First long-term observation of emperor penguins documents global population trend | Polarjournal
An emperor penguin colony near the ice edge photographed during an aerial survey. Photo: Sara Labrousse

High-resolution satellite images enabled an international research team to document the global population development of emperor penguins for the first time: between 2009 and 2018, the number of penguins declined by almost ten percent.

The total population of emperor penguins was studied by an international research team using high-resolution satellite images over a period of several years for the first time. The results of the long-term study were published on 13 March in Proceedings of the Royal Society B: Between 2009 and 2018, the number of penguins decreased from around 252,000 to around 226,000, with the overall population recovering somewhat by the end of the study period after a sharper decline in 2015 and 2016.

Scientists are still speculating about the causes of the negative population trend. It is not clear whether the expansion of fast ice, the usual breeding area for emperor penguins, is linked to the decline in the population – the analyses revealed only a weak correlation. In an email, Peter Fretwell, who is a mapping and geographic information systems scientist at the British Antarctic Survey and co-author of the study, writes that this may be because of a lag in the population between the number of breeding animals and the ice conditions, as chicks do not reach sexual maturity until they are five years old. “There are a whole range of other climate related threats, such as increase competition and predation, rainfall, changes to prey availability that may also be a cause, but these are difficult to assess at this moment,” he explains.

Known colonies of emperor penguins along the entire coast of Antarctica. The red dots mark colonies for which population size estimates are available. Teal dots mark other colonies. The graph in the center shows the global abundance index of emperor penguins, which represents the number of adult birds present in the colonies in spring and is calculated annually. Graphic: Natalie Renier, Woods Hole Oceanographic Institution

In order to determine the causal factors of population development, further research will be necessary, combining satellite images with animal tracking, field observations and genetic analyses.

Some colonies are stable

The population decline during the observation period does not apply equally to all 66 known colonies, several of which were newly discovered during the study. The size of the colonies in four of the eight fast-ice sectors – Amundsen Sea, Bellingshausen Sea, Dronning Maud Land and Australia – remained stable or even grew on average over the study period. However, there was a sharp decline in the Bellingshausen Sea-Amundsen Sea region between 2009 and 2012, which was followed by a significant increase from 2016 to 2018.

In the Weddell Sea and the eastern Indian Ocean, on the other hand, the probability of a decline in regional populations was highest, as the model calculations showed. For example, the colony in Halley Bay in the Weddell Sea declined from around 20,000 penguins in 2009 to less than 1,000 in 2018. In both sectors, the extent of fast ice has decreased in recent decades.

The dynamics of regional populations in the various fast-ice sectors between 2009 and 2018. The decline is particularly evident in the Weddell Sea and the eastern Indian Ocean. Graphic: LaRue et al. 2024

“We don’t understand the population trend in full, and we need to do more research. However, the fact that we have detected a population trend in such a short data set is really important because it potentially has major implications for the future of the species,” Philip Trathan, Emeritus Associate and former Head of Conservation Ecology at the British Antarctic Survey and co-author of the study, said in a press release by the Woods Hole Oceanographic Institution (WHOI). “Projections indicate that emperor penguins are going to be struggling to survive into the future. So, having tools that allow us to ground truth some of the population models in the future will be really important.”

Difficult monitoring

Due to their life cycle and extremely remote breeding colonies on fast ice, emperor penguins are one of the most complex and difficult seabirds to observe and study. However, since the availability of high-resolution satellite images, it has become possible to comprehensively monitor population status and trends on a global scale, mainly because around half of the colonies were discovered on satellite images.

“Our work documents change in an iconic polar seabird and shows how useful remote sensing can be to understand animals that live in wild places,” Michelle LaRue, associate professor at the University of Canterbury in New Zealand and research associate at the University of Minnesota Twin Cities in the US and co-lead author of the study, said in the press release. “In a rapidly changing world, we have to constantly push the envelope to combine new approaches with tried-and-true methods if we want to understand the consequences of that change, especially in places we cannot get to.”

The authors describe the method as an “invaluable tool for adaptive conservation planning in a changing Southern Ocean”. Among other things, aerial photography, which the team carried out in the Antarctic spring of 2018, was only necessary to validate the data obtained from the satellite images.

“What we did in this paper, and through international collaborative research, was to develop the state-of-the-art approach to monitoring emperor penguins across all of Antarctica, including in remote places that are inhospitable to people. Having the very high resolution satellite imagery is a breakthrough in our understanding of the spatial distribution of emperor penguins, and having this global population trend is very important for conservation,” Stephanie Jenouvrier, senior scientist in the Department of Biology at Woods Hole Oceanographic Institution and senior author of the study, said in the press release.

But even with this advanced method, the researchers can only record the penguins that are present on the fast ice in late spring. Pre-breeding juveniles that spend the first five years of their lives in the Southern Ocean, as well as non-breeding adults, escape observation. “Its very difficult to study the animals that do not come onto the ice. We may be able to get some tracking data, but overall this gap is still something that is very hard to fill,” Peter Fretwell tells PolarJournal.

He sees the next step as extending the survey to 2024, whereby the team will try to improve the method, which is “imperative if we want to understand future population trajectories”.

Emperor penguins in a warming world

Their highly specialized life cycle makes emperor penguins particularly vulnerable to the effects of climate change and it is very unlikely that they will be able to adapt to the predicted changes in their habitat.

Some Antarctic regions are already undergoing major changes, while others still have relatively stable conditions. According to Peter Fretwell, the high latitudes of the Weddell Sea and Ross Sea could possibly be refuges for emperor penguins, but that remains to be seen.

Whether the emperor penguins can continue to successfully raise their chicks in the future depends very much on us humans. Photo: Michael Wenger

When asked whether emperor penguins as a species have a chance of surviving global warming, Peter Fretwell has a clear answer: “It really depends on us. People often ask if emperor penguins can adapt, but really the question is can we adapt? Overall, if we do not change our carbon emissions the future looks bleak, but if we can change in time, they should survive, what is difficult to tell is how long “in-time” is.”

Julia Hager, PolarJournal

Link to the study Michelle LaRue, David Iles, Sara Labrousse, Peter Fretwell et al. Advances in remote sensing of emperor penguins: first multi-year time series documenting trends in the global population. Proceedings of the Royal Society B: Biological Sciences, 2024; 291 (2018) DOI: 10.1098/rspb.2023.2067

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