A new study shows that penguin populations have declined by as much as 50 percent during the past three decades in the West Antarctic Peninsula and Scotia Sea.
The recent decline seems to be due to a shortage of krill, the primary meal of these penguins. The decline in krill is a result of rising regional air temperatures and rebounding populations of whales who also depend krill as a primary food source.
Wayne Z. Trivelpiece, fisheries biologist of the National Marine Fisheries Service in La Jolla, California, has been banding and monitoring colonies of chinstrap and Adélie penguins since the mid-1970s. He has been able to discover a key factor in the population collapse: fewer and fewer young penguins are surviving their first independent winter because they can’t find enough 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.”
Krill represent an enormous part of the Antarctic food web, feeding the smallest to the largest creatures. The krill themselves feed on phytoplankton and their decline is due to both temperature and a rebounding whale population.
Regional air temperatures are about 5-6 degrees Celcius higher than they were in the 1940s and 1950s. The air temperature directly controls how much sea ice forms which changes the population of phytoplankon and the population of krill.
“If the ice no longer forms, phytoplankton in that sea ice aren’t available to provide a winter food source for the young krill that spawned the summer before,” said Trivelpiece. “Without that food, the young krill don’t survive.”
The second reason, while bad for krill, is actually a success story. According to Trivelpiece, “stocks of krill-eating whales are beginning to return, and their numbers are growing.”
“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.
Now the question remains: with whale populations rebounding, will penguins be able to switch back to a diet of 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 their 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.”
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC