South Georgia cormorants continue to disappear | Polarjournal
Juvenile South Georgia cormorants are becoming an increasingly rare sight on the sub-Antarctic island of the same name, according to a study by the British Antarctic Survey BAS. More than half of the nests on Bird Island are now unoccupied compared to 30 years ago. Picture: Michael Wenger

In our country cormorants are not welcome birds. Their populations have increased and they are said to cause massive damage to fisheries. The situation on the sub-Antarctic islands of South Georgia and Signy Island is quite different. South Georgia’s resident cormorants have been dwindling for about 30 years.

A research team from the British Antarctic Survey (BAS) and Amanda Lynnes from the International Association of Antarctic Tour Operators (IAATO) came to the conclusion in a newly published study that appeared in the journal Polar Biology. According to the team, the number of breeding pairs on Signy Island, one of the South Orkneys, decreased by 40.9 percent in 43 years and on Bird Island, which belongs to South Georgia, by as much as 58.3 percent in 33 years. The decline of this bird species, which is endemic to this area of the sub-Antarctic, is causing alarm bells to ring among researchers. Lead author of the study, Dr Mike Dunn of BAS explains, “Identifying these declining population trends is essential for the effective future conservation of this species. The majority of the world population of South Georgia shags are located in the South Orkney islands and South Georgia, including Bird Island. Consequently, continuation of the declines shown in this study will be of important conservation concern.”

South Georgia cormorants are monogamous birds, that can become relatively old with approximately 11 years of average life-expectancy. Males and females are similar in appearance, but different in size. They belong to the blue-eyed cormorants, which all have a blue ring around their eyes. Antarctic cormorants are also among them. However, these live further south in the area of the Antarctic Peninsula and the surrounding islands. Picture: Michael Wenger

The total population size of South Georgia cormorants has not been accurately known. But the animals are known to occur only on South Georgia and the surrounding rocks and islands, on the South Orkneys further southeast, and some on the South Sandwich Islands. Currently, South Georgia cormorants are listed as Least Concern on the IUCN Red List. The species is related with the Antarctic shags living even further south and forms the group of the blue eye-cormorants with these.

Long-term monitoring programmes do exist on both the South Orkneys and South Georgia, and population size figures had been published. But the research team behind the new study cautioned against viewing these numbers as absolute. They gave the different observation methods over the years and the difficult conditions due to the location of the nests as reasons. However, with the appropriate analytical methods and more accurate counting methods like the use of drones, the researchers were able to approximate the population sizes. This allowed them to see that although the cormorant population had increased on Signy Island in the 1980s. But from the 1990s onwards, the curve showed a fluctuating downward trend.

Cormorants on South Georgia usually breed on rocky outcrops and other hard-to-reach places. But apparently a shipwreck is also considered a suitable breeding site. Nevertheless, such places are difficult to observe. Picture: Michael Wenger

In their work, the researchers looked at both nest occupancy and breeding success. This showed that although breeding success varied massively, it did not really correlate with declines. In contrast, adult nest occupancy was subject to a clear decline. Between 2000 and 2021, there was a 36 percent decline on Signy Island. For South Georgia, the numbers were even higher, nearly 60 percent between 1990 and 2021.

The South Orkneys are a small group of islands located about 850 kilometres southeast of South Georgia on the southern edge of the Scotia Arc. On the two largest islands, Signy and Laurie, there is a British and Argentine station. The islands are important breeding grounds for birds and an iceberg graveyard. Picture: Michael Wenger

Interestingly, the research team found different changes for the only two breeding colonies of cormorants on Signy. On the one hand, the larger of the colonies experienced a massive decline. However, the colony, which was about 10 times smaller at the beginning of the study period, flourished over time, but not enough to offset the overall decline. On South Georgia and the surrounding islands, however, the trend was much more apparent, namely only downward. Reasons for the decline could be declining numbers of prey fish and climate change, the science team explains in their study. However, only a closer look at the factors influencing the populations and their changes will provide more detailed information. We can only hope that by then the black and white birds with their blue eye rings will not have disappeared even further.

Dr Michael Wenger, PolarJournal

Link to the study: Dunn, M.J., Adlard, S., Lynnes, A.S. et al. Long-term population size and trends of South Georgia Shags (Leucocarbo [atriceps] georgianus) at Signy Island, South Orkney Islands and Bird Island, South Georgia. Polar Biol (2021).

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