Iconic Antarctic Seabirds Suffer Population Decline and Reduced Breeding Success, Researchers Say

Over the last 50 years, southern giant petrel seabirds living on the Antarctic island of Signy have experienced some ups and downs. Since 1996, the birds have struggled to breed. Despite some success from the mid-1980s to the mid-2000s, overall populations have declined by fifty percent in the last ten years, report researchers from the British Antarctic Survey (BAS).

"In the fifty year study we found that although the population fluctuated periodically over this time, in the last 10 years, both numbers and breeding success have declined," Mike Dunn, lead author from BAS, explained in a news release. "Since the South Orkney Islands, of which Signy is part, represent nearly 10 percent of the global population of this species, continuation of such a decline both at Signy and elsewhere in this island group would be of conservation concern."

Southern giant petrels are large brown or white nesting seabirds with wingspans of over two meters long - a characteristic that allows for long-distance ocean travel. These birds prefer to prey on crustaceans, such as Antarctic krill, but they will also eat squid and fish while at sea and penguin or seal remains on land. Generally speaking, most birds start breeding between the ages of six and ten and produce one chick per year. Southern giant petrels breed on the Antarctic continent, Antarctic Peninsula and on subantarctic islands and tend to return to the same nesting sites each year. After laying their eggs, parental birds provide full time care to protect their young from the potentially fatal cold.

BAS researchers have been studying giant petrels breeding at Signy Island since 1968, although a program to monitor how many chicks fledged each season was set up only 20 years ago. This allowed scientists to keep a closer eye on these iconic birds and revealed that their overall breeding success had declined from 60 percent in 1996 to 40 percent in 2015. Furthermore, since 2005 populations have declined from 5800 individuals to merely 2600.

But why?

"The results are surprising because this species seemed to be doing well on Signy. We really don't know what's causing this decline. It could be a reduction in sea ice or other factors affecting food availability and we don't know if it's affecting the species regionally or more widely," Dr. Richard Phillips, BAS seabird ecologist and co-author, said. "This study illustrates how the conservation situation of a species can change in only a few years. Formerly considered to be in the IUCN Red List category of Vulnerable, the southern giant petrel was re-categorized as 'Least Concern' in the late 2000s because positive trends at some sites counteracted negative trends elsewhere. However, if breeding numbers and success at Signy do not improve, and populations elsewhere in the Antarctic have continued to decline, then the IUCN status of this species will need to be re-assessed."

Similar inconsistencies have been found in populations of southern giant petrels living on other islands in the Southern Ocean. Variations in breeding pair numbers can ultimately impact breeding success, simply because when there are fewer individuals to mate there are less offspring produced to revive struggling populations.

Their study was recently published in the journal Polar Biology.

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