How do superswarms of krill form?

In the ocean, krill live together in swarms, some of them stretching for tens of kilometres. Krill swarms are some of the largest gatherings of life on the planet and this naturally poses some puzzling questions to science: Why are krill living together? How do they find each other? Why are some swarms enormous when others are more moderately sized?

In an effort to shed some light on the mystery, a team of British Antarctic Survey (BAS) researchers headed by Dr Geraint Tarling set out to study the composition and structure of 4525 separate krill swarms in the Scotia Sea. Despite its name, the Scotia Sea is not located close to home for these British scientists – it is a vast expanse of water situated partly in the Southern Ocean and partly in the Atlantic; between Argentina and the Antarctic Peninsula.

Using echo-sounding equipment, the Tarling team tracked down the krill living in this 900,000 km² area and what they found surprised them. According to this new research, krill normally gather into two different types of swarms. The first type is relatively small, typically not exceeding a length of 50 meters and a depth of 4 meters. In this comparatively small type of swarm, the density of krill isn’t very high – you will just find an average of ten krill per cubic meter.

The other type of swarm – dubbed “superswarm” by the researchers – is on the other hand a very densely packed group with up to 100 krill per cubic meter. These dense congregations are the ones that grow really big, often stretching over one kilometre in length and averaging almost 30 meter in depth.

“I was coming at it thinking there might be small swarms tightly packed, and then large swarms that were a bit more diffuse,” says Dr Tarling. “But what we actually found was the opposite. There were small swarms that were quite diffuse and large swarms that were tightly packed.”

This means that a majority of the krill living in the Scotia Sea at any one time will be found within one of just a few enormous superswarms.

“We talking trillions of krill in one aggregation,” explains Dr Tarling. “Ten or 12 swarms could explain 60 or 70% of the biomass in an area the size of the eastern Atlantic. It was astonishing how much biomass could be concentrated into such a small area.”

A fishing flee scooping up a whole swarm of krill may therefore be removing the majority of krill from the Southern Ocean in just one short fishing trip if they happen to target one of the superswarms instead of a small swarm.

How does a superswarm come about?

Although they weren’t able to fully answer this question, Tarling and his colleagues managed to pinpoint certain factors that make superswarms more likely to appear.

“The factors we identified included whether there was more likely to be a lot of food around or not, and when there wasn’t that much food around, they tended to form larger swarms,” says Dr Tarling.

Age is also of importance. The smaller, diffuse swarms typically contained adult krill, while the enormous superswarms consisted of densely packed juvenile individuals.

“Where the animals were less mature, they were more likely to form the larger swarms,” says Dr Tarling, adding that he doesn’t know why.

It might be a question of safety in numbers; it is common among prey animals to live in large groups to reduce the risk of getting eaten, and krill is after all a favoured meal by a long row of sea living creatures.

“All types of swarms are probably to a greater or lesser extent an antipredator response,” Dr Tarling says.

But although living in a swarm reduces the risk of being eaten, it also means having to compete with all the other members of the group for food. Juvenile krill are more buoyant than adults, which mean that they spend less energy swimming. Perhaps this is why adult krill prefers to live in smaller congregations; their negative buoyancy forces them to eat more so they can’t afford living in a huge swarm densely surrounded by competitors.

On the other hand, being in a swarm has been shown to be more energetically efficient than being isolated.

“For a juvenile that wants to grow very quickly, saving energy could be a bonus for them,” says Dr Tarling.

Night-time mystery

As a scientist, you often find yourself in a situation where new findings answer one question but simultaneously create three new ones. One of the new conundrums that Dr Tarling has brought back home from his research trip is the following: Why are superswarms more likely to form at night?

“That is more puzzling for us to explain,” says Dr Tarling. “Up until this point, most polar biologists believed that the swarms dispersed [at night], because that’s the time they feed. When daylight comes they get back into the swarm again for the antipredator benefit. But we found the opposite to that.”

The research has been published in the journal Deep Sea Research I.