The krill population is being undermined by salps, simple pouch-like creatures that are eaten by only a few marine animals, but which produce dense blooms which interfere with krill reproduction and kill off their larvae.
Krill look like small prawns, with a pair of swimming legs and large black eyes. They grow up to about 6 centimetres long during their life of between five and 10 years, maturing after two years. There are estimated to be about 500,000 billion individual krill - comprising 500 million tonnes of biomass - and they occur in vast swarms which can seem to turn the water red. They have been harvested since the 1970s by Russia, the Ukraine and Japan, with about 300,000 tonnes caught annually: their uses include feed for fish farms, domestic animals and human food.
By contrast, salp only live about a year, and their numbers can explode if conditions are favourable.
Data collected in the Antarctic suggests that there are fewer krill because salps flourish in years when there is less sea ice, whereas the krill do better in colder years - probably because they can live off algae that forms on the ice. Krill form food for predators including "baleen" whales - the class of whale with a sieve-like mouth for filtering food from the ocean - as well as Adelie penguins, petrels, fulmars, squid and fish.
Over the past 50 years, records show that there have been progressively fewer winters with extensive sea-ice coverage, while average air temperatures have risen. This will help the salp and hurt the krill. That, in turn, will affect the marine food web, and could lead to falling numbers of whales.
At the same time, on King George Island in the Antarctic the number of Adelie penguins - which forage for young krill - has fallen by 30 per cent since the 1970s, and fewer fledglings are surviving.Reuse content