Scientists have discovered that there are two main types of swarms. The smaller ones may be up to 50 metres long and 4 metres deep, with an average density of 10 krill per cubic metre. These are usually made up of adult krill, which are negatively buoyant. This means they have to swim to keep afloat, which takes up energy and means they need to eat more. But while a swarm offers some protection against predators, it means they are also competing for food, which could be why these adult swarms are not larger or more dense.
The second type of swarms are much bigger and also much denser. These tend to be formed of juvenile krill which are buoyant and so need less energy.
One worrying aspect is the possible impact of overfishing, if most krill in the Southern Ocean are gathered in a few very large superswarms. Fishing fleets can effectively locate these superswarms and by fishing them out could remove the majority of krill living in the ocean, which would have a dramatic effect on other species and the environment.
The next thing scientists may have to look at is why the largest swarms form at night, when the animals would typically be feeding and people had expected them to disperse.
The above photo comes from an article on the BBC’s website.