Actually, the ORCA is at the top of the ocean food chain. Even Great White Sharks do not attack a young and healthy orca. They may attack one that is already weak and dying or will eat flesh of a dead Orca. But Orcas have no natural enemies at all.
A killer whale can destroy even the largest Great White....It was always thought they avoided each other..until someone caught a battle between the two in San Francisco bay years ago...whale ripped the mouth off the shark