There was an incident in Monterrey Bay, California when orca ganged up on a mother gray whale and her calf for hours on end.

I watched a nature show that had footage of them doing this same thing. in fact, they know when the mother gray whales will be migrating with their calves and they wait for them to come through. The male gray whales go through earlier, so it's only the mother to defend. They finally were able to separate the mother and baby, and killed the baby. Then, insult to death, they only ate the tongue. Apparently the rest of it sinks to the floor and feeds a whole different ecosystem, but still, it was hard to watch.