What is the main prey for an orca?

A male great white shark can eat a full-grown male killer whale. Great White Sharks are apex predators. Other sharks can kill young killer whales.

So sorry, but it's the other way around. Orca eat Great White Sharks and, so it appears, solely for their livers.

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Nothing is known to eat Orcas although the animal appears to be rather fragile and prone to starvation.