Penguin populations have plunged by as much as 50 percent during the past three decades in the West Antarctic Peninsula and Scotia Sea, scientists report.

The problem appears to be a shortage of krill, the seabirds' primary fare, caused by rising regional air temperatures and rebounding populations of hungry whales.

Fisheries biologist Wayne Z. Trivelpiece of the National Marine Fisheries Service in La Jolla, California, has been monitoring colonies of chinstrap and Adélie penguins since the mid-1970s.

Because Trivelpiece regularly bands and monitors individual penguins, he's been able to uncover a key factor in the collapse: Far fewer young penguins are surviving their first winter on their own, because they're having a hard time finding krill.

"It's gone from about half of the chicks surviving in the 1970s and mid-1980s to only about one tenth now," Trivelpiece said.

"And we see from direct measurements of krill that there's about 80 percent less out here than there was just 20 years ago. So the probability of young penguins finding it often enough to survive during those first months of independence is much reduced."

Penguins at Risk as Krill Vanish

Krill are tiny, shrimplike animals that live in enormous numbers and represent a large part of the Antarctic food web. Like flocks of herbivores on land, krill feed on single-celled plants called phytoplankton and are in turn gobbled up by many marine predators, including penguins.

The local krill collapse is probably due to a pair of factors, Trivelpiece said.

One is regional air temperatures, which are some 10 degrees Fahrenheit (5 or 6 degrees Celsius) higher than they were in the 1940s and 1950s. Those temperatures drive how much ice forms at the sea surface.

"If the ice no longer forms, phytoplankton [growing on the bottom of] that sea ice aren't available to provide a winter food source for the young krill that spawned the summer before," Trivelpiece said. "Without that food, the young krill don't survive."

The second krill killer is actually a conservation success story—rebounding populations of baleen whales, such as humpbacks.

"From what information is available, stocks of krill-eating whales are beginning to return, and their numbers are growing," Trivelpiece said. (Related: "Whale Hunting to Continue in Antarctic Sanctuary.")

Nineteenth- and 20th-century whale hunts, which severely impacted populations of the giant marine mammals, appear to have ushered in a penguin heyday.

"We don't have good data prior to the 1930s, but it appears that at least the 1930s to the 1970s were a real boom time for penguins, primarily because of the removal of competition in the form of whales."

"Population data from that period is largely anecdotal and provided by the rough counts of British Antarctic workers. But even if you're counting by the seat of your pants, the difference between 100,000 penguins in the 1930s and 500,000 or 600,000 in the 1970s is enormous."

Marine ornithologist Steve Emslie also provided valuable evidence of the boom with his studies of historic penguin colonies. Chemical analyses of old tissue sources, such as eggshells, found that Adélie penguins actually had been fish-eaters before whale numbers dropped.

"Only in the last hundred years or so did krill come into their diet, when the whales were taken out of the system and there was a krill surplus," Trivelpiece said.

Can Penguins Survive Without Krill?

With krill now dwindling, the previous shift in penguin behavior begs a question: Can the birds simply switch back to eating fish?

"From everything we've seen over a 30-year period, while krill has declined 80 percent, we haven't seen an increase of fish in [penguin] diets," Trivelpiece said.

"But the fish stocks have also been heavily fished out by Russian trawlers, so we don't even know how much of that prey is available to them at this point."