Each Killer whale population has a different niche in their habitat and consume different resources, however all are the apex predators of the place they live.
Dolphins are sometimes victims to sharks or killer whales. However, their most dangerous threat is the man.
The Orca, Killer Whale (Orcinus orca) is the most widely distributed mammal after the humans. It is an apex predator that weighs up to 6 tons and reach up to 26 feet in length.
Large species of sharks, eat small species of dolphins or calves. Also, occasionally killer whales eat small dolphins too.
The only predator that the small species of dolphins or calves have in the ocean are the large species of sharks.