Krill At Risk from Climate Change

All you need to know about Krill, the most important shrimp-like crustacean in the marine food web. We examine its life cycle, diurnal migration, and the threats posed to it by global warming to sea temperature and stocks of phytoplankton, its favorite prey.
Antarctic Krill in deep sea water
Antarctic Krill. Individuals weigh less than one ounce and grow to about one inch in length. Photo: © CC BY-SA 3.0

Krill are small shrimp-like crustaceans found in all the world’s oceans. They are one of the most abundant marine species on the planet and are known for their huge swarms. One species alone – Euphausia superba, based in the waters around Antarctica – has an estimated biomass of 370 million tonnes – more than twice that of the human race. 1

Krill are a ‘keystone species’ in the marine food web and in the Southern Ocean ecosystem. They are a voracious predator of drifting micro-algae – that is, phytoplankton – and in turn are preyed upon by many larger animals, including whales, seals, penguins, squid, and fish. Unfortunately, krill populations have declined by more than three quarters since the 1970s, partly due to global warming. 2 3

Are Krill Classified As Zooplankton?

Zooplankton are defined by their self-propulsion. Plankton can’t swim against the tide, and instead drift with the currents. Adult krill are capable of swimming against the current and therefore are not counted as zooplankton. However, their tiny larvae and eggs are not capable of self-propulsion and thus fall into the zooplankton category. 4

Why Are Krill So Important?

Krill perform a critical role at the bottom of the marine food web, where they serve as a food bridge between microscopic life forms and larger marine animals. It works like this. Phytoplankton, the microscopic plant-like “primary producers” who create food out of sunlight, are eaten by zooplankton, a slightly larger carnivorous organism.

But neither of them is big enough to be considered a food source by much larger creatures. But krill are big enough. In fact, once krill have grown fat on phytoplankton and (to a lesser extent) zooplankton, they become a very convenient nutritious food source for large creatures higher up in the food chain.

In a nutshell, by grazing lots of microbial phytoplankton, krill re-package huge amounts of primary food energy into their bodies, thus making it easily available to larger predators.

For example, in the Antarctic, krill are a major food source for penguins (Adélie, chinstrap, emperor, gentoo, king, macaroni, and rockhopper); seals (crabeater, fur, elephant, leopard, and Weddell), finfish, squid, albatrosses and numerous other species of seabirds. Krill are also the main food source of humpback and blue whales. 5

Krill plays the important role of re-packaging vast amounts of primary production into their own body by grazing micro-size phytoplankton to make them available for marine predators. Because of this role they are called the ‘keystone species’ in the Southern Ocean ecosystem.

Just how much krill predators eat, is a question scientists have been grappling with for some time, it is a complicated jigsaw. Even something as simple as counting how many penguins there are, and how much krill they eat is very complex. The more recent use of satellites to count penguins from space has certainly helped. That said, studies indicate that fish actually eat more krill than penguins and whales put together. But final numbers are based on a lot of estimates – which still adds up to a lot of uncertainties. Knowing how much krill is in the sea is important for fishery management.

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Larval and Adult Krill Feed Especially on Ice Algae in Winter

All krill, but especially young larvae, feed heavily on the algae that accumulates beneath sea ice in winter. In fact, part of the reason that Antarctic krill are so plentiful is that the seas around Antarctica are very rich sources of phytoplankton and algae that grow underneath the ice.

Diagram of Euphausia Superba Anatomy
Diagram of krill anatomy (Euphausia superba). Note the compound eye, krill don’t have great vision but they do have a wider angle viewpoint and are good at detecting fast movement. Photo: © Uwe Kils (CC BY-SA 3.0)

How Big Are Krill? What Do They Look Like?

Krill are crustaceans (part of the crab, lobster, crayfish, shrimp family) and have an external skeleton (exoskeleton) consisting of three segments: head, thorax, and abdomen. The exoskeleton has a bright red tinge and is transparent in most species. Krill have two antennae as well as various pairs of legs connected to the thorax. All species have five pairs of swimming legs, similar to those of a crayfish. They have large black eyes and externally visible gills.

Adults are usually about 1–2 centimetres (0.4–0.8 inches) in length, although some species (e.g. Thysanopoda spinicauda) can grow to between 6 and 15 centimetres (2.4–5.9 inches).

How Long Do krill live?

Polar species (e.g. Euphausia superba) can survive for as long as 6 years. Mid-latitude types (e.g. Euphausia pacifica), live for about two years, while those in the tropics (e.g. Nyctiphanes simplex) normally live for about 6 months.

Main concentrations of krill in the Scotia Sea: Map
Main concentrations of krill in the Scotia Sea – the area to the north of the continent, bounded by Tierra del Fuego, South Georgia Island and the South Sandwich Islands. Image: © NASA SeaWIFS

Where Do Krill Live? Where Are They Fished?

