Recent years have seen significant increases in Antarctic penguin populations. Some have argued that this is the result of reduced competition from whales who, like penguins, also feed on krill. Palmer ornithologist Bill Fraser and his research team have challenged this assumption, suggesting that "penguin populations are increasing as a result of a loss of sea ice due to environmental warming." Supporting data has come from a winter expedition to the Scotia and Weddell Seas, recent satellite images of ocean ice cover, the analysis of long-term surface temperature records and penguin demographics.
Krill are the main diet of penguins. In January 1995, Antarctic krill were scarce in the area studied by the LTER team. The main prey item available that year was Thyanoessa, a smaller euphausiid, which provided the penguins with much less energy per bite than the krill they favored. So that year, the Adelie penguins had very long foraging times (the hours away from the nest foraging for food for their chicks.) At one point both parents were observed simultaneously leaving the chicks, a previously-unknown behavior suggesting a high level of desperation.
Here are some real world "Foraging Facts":
Prey items are picked up individually, so getting a large item gives more return for effort expended than a small one if the items are of equal value per gram.
But, size is not everything. In other years (1994, for example) Palmer researchers were finding both Antarctic krill and salps. Salps are larger than krill (up to 120 mm in size, with krill around 40mm), but they have a much higher percentage of water and so are not as economical as food. It's like filling your stomach up with water instead of a thick vegetable and chicken soup!
Penguins are restricted in the amount of time they can spend on each foraging trip because they need to get back to their chicks. Each foraging trip must also include time for swimming to the prey, time to feed for themselves, and time to fill up their stomachs with food to bring back to the chicks in the rookery.