Big boiled shrimps close up. Seafood concept. Food photography

Tiny shrimp-like crustaceans that are at the heart of the Southern Ocean food chain are in decline, and that could threaten other Antarctic animal life, scientists warn.

The new research, published in the journal Nature, shows that Antarctic krill numbers have dropped by about 80 per cent since the 1970s. Behind the decline in krill, scientists suggest, is a dramatic decline in sea-ice.

This is the first time that we have understood the full scale of this decline,” lead author Dr Angus Atkinson from British Antarctic Survey, says.

Global warming may be the culprit behind the decline in sea ice.

The ocean around the Antarctic Peninsula, a key breeding ground for the krill, is warming relatively quickly as the planet heats up. In the past 50 years, the Peninsula has warmed by 2.5°C, about 5 times faster than the global mean rate. That has led to a striking decrease in sea-ice in the winter months.

“We don’t fully understand how the loss of sea-ice here is connected to the warming, but we believe that it could be behind the decline in krill,” says Atkinson.

The link could be that the sea ice provides food and shelter from predators for krill larvae. “Krill feed on the algae found under the surface of the sea-ice, which acts as a kind of ‘nursery’,” says Atkinson. The crustaceans grow up to a length of 6 centimeter’s and can live for 5 to 6 years.

And in the meantime, transparent tube-like creatures called salps, which tend to live in warmer, less food-rich areas, are on the increase in the Southern Ocean.

However, the decline in krill may have a negative impact on other parts of the food chain as krill are one of the most important animals in the Southern Ocean. Krill feed on phytoplankton and are in turn eaten by a wide range of animals including fish, penguins, seals and whales.

The Nature article noted that it is not just the magnitude of change in the level of krill, but that is also the wide spread of that change through the Southern Ocean. “Penguins, albatrosses, seals and whales have wide foraging ranges but are prone to krill shortage,” the authors write. “Thus, the wide extent of our indicated change in krill density — not just its magnitude — is important.”

That may have a commercial impact — thousands of tourists are also attracted to Antarctica to enjoy the spectacular wildlife, most of which feed on krill and their decline may also impact on the other fisheries in the Southern Ocean.

There has been previous speculation that krill stocks might have declined based on localized surveys spanning a shorter duration than the current study. The survey gives for the first time a large-scale view of change across the Southern Ocean. Nine countries working in Antarctica pooled their data spanning 40 Antarctic summers, over the period 1926 to 2003.

But it is not the first time the food chain in the Antarctic has been out of balance. Early last century, the over-exploitation of whales preceded a rapid increase in krill predators such as fur seals. Now whales are back to pre-exploitation levels but they face a new challenge of having fewer krill to eat.

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