Warmer Waters Cause Overlap Between Adelie and Gentoo Penguins In Antarctic

Warmer waters are encouraging gentoo penguins to relocate to the West Antarctic Peninsula (WAP), and scientists believe they may be competing with native Adelie penguins for the same kinds of food. 

Scientists at the U.S.'s Palmer Station Antarctica, a research center on the WAP, have noticed recent population declines among Adelie penguins: In 1975 there were nearly 15,000 breeding pairs, but today there are only a few thousand remaining. In a recent study researchers from the University of Delaware (UD) investigated whether or not the arrival of gentoo penguins over the last two decades has contributed to the population decline of Adelie penguins by increasing food competition, according to a news release. 

To test whether the two species were "eating out of the same lunch box," researchers fitted penguins with small satellite transmitters and depth recorders to track where they swam and how deep they were diving for food. The tags were attached to a different penguin every three days over a month-long period, specifically during the chick-feeding phase of the breeding cycle when adults fed chicks and parental foraging ranges of both species overlapped. This allowed researchers to collect data on several foraging trips and thousands of dives.

Paired with the satellite tags, an autonomous underwater vehicle (AUV) called REMUS was used to sample the water's temperature, salinity, how much light was in the water (important for visual predators like penguins), and the amount of krill and phytoplankton -- the main food source for both species. 

"Gaining an understanding of where krill are in relation to their food, and where penguins are in relation to their food was an important part of the study," Megan Cimino, lead author of the study and a doctoral candidate in UD's College of Earth, Ocean, and Environment, explained in the release. "Without the REMUS -- which can swim at about the same speed and dive almost as deep as a penguin -- we would not know what's going on in the waters where the penguins are, we would only know that the penguins were there."

In general, researchers found Adélie and gentoo penguins foraged in different areas. However, there was a small area of overlap between the two populations, which caused gentoos to dive nearly 35 percent deeper than Adélie penguins for food. 

Both penguins are able to dive 150 meters below the ocean surface, yet throughout the study Adelies foraged in the upper 50 meters of the water and didn't change their behavior when their foraging territory overlapped with the gentoos. Researchers suggest this may be the penguins' way of limiting competition.

"It was unexpected to see the Adelies foraging much shallower than the gentoos when we know they are capable of deeper dives," Cimino added in the university's release.  

Despite being in close quarters, penguins seemed to be getting enough food, since adults were able to provide for their chicks during feeding cycles. The location of krill, on the other hand, may be the culprit behind Adelies population declines. However, researchers note further research is required to be sure. 

"The novel aspect of the study was that the environmental sampling done by the robot was informed by the location of the penguins. By doing this, we could couple the behavior of these two species with the distribution of their prey and make distinctions that were not possible beforehand," co-author Mark A. Moline, UD's director of the School of Marine Science and Policy, explained.

Furthermore, climate change may be altering the penguins' breeding habits, the marine foraging environment and foraging trip duration. For instance, if the WAP climate continues to warm, sea ice extent and coverage duration will continue to decrease, potentially altering the food web and migratory patterns of Adelie penguins, who regularly leave their breeding colony in winter and stay out at sea. Gentoo penguins, however, are non-migratory and remain at the breeding colony all winter.

"It is cool to see that two species can exist in very close quarters - less than 20 kilometers apart - and have different foraging habitats. But if their winter habitats are changing as well, for better or worse, it will likely have a direct effect on their population and how many penguins come back to breed each season," Cimino concluded.

Their study was recently published in the journal Scientific Reports. 

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