Can a killer whale eat a stingray?

Marine mammals : Environmental PCB toxin threatens the survival of killer whales

For 25 years, researchers and conservationists from the Hebridean Whale and Dolphin Trust have been observing the killer whales that live off the Atlantic coast of Scotland. In this quarter of a century, not a single calf was born in this group. It currently has eight animals. When, in January 2016, the ninth member of the group, the female orca Lulu, was washed up dead on the beach on the Scottish island of Tiree, an autopsy confirmed a long-held suspicion. The animal was tangled in fishing lines and suffocated. In his body, however, the researchers found high concentrations of the long-banned polychlorinated biphenyls.

Useful, toxic plastic

These substances, better known under the abbreviation PCB, could exterminate half of the world's killer whale populations in the coming decades. This is the conclusion reached by Jean-Pierre Desforges and Rune Dietz from the university in Aarhus, Denmark, Ailsa Hall from the university in St. Andrews, Scotland, and their colleagues in the journal Science (volume 361, page 1373).

"We have known for a long time that these PCBs not only endanger orcas, but also many other large animals in the sea - from dolphins to sharks to tuna", comments the marine biologist Fabian Ritter from the Berlin whale protection organization M.E.E.R. e. V. ("Mammals Encounters Education Research"). In total, well over a million tons of these substances have been produced since 1929, which were used wherever particularly high chemical stability was required: PCBs insulated in transformers and capacitors in the electrical industry, sealed joints in prefabricated buildings and were in demand as hydraulic oils in mining.

Natural accumulation in the whale

In the environment, however, PCBs do not seem to degrade at all. The concentration is actually extremely low, partly because the production of PCBs has been increasingly restricted since the 1970s. However, tiny organisms ingest these substances with their food and hardly give them off. If they are eaten by larger ones, the PCBs continue to accumulate in them until the really big ones such as sharks, tuna or killer whales have millions of times higher concentrations than the water in which they swim. In an orca, researchers then found the record level of 1.3 grams of PCB in one kilogram of fat, in which the substance is mainly concentrated.

Even in much smaller quantities, however, PCBs prove to be very dangerous. In mammals, they not only damage the immune system and thus significantly increase the risk of infections, but also disrupt the endocrine system, delay intellectual development and can render them sterile. Animals that are at the top of the food chain are particularly affected. And that includes not only killer whales and sharks, but also humans. That is why strict limit values ​​for the concentration of PCBs in indoor areas or in food such as fish have long been in force in the European Union. β€œNevertheless, I would advise not to eat predatory fish at the upper end of the food chain, such as tuna, too often,” recommends marine biologist Fabian Ritter.

High-fat, PCB-rich milk

There is another problem with orcas and other toothed whales. "The milk of the females, which is extremely high in fat (up to 30 percent), contains a great deal of PCB, so the mothers pass large amounts of these substances on to their offspring when they suckle," explains Fabian Ritter. But this means that the orcas off the Scottish coast, for example, get into trouble twice: On the one hand, the PCBs severely limit fertility. If an orca calf is born, it takes over many PCBs from its mother, which weaken its immune system. The few calves therefore often die early from infections.

When the researchers led by Rune Dietz examined orca populations worldwide and simulated their development over the next hundred years in a large number of model calculations, very different developments emerged for different marine regions: Scientists found particularly high PCB concentrations in the orcas off the coasts of the highly industrialized regions: off Scotland and near Gibraltar or off Japan and off the Pacific coast in southern Canada. There, in turn, the killer whales, which feed on large animals such as seals, tuna or sharks, are particularly affected, while the orcas, which rely on herrings and other smaller prey, have significantly fewer PCBs in their fat. The model calculations therefore show that killer whales in the Southern Ocean or off Iceland and Norway will not do so badly in future with regard to PCB pollution, while populations like those in Scotland suffer particularly badly and are threatened with extinction.

Deadly Traditions

With the orcas, the situation is aggravated somewhat by the fact that their behavior is very much determined by traditions within the respective population: the young learn from the experienced killer whales, of which the females can live to be 80 or 90 years old, how to prey with sophisticated hunting techniques makes. The groups specialize in different animals: some primarily hunt herrings, others become specialists in salmon, prey on seals or even overpower large sharks or stingrays. The young learn from the old, which also makes them inflexible: "With this strong specialization, the animals cannot simply switch to herrings, for example, when their previous salmon prey is running out," explains Fabian Ritter. So when the fishermen on the west coast of North America strongly decimate the salmon stocks, the orcas, which specialize in salmon, are also starving and are also heavily contaminated by PCBs and other environmental toxins. And because not only orcas, but also other toothed whales have a number of other problems, from overfishing to underwater noise, the situation for the killer whales as a whole is likely to be and will be considerably more difficult than the model calculations on PCB contamination alone suggest.

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