Waddling over the rocks, legions of penguins hurl themselves into the icy waters of Antarctica, foraging to feed their young.

Like seals and whales, they eat krill, an inch-long (2.5 cm) shrimplike crustacean which forms the basis of the Southern Ocean food chain.

But penguin-watchers said krill are becoming scarcer around the western Antarctic peninsula, under threat from climate change and fishing.