FOR conservationists it is the ultimate dilemma. Marine biologists are discussing a cull of killer whales because the predators are destroying other endangered sea mammals.

They are concerned by new research linking a huge population slump in species such as sea otters, Steller’s sea lions and harbour seals to the changed feeding habits of some killer whales, or orcas, as they are also known. The main prey of these orcas has traditionally been great whales such as grey whales and sperm whales, but hunting by humans has cut the numbers of those species to far below their natural level.

Professor James Estes, an expert in the population dynamics of sea mammals at the University of California, Santa Cruz, believes that, faced with a shortage of food, some groups of Pacific orcas have altered their diets. Each killer whale is capable of eating several otters or seals a day.

Estes, whose research will be published in the Philosophical Transactions journal of the Royal Society, said: “Killer whales are the world’s largest carnivores. They are fast and effective killers and they need a lot of food to keep going.

“They live in groups and I believe some of those groups are now preying on small marine mammals like sea otters and sea lions.”

His views are backed by many eyewitness reports of killer whales attacking smaller mammals, some of which can be swallowed whole. Other experts have been prompted to consider culling killer whales.

Callum Roberts, professor of marine biology at York University and author of The Unnatural History of the Sea, said: “Steller’s sea lions and sea otters are high on the conservation agenda, and seeing past success in rebuilding populations slipping away is agonising for those who have battled to bring them back from the edge of extinction. The possibility that killer whales are responsible creates the ultimate conservation dilemma: should we kill the killers?”

Estes is very cautious about such ideas, partly because he believes the science needs to be more certain and because pred-ator control programmes are seldom successful. A cull would also cause an outcry among conservationists.

“Culling killer whales might solve the problem but it would have a huge political dimension. A lot of people involved in conservation are nervous about this issue,” he said.

Estes, however, remains deeply concerned that the current population levels of sea otters, Steller’s sea lions and some seal species is so low that they are at risk of extinction. Although exact numbers are unknown, the population of these species has dwindled to about 10,000-20,000, a fraction of their natural levels.

Such animals were intensively hunted for their fur, oil, meat and blubber from the 18th century onwards but their numbers began to recover after they were all given protection in the early 20th century.

Last century, however, industrial whaling wiped out most of the great whale populations in the Pacific, Atlantic and Southern Ocean. “It seems likely that killer whales expanded their diets to include a higher percentage of sea otters and other sea mammals after the reduction in great whale numbers caused by postworld war two industrial whaling,” said Estes.

Killer whales are found in oceans around the world. Their global population is thought to be 100,000-200,000 but around 90% of these live mainly on fish. It is the remaining 10%, a separate subtype, that prey on whales and other sea mammals.

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