According to About.com, penguins are carnivores with piscivorous diets, and their diet consists mainly of fish, crustaceans and cephalopods. Penguins are opportunistic feeders, and their diet is largely based on what is available seasonally and in a particular location.
Some of the most common fish that penguins prey on include anchovies, sardines, silverfish, cod, lantern fish, sprats and pilchards. Krill, shrimp and crabs are regular eaten by penguins, and squid and cuttlefish are occasionally preyed upon as well.
Penguins forage for food in the ocean, using keen underwater vision and excellent swimming abilities to prey on ocean creatures. Changing ocean patterns and weather conditions can cause changes in penguin's diets. Different species hunt in various ways and places. Some penguins prefer shallow water while others dive deep for their prey. Galapagos penguins usually do their hunting within a mile of their nest, whereas emperor penguins have been known to travel 500 miles or more to forage food for weeks at a time.
In captivity, penguins are fed fish, either chopped or whole, and they are given supplements to keep them healthy. Regular feeding patterns mean that they do not experience weight changes that are common with wild penguins due to long periods of time when food is unavailable.