Krill (Euphausia superba) are the small fry of the oceans, the bottom of the food chain. Translucent, with blotches of orange and massive eyes, krill grow to 2cm in length, or smaller than your thumb. Humans have nibbled on them for centuries. The name krill comes from the Norwegian for “small fry of fish”.
Names aside, krill aren’t about the individual. Swimming in massive schools, they are, by mass, the most plentiful species of animal on earth. They are also a crucial element in a healthy ocean ecosystem. Their faeces drop down into the ocean, causing bubbles of carbon dioxide. While moving and eating, they also disturb the water around them, releasing trapped iron and other particles, which, among other things, help to fertilise phytoplankton blooms.
Phytoplankton are what krill feed on; they are too small for other fish to eat. But when tonnes of phytoplankton are eaten by thousands of krill, they get concentrated in one delectable moving buffet for creatures such as whales. Baleen whales have evolved so that they can swim through a school and have thousands of krill filtered by their teeth that then go straight into their stomachs.
At its thickest, a school of krill can be seen from space, with up to 30 000 packed into a square metre of ocean. Surrounded by predators — whales and dolphins under the waves and albatrosses hovering above — the swirling mass becomes a frantic concentration of jittery krill.
But, because there are thought to be 300-trillion individual krill in the oceans, the predators can smash through a school and eat tens of thousands without even making a dent. Numbers are the krills’ key evolutionary advantage.
But new research in the journal Nature Climate Change shows that climate change is dramatically reducing their numbers, and this advantage. The research — Krill Distribution Contracts Southward During Rapid Regional Warming — looked at 90 years of data on krill population levels. This data started out by recording krill numbers, to see how fishing affected this foodstock of the oceans. It has since morphed into researching how warming oceans (the oceans store over two-thirds of the heat trapped by global warming) are affecting krill populations.
In this new research, a team from research institutions such as the British Antarctic Survey found that the number of krill in the ocean had dropped by 70% in just 40 years. A big reason for this is that the ocean is getting warmer. This is forcing krill to move further south, towards the Antarctic, and to live in a much smaller space, because the ocean is less deep there.
Krill schools have, on average, moved 440km further south.
The crustaceans need cold water — preferably with icebergs floating about — and deep ocean to breed and grow from really tiny to small. They aren’t getting either of these conditions as they move further south and the oceans warm. So fewer krill are being born and the population is getting older and larger. The scientists can work this out by looking at the average mass and size of the krill catch — krill are now 6mm longer and have a 75% larger body mass than krill from the 1970s.
As a bonus threat, the world’s oceans are also getting more acidic, which threatens the long-term survival of krill even further.
The scientists warn that these changes to the krill population will “have a series of profound implications”. Less krill means less food for other fish. It also means less krill for people to catch: nearly a quarter of a million tonnes of krill are fished out each year around the Antarctic. That means less food, less fish meal for cattle, and also less oil being sold for skin care (krill oil is rich in Omega-3).
While the oceans continue to warm, with little sign of humans reducing carbon emissions, the researchers say that the way krill is being fished needs to change. Right now, there is an annual tonnage limit on how much is caught. But the limit isn’t linked to research on how krill populations are doing, so it ignores the long-term decline in krill numbers. That means krill will continue to be fished in unsustainable quantities, threatening what’s left of them.