Many krill are filter feeders using their first appendages which have been modified to form very fine combs to filter their food from the water. Most krill are omnivorous eating both phytoplankton and zooplankton. A few species are carnivores eating small zooplankton and fish larvae. Many animals feed on krill, ranging from smaller animals like fish, penguins and
to larger ones like seals, squid and even whales.
Krill play a crucial role in many marine food webs, feeding on phytoplankton and also zooplankton, then eaten by whales, seals, penguins, squid and fish. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of over 500,000,000 tonnes roughly twice that of humans. Of this biomass, over half of it is eaten each year and is replaced by growth and reproduction.
Most krill species migrate large distances up and down he water column every day. (up to 100 meters below the surface) tThis means they are eaten by predators near the surface at night and in deeper waters during the day.
Most krill are found in large schools or swarms which can stretch for kilometres in every direction. For Euphausia superba, there have been reports of swarms of up to 10,000 to 60,000 individuals per cubic metre turning the water red or orange. Swarming is for defense , confusing predators that try to catch a single individual.
Recent studies that show Antarctic krill stocks may have dropped by 80% since the 1970s when ommercial krill fishing began. Scientists believe these declines in part to ice cover loss caused by global warming. The loss of ice ice has led to a reduction in a major food source for them - ice-algae. The fishery is managed through an international body (CCAMLR) which sets limits on the krill catch.