The babies are separated from their mothers and held underwater until they drown. Then, after a good feed, the killer whales store the carcases in shallow water and come back days later to eat again.
These hunting methods are unique and carried out by a group of perhaps 150 killer whales, with the result of nearly 1/3 of eastern Pacific grey whale calves being eaten each year, from a total grey whale population of around 20,000.Tweet
Sharks and brown bears benefit by eating the scraps underwater or the carcases that wash up on the shore. One baby whale carcas quickly attracted a group of 19 brown bears. In this way, healthy grey whale populations provide an important food resource for ocean and land animals.
All of this happens around Unimak Island in Alaska, a group of killer whales specialize in grey whale calves, even though their prey is close to their own size and their protective mothers are much larger than the killer whales.
Here's an unrelated video of a killer whale attack on a baby grey whale. Yikes.