, they only dissolve and accumulate in fatty tissue,” says Dr Paul Jepson from the Zoological Society of London.

Dr Jepson says this fat solubility is a considerable issue for female cetaceans such as killer whales who feed their young for up to a year on high fat milk to kick-start their development.

“You get this huge maternal transfer. It’s been calculated that in whales and dolphins about ninety percent or more of the mother’s body burden of PCB can be offloaded, particularly to the first calf,” he tells BBC Nature.