Krill are eaten by whales, penguins, seals, squid, fish and people. Although krill are also eaten by hundreds of other animals, these are their greatest predators. Whales eat the larges amount of krill; the blue whale can eat up to four tons of krill each day. The diet of Adelie penguins mainly consists of krill, although all 17 species of penguin depend on krill to survive.
Smaller species of seals depend on krill to survive, while larger seals, such as the leopard seal, mainly eat other animals, such as penguins and younger seals. The majority of krill are caught in the Arctic, but their numbers have decreased by at least 80 percent since the 1970s. Because of this, krill fishing is now monitored to protect the dwindling population.
The decline in krill is caused by disturbances in their ecosystem, such as global warming and climate change. Increasing Arctic temperatures due to global warming have caused less sea ice to form, resulting in less food for the krill because they eat the algae that grows beneath the sea ice. Another cause of the decline in the krill population was caused by a diatom concentration drop in the Bering Sea that occurred in 1998 during a coccolithophore bloom. Coccolithophores are also part of the krill's diet, and the concentration drop caused them to grow smaller. Kill are unable to feed on the smaller coccolithophore, resulting in less available food for the krill.