I've been on whale watch cruises. Small fish congregate in what the captain called "bait balls". The whales come up from underneath, open wide, and let the resulting vacuum suck up all the small fish. So yes, they eat krill, but they also eat all manner of small fish. They separate the fish (or krill) from the seawater with their baleen plates, and allow the seawater to leave their enormous mouths.
