Chasing the mysterious Ross seal in Antarctica | Polarjournal
The Ross seal (Ommatophoca rossii) is the smallest of the true Antarctic seals and also the least studied species. On average 1.8 metres long and weighing around 180 kilos, the seal is recognisable by its dark throat stripes, large eyes and squat shape. Little is known about this seal and it is difficult to spot. Picture: Michael Wenger

The effects of climate change are much more difficult to detect in the Antarctic than in the Arctic. This is not least due to the size of the Antarctic continent and the fact that the total area of Antarctica is almost twice as large. In order to learn more about the effects, scientists are therefore increasingly relying on animal helpers. A South African project now wants to kill two birds with one stone (not literally): to investigate the effects of climate change on the ecosystems on and under the sea ice and, at the same time, to finally learn more about Antarctica’s most elusive seal species, the Ross seal.

The project, led by Professor Trevor Mcintyre of the University of South Africa and Doctor Mia Wege of the University of Pretoria, aims to study the foraging and feeding strategies of Ross seals in the marginal ice zone, and thus find out not only about the seal itself, but also about its prey and the food web in general. They are supported by the South African Antarctic Programme. Ross seals are quite unique in many ways compared to their relatives such as Weddell or crabeater seals. Little is known about their population size and the animals are very difficult to spot.

Spotting a Ross seal in the marginal regions of the Antarctic sea ice takes a lot of luck. Because the small seals can hide well behind small ice hummocks. Rarely are the animals as visible as here. Therefore little is known about the species. Image: Michael Wenger

A study by Mia Wege published in May was able to show that Ross seals move far into the open ocean to forage during the Antarctic summer. Then, when the sea ice grows northward in winter, the seals stay true to their location and hunt from the marginal ice zone. “Ross seals preferred to forage in waters ranging between −1 and 2°C, where the mixed-layer depth was shallower in summer and deeper in winter, where current speeds were slower, and away from the ice edge in the open ocean,” the study authors write. The depth of the mixed-layer, where different water masses mix, also plays a role.

The animation tracks transmittered Ross seals east of the Weddell Sea (colored dots) and the course of the pack ice (blue line). The animals move far into the open Southern Ocean in the summer and stay more or less in the same region in the winter. Video: Wege et al (2021) Fron Mar Sci

The findings Mia Wege and the team made in the study show that, on the one hand, Ross seals could benefit from the changes that climate change is causing to the pack ice, as the pack ice edge retreats southward and the mixed-layer depths moves upward. This would mean that the seals would not have to spend as much time foraging, which is an energy gain. But at the same time, researchers warn that water currents are accelerating and water temperatures are rising, which could have a negative impact on the seals and their prey.

To learn more about diving habits and physiological processes, South African researchers plan to fit Ross seals with transmitters that will record data. The researchers also want to collect tissue samples to find out more about the prey. So far, the only known fact is that the animals preferably hunt small Antarctic silverfish, especially when the seals are in a moulting phase. Squids are also on the menu. But more details are not known. Most importantly, there is a gap of year-round data, which the new project aims to fill. Furthermore, it is not known whether the seals have a broad or narrow prey spectrum. If the predictions regarding changes in Antarctic waters come true and the seals only have a narrow range, the animals could be in trouble.

Ross seals have enormously large eyes, which indicates that the animals can dive deep. Previous results from Mia Wege show that the seals probably hunt along the shelf slope. Whether