Adelie penguin

They used to feed mainly on fish, but that changed about 200 years ago.

An excerpt from the article 14 facts about Adelie penguin

Today, krill forms the basis of their diet. They also feed on fish and cephalopods. The change in diet was inflicted by the decline of the Antarctic fur seal and baleen whales population. These animals fed on krill, significantly reducing its numbers significantly.

Now, with less food competition, krill became easier prey for the Adelie penguin. Semi-digested krill is usually what they feed to their hatchlings.