Krill’s role in global climate should inform fishing policy in Antarctica

Krill – small crustaceans eaten by whales, seals and penguins – play a vital role in removing carbon from the atmosphere, according to a new study.

A study on how krill affect the Southern Ocean’s ability to take in carbon from the atmosphere and bury it on the seafloor has revealed the small crustaceans play an outsized role in the process.

Krill are eaten by larger animals in Antarctica such as whales, seals and penguins, but are also fished for food and fishing bait, and for use in aquaculture and dietary supplements.

The Southern Ocean is one of the largest carbon sinks globally, so krill have an important influence on atmospheric carbon levels and therefore the global climate. Dr Emma Cavan

The findings suggest those managing how krill are fished in the Southern Ocean, which surrounds Antarctica, should take krill’s impact on carbon into account, although more research needs to be done to pick apart the process.

The findings are published today in Nature Communications.

Lead author Dr Emma Cavan from the Department of Life Sciences at Imperial completed the study while at the University of Tasmania. She said: “The Southern Ocean is one of the largest carbon sinks globally, so krill have an important influence on atmospheric carbon levels and therefore the global climate.”

The effect of harvesting krill

Krill eat phytoplankton – microscopic plants in the ocean that perform photosynthesis. Phytoplankton take carbon out of the atmosphere during photosynthesis, and are then eaten by krill, which excrete carbon-rich faecal pellets that sink to the bottom of the sea, locking the carbon away.

The pellets are also rich in iron and other nutrients, which help enrich the waters for other organisms.

Antarctic krill can grow up to six centimetres long and weigh around one gram, but they swarm in such vast numbers that their combined contribution to the movement of ocean carbon and other nutrients can be huge. Krill faecal pellets constitute the majority of sinking carbon particles that scientists have identified in both shallow and deep waters in the Southern Ocean.

Management of krill fishing in the Southern Ocean is currently very sustainable, but centres on making sure there are enough krill to support megafauna such as whales, and that the fishing of krill itself is able to continue year after year. However, little attention has been given to assessing the significance of krill to the carbon cycle and ocean chemistry.

Dr Cavan said: “Today the fishery takes less than 0.5 per cent of the available krill and only adults are targeted. But there is no consensus on the effect that harvesting Antarctic krill could have on atmospheric carbon and ocean chemistry.

“Southern Ocean ecosystems and chemical processes are highly complex and poorly understood, and our lack of knowledge about the extent of krill’s ability to affect the carbon cycle is a concern, given that it is the region’s largest fishery.”

Pressing need for further research

The picture is complex, say the authors, as a decline in krill might lead to an increase in phytoplankton, themselves important in transporting carbon to the seafloor. However, as krill also provide important nutrients like iron, their decline might mean instead that phytoplankton and other organisms are unable to thrive.

Dr Cavan added: “Our study has shown there is a pressing need for further research to address these and other questions about the significance of krill, as well as for more accurate estimates of their biomass and distribution.

“This information would inform both our understanding of biogeochemical processes in the ocean and the management of the krill fishing industry.”

Funded by The Pew Charitable Trusts, the research also included scientists from the Australian Antarctic Division, the British Antarctic Survey and research institutes and universities from the UK, Germany, and the US.

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‘The importance of Antarctic krill in biogeochemical cycles’ by Emma Cavan et al. is published in Nature Communications.

Read more about krill and their role in the ocean on the Grantham Institute's blog.

Based on a press release by the University of Tasmania.

Image credits: I. Noyan Yilmaz/Shutterstock and Tarpan/Shutterstock