Krill are found in all the oceans, although their greatest numbers are in the Southern Ocean around Antarctica and South Georgia. There are 85 known species of krill, divided into two families. The larger is Euphausiidae, which includes the Antarctic Euphausia superba, the Pacific Euphausia pacifica, and the Arctic Meganyctiphanes norvegica. The other family is a primitive, deep-water species, known as Bentheuphausia ampblyops.

Krill are harvested mostly in the Southern Ocean and (to a lesser extent) in the waters around Japan. As a defence mechanism against predators, they often swarm in dense shoals. These can stretch up to 20 km (12 mi) in length, and contain 10,000-60,000 individuals per cubic meter of water – that’s about 3 million tonnes or 30 trillion crustaceans. These vast swarms are the largest aggregation of animal life on Earth. Not surprisingly, they provoke a feeding frenzy among fish, birds and mammal predators, near the surface.

How is Krill Density Measured and Calculated?

Krill biomass is measured by using hydroacoustic surveys, using standard protocols laid down by the Conservation of Antarctic Marine Living Resources (CAMLR). Ships move along a series of coordinates using echosounders and record the signals reflected from krill in the water. Targeted trawls are also conducted in the area to confirm the signals. Based on this information and a set of mathematical calculations, the echo signals will be translated into numbers or biomass.

Large-scale krill fishing first developed in the late 1960s, led by the Soviet Union. In 1983, the annual catch peaked at around 528,000 tonnes for Antarctic krill alone. Since then, in accordance with the 1993 Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR), catches have been managed and now total around 250,000 tonnes. The market is currently dominated by Norway. 6

Most krill are processed for use as aquaculture and aquarium feedstocks, or in the pharmaceutical industry, but krill oil – rich in omega-3 – is becoming popular for human consumption. Overall, the world catch, which is dominated by Norway, is up by more than 50 percent, over the last decade.

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Diurnal Migration and Fecal Carbon Cycle

In order to avoid predators during daylight, Krill spend much of their time drifting in dark waters about 100-150 meters (300-400 feet) below the surface. At night time, they rise to the near-surface to filter-feed on phytoplankton, notably unicellular algae known as diatoms. After feeding all night, they swim much more sluggishly and so sink back down into the depths. 7

As they sink, they expel feces thus contributing to the Antarctic carbon cycle. 8 When their stomachs are empty, they begin to swim more actively, and head towards the surface. Some species may return to the surface once or twice more during a 24-hour period, even during daylight.

Whales Feeding on Krill
Two humpback whales bubble-net hunting krill. A humpback whale (its partner is just visible below the surface) have encircled a swarm with columns of bubbles blown through their blowholes, creating an optical barrier which the crustaceans are unwilling to swim through. When they are tightly bunched together, the whales will lunge upwards into the swarm to feed. Photo: © Duke Marine Robotics and Remote Sensing Lab (NOAA Permit 14809-03, ACA Permit 2017-034)

What Types of Fish Eat Krill?

An enormous number of species depend upon krill as a diet staple, ranging from smaller fish or penguins, to larger sea animals such as seals and whales. Whales in particular are voracious feeders, with blue whales taking in up to 4 tons of the tiny creatures per day. They include skim feeders such as right whales, bottom feeders such as gray whales, and filter feeders (also called gulp feeders) such as blue whales and humpbacks.

When blue whales are feeding on a swarm, they lunge at it with an open, specially extended mouth, taking in a huge mouthful of water – up to 130 percent of its total weight. The whale then closes its mouth and uses its tongue and throat muscles to force the water back out. Whales typically gorge on krill in order to accumulate blubber for the winter. A single blue whale, for instance, eats up to 8,000 pounds (nearly 4 tonnes) every day for four-months.

How Do Krill Reproduce?

In the Southern Ocean, Krill reproduce during the southern hemisphere’s late summer and early autumn (that is, in January, February and March). Female krill lay up to 10,000 eggs at a time, sometimes several times a season, at intervals lasting days rather than weeks.

One of two methods are used: broadcast spawning or sac-spawning. Female ‘broadcast-spawners’ release their fertilised eggs into the water, allowing them to sink or disperse on their own. In the open ocean, the eggs typically sink for a week, before hatching at a depth of about 700–1000 meters (2,400-3,400 feet). Female ‘sac spawners’ carry their eggs with them, until they hatch. 9 10

Krill Scraping Ice Algae
Antarctic krill scraping ice algae in Antarctica. They scrape algae off the surface of the ice with rake-like setae bristles. The packed ice also provides protection from predators, this safety zone is particularly important for baby krill. Photo: © ATOLL laboratory

Krill Recruitment Harmed By Climate Change

What does the term ‘krill recruitment’ mean and how is it affected by global warming? The term recruitment is commonly used to describe the process of juveniles joining the adult population. This happens when a krill larva survives through the winter to join the adult cohort during the following spring. 11

In order to survive their first winter, they congregate on the underside of sea-ice, where there are lots of algae phytoplankton upon which they can feed. The ice serves as a valuable nursery and habitat for larval krill and represents a key stage in the abundance of the new generation.

