The types of fish penguins eat vary between species. Some species only go for small fish such as anchovies, cod, snailfish, eelpouts, ice fish and mackerel. Size matters because penguins do not have teeth and must swallow their food whole. Fish make up about 40 per cent of many penguin diets, though many tend to forage for krill, squid and crustaceans before pursuing fish.
Young cod is a favourite of the Gentoo penguin. The Gentoo will eat the young cod while they are still small enough to be able to swallow whole.
The Liparidae, commonly called the snailfish, is similar in shape to a tadpole. They school in warmer oceans but are found in oceans throughout the Southern Hemisphere. Scientists have observed the Macaroni penguin hunting snailfish when they school.
Eelpouts are slender fish with an eel shape. They exist worldwide, but they stay in the cooler parts of oceans with warmer surfaces. King penguins love to dive to deeper ocean waters to hunt eelpours.
The Antarctic ice fish belong to the Channichyidae family. Found mostly in Antarctica, this fish can survive in temperatures that would freeze other fish. Emperor and Adelie penguins that dwell mainly in the Antarctic dine on these fish when they are available.
Anchovies are a small saltwater fish that range anywhere from 2cm to 40cm in length. Their largest concentration is in temperate waters. The South American Humboldt penguin and African penguins like to dine amongst the anchovy schools.
Mackerel live mostly in cold and temperate oceans. When they are still young and small, they provide another food source for penguins inhabiting the Antarctic such as the Emperor and Adelie species.
Sardines, also known as pilchards, are another favourite of the Humboldt and African penguin. Hundreds of thousands of sardines will travel in a single school. The penguins can just take their pick. Their habitat is generally warmer temperate oceans rather than the Antarctic, so they are not a food source for the Emperor and Adelie penguins.
Zoologists strive to create a habitation that closely resembles the natural habitat of the penguins in their care. At the Philadelphia Zoo, Humboldt penguins receive five types of fish. Trout and silversides were their favourites as of 2011. At the Denver Zoo, the caretakers give the Humboldt and African penguins their favourite natural foods, including sardines and anchovies.