Penguin, Krill Populations in Freefall

By Jessica Marshall Mon Apr 11, 2011

Chinstrap penguins walking on Antarctic ice. Numbers of chinstrap and Adelie penguins are in steep decline.

Numbers of Chinstrap and Adélie penguins in the Antarctic Peninsula region have dropped by more than 50 percent in the last 30 years, driven mainly by dramatic declines in supplies of tiny, shrimp-like krill, their main prey, says a new study.

Krill, meanwhile, have declined by 40 to 80 percent, due primarily to rapidly warming temperatures in the area -- the South Shetland Islands near the tip of the Antarctic Peninsula and nearby sites.

This is one of the fastest-warming places on the planet with winter mean temperatures some 10 degrees Fahrenheit warmer now than in pre-industrial times.

Researchers previously thought that chinstrap penguins would flourish as temperatures warmed because they winter in the open water near ice edges, unlike Adélie penguins, which winter on pack ice. In earlier years, chinstraps did better in warmer winters, while Adélie penguins grew their numbers in cold winters with lots of ice.

But since around 1980, both types of penguins have declined dramatically and now researchers believe that they can point to plummeting populations of krill.

In a paper published today in Proceedings of the National Academy of Sciences, researchers report long-term monitoring of krill and penguins in the South Shetland Islands and data from other sites throughout the Scotia Sea and the West Antarctic Peninsula, the northernmost finger of the Antarctic continent.

"The way we really learned about this story is we saw the huge changes in the penguin populations and went looking for reasons as to why," said study lead author Wayne Trivelpiece, of the National Oceanic and Atmospheric Association's National Marine Fisheries Service in La Jolla, Calif.

The team looked at the data they had amassed over the years and noticed that while penguins used to eat a wide distribution of krill sizes, in recent decades, the krill were a similar size in a given year, as though one cohort of krill were growing up and being eaten, but older and younger krill were absent.

"We went back and looked at environmental data and saw that years with big krill populations were summers that had followed winters with lots of ice," Trivelpiece said. "The ice was suddenly disappearing. I wasn't forming as extensively."

The krill rely on phytoplankton growing in mats on the underside of sea ice for food at critical stages, he said.
"The young krill that are spawned in the Antarctic summer can't survive the winter without food," he said.

"Once they get one year or older they can fast through the winter. We would get these long stretches of two, three or four warm, ice-free winters and there would be no survival of krill from the year before."

"We put together the pieces of the puzzle and said what's driving the penguin declines is a change in climate," he concluded.

The krill loss seems to be hitting the youngest penguins the hardest. Previously about half of penguins returned in their second or third year to breed. Now only about 10 percent survive.

Although neither penguin species is classified as endangered, researchers are concerned about the chinstrap penguins' future.

"Their entire world population is pretty much contained exactly in the region that is warming dramatically," Trivelpiece said. Adélie penguins, meanwhile, live throughout the Antarctic.

The penguins did not always eat krill. Until about 200 years ago, they ate a diet largely of fish. But as whaling and seal hunting wiped out other top predators of the Antarctic that feasted on krill, the penguins moved in on the easy-to-catch plankton -- and their populations swelled.

The recovery of whales and seals puts additional pressure on the krill populations, and so far there is no indication that the penguins have returned to eating significant numbers of fish, mainly because the fish stocks do not seem to have recovered from heavy depletion in the middle of the 20th century, Trivelpiece said.
The krill losses have more than erased the gains penguins made when they switched to krill, he said.

"The data are now indicating that the warming trends have added a monkey wrench to the whole system and things are now changing more rapidly than they have historically and through geologic time," said Steven Emslie of the University of North Carolina Wilmington.

"The penguins of the peninsula have been the most telling as far as the effect of warming and what may happen in the rest of the Antarctic if warming continues," he said.

"Everything comes back to the decline in sea ice," added Hugh Ducklow of the Marine Biological Laboratory in Woods Hole, Mass.

Although researchers previously predicted Adélie penguins would suffer without the pack ice they needed for winter, the new paper suggests what's more important is krill.

"But of course, the decline of krill basically comes back to the decline of sea ice as well."

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