Walrus counts in satellite images validated by drone images | Polarjournal
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Walrus counts in satellite images validated by drone images

Julia Hager 31. May 2024 | Arctic, Science
The habitat of walruses is changing drastically due to the rapid warming of the Arctic and it is essential to better understand them to ensure their survival as a species. (Photo: Julia Hager)

Counting wild animals with the help of satellite images is probably the gentlest method of determining population sizes. But are the counts accurate enough? A recent study on walruses now shows that both scientists and citizen scientists only slightly underestimated the number of animals in the satellite images.

One, two, three, …, 73 walruses resting on a narrow sandbank in Svalbard, clearly visible and easy to count in the drone image taken from 55 meters above. However, counting the same group of animals on a satellite image is somewhat more difficult due to the poorer resolution and in the new study, both citizen scientists and walrus experts slightly underestimated their numbers. Here, the accuracy was highest at a resolution of 30 centimeters, compared to a resolution of 15 or 50 centimeters.

“Counting walruses from space could revolutionise the way we currently monitor them. It can increase the extent and frequency of current surveys, helping us gain a better insight into how walruses are faring in a changing world. But we need to know how accurate walrus counts from satellite imagery are,” said Dr. Hannah Cubaynes, a wildlife from space scientist at the British Antarctic Survey and lead author of the study, in a BAS press release.

Scientists from the British Antarctic Survey, the Norwegian Polar Institute and WWF took the drone images on the Sarstangen sandbank on the west coast of Spitsbergen in the summer of 2022. The satellite image, which comes from Maxar Technologies, was taken just 15 minutes later.

“Thanks to the team’s perseverance, we were able to get an on-the-ground drone image within minutes of a satellite image taken from space. This was totally unprecedented and the best result we could have hoped for. This is a really exciting development for the project – giving us almost like-for-like satellite and drone images that we can use to calibrate walrus counts from both experts and citizen scientists,” explains Dr. Cubanyes.

The red dot marks the Sarstangen resting place investigated in the study. The gray dots mark other resting places. Map: Norwegian Polar Institute via Cubaynes et al. 2024

Despite the discrepancies found in the counts, the study, published last week in Remote Sensing in Ecology and Conservation, represents a major advance in the use of satellite imagery as a non-invasive method for monitoring walruses in the Arctic.

Comparing the counts of citizen scientists and experts allows the research team to calibrate the results so that in future the walrus counts on satellite images will also provide reliable data and improve our understanding of walrus populations.

During Walrus from Space’s fieldwork in Svalbard, Norway, in July 2022, Peter Fretwell (center) pilots a drone, Hannah Cubaynes (left) notes important information such as timing, and Rod Downie from WWF-UK is seen in the background. (Photo: Emmanuel Rondeau, WWF)

“All Arctic endemic marine mammal species are being negatively impacted by global warming through losses in their sea ice habitats. This increases the need to monitor populations more closely to provide data to management authorities for mitigation and conservation actions. However, these animals are spread over large areas and are thus logistically challenging and very costly to count using ships or aircraft. Being able to monitor walruses (and other Arctic wildlife) using satellite imagery would be a giant step forward,” say Kit Kovacs and Christian Lydersen, senior research scientists at the Norwegian Polar Institute.

More than 11,000 citizen scientists from 68 countries have already helped the researchers to count walruses on satellite images. At Walrus from Space you can still sign up and help the researchers and thus also the walruses, which are threatened by the rapid warming of the Arctic.

Julia Hager, Polar Journal AG

Link to the study: Cubaynes, H.C., Forcada, J., Kovacs, K.M., Lydersen, C., Downie, R. and Fretwell, P.T. (2024), Walruses from space: walrus counts in simultaneous remotely piloted aircraft system versus very high-resolution satellite imagery. Remote Sens Ecol Conserv. https://doi.org/10.1002/rse2.391

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