Obviously, any reduction of sea-ice is likely to have an immediate impact on krill habitat and population numbers. Based on projections of greenhouse gas emissions and the resulting distribution of CO2 around the Antarctic, studies suggest that 20-80 percent of certain traditional habitats could become unsuitable for spawning by the year 2100. 12 11

Global Warming Hits Phytoplankton & Sea Ice

Krill need two things to thrive. The correct ocean temperature and an abundance of its main food source, phytoplankton. They can’t tolerate temperatures above a certain point, and neither can their main prey, phytoplankton. So ocean warming is bad news on two counts.

Lack of prey also arises when sea ice melts. This is because juvenile krill congregate under the ice for shelter and to eat the phytoplankton who also gather there. So if sea ice disappears, it’s bad news on two counts again.

Unfortunately, according to a recent study, climate change is warming the northern fringes of the Southern Ocean, causing phytoplankton to head south. Actually, it’s a combination of global warming and the influence of the Southern Annular Mode weather cycle that is driving phytoplankton southwards.

By 2100 the study forecasts that waters around South Georgia island in summer (Dec, Jan, Feb) will warm by 1.8°C – a level which is predicted to result in a 50 percent drop in the number of available phytoplankton.

In addition, the study found that the krill’s ice habitat deteriorated in summer and autumn (December to May). This early end to the growth season could have profound consequences for population growth. Female krill need access to plenty of food during the summer (Dec, Jan, Feb) in order to spawn. A decline in summer growth habitat could lead to smaller-sized females and significantly fewer eggs. 13

This combination of rising sea temperatures (which reduce the abundance of phytoplankton) and loss of sea ice (a habitat that sustains both krill and the ice-algae they eat), is exacerbated by overfishing, caused by ‘suction’ harvesting. This fishing technique is conducted by large trawlers which are now able to suck up vast amounts of crustaceans and other small fish. In response, a global campaign has been launched to create a 1.8m sq km fishing-free sanctuary in the Weddell Sea off Antarctica.

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Knock-On Effect on the Sub-Antarctic Ecosystem

If the krill migrate south to follow the phytoplankton, less food will be available on sub-Antarctic islands (e.g. South Georgia) for their predators like penguins, fur seals, and albatrosses who rely heavily on the crustaceans as a food source. In addition, each year, humpback whales migrate to the poles to feed on the vast swarms of summer krill. If the numbers of these crustaceans peak earlier in the season, the whales must adapt accordingly, or risk going hungry.

Another report warns that global warming could reduce krill size by up to 40 percent in some parts of Antarctica’s Scotia Sea causing a dramatic reduction in predator numbers. 14


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  2. Decline in krill threatens Antarctic wildlife, from whales to penguins.[]
  3. Evidence for a decline in the population density of Antarctic krill Euphausia superba Dana, 1850 still stands. A comment on Cox et al.” Simeon L Hill, et al. Journal of Crustacean Biology, Volume 39, Issue 3, May 2019, Pages 316–322 []
  4. Krill Fisheries of the World. UNFAO. []
  5. Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) []
  6. Source: Conservation of Antarctic Marine Living Resources. 2015 []
  7. Satiation gives krill that sinking feeling“. Geraint A. Tarling; Magnus L. Johnson (2006). Current Biology. 16 (3): 83–84. []
  8. The importance of Antarctic krill in biogeochemical cycles.” E. L. Cavan, et al. Nature Communications volume 10, Article number: 4742 (2019). []
  9. Hatching mechanism and delayed hatching of the eggs of three broadcast spawning euphausiid species under laboratory conditions“. J. Gomez-Gutierrez (2002). Journal of Plankton Research. 24 (12) []
  10. Lifespan of Krill []
  11. Projected changes of Antarctic krill habitat by the end of the 21st century.” Andrea Pinones et al. Geophysical Research Letters. 16 August 2016. [][]
  12. Potential Climate Change Effects on the Habitat of Antarctic Krill in the Weddell Quadrant of the Southern Ocean.” Simeon L. Hill, Tony Phillips,Angus Atkinson. Published: August 21, 2013 []
  13. Circumpolar projections of Antarctic krill growth potential.” Devi Veytia, Stuart Corney, Klaus M. Meiners, So Kawaguchi, Eugene J. Murphy & Sophie Bestley. Nature Climate Change volume 10, pages 568–575 (2020) []
  14. Impacts of rising sea temperature on krill increase risks for predators in the Scotia Sea.” Emily S. Klein, Simeon L. Hill,Jefferson T. Hinke,Tony Phillips,George M. Watters. January 31, 2018. []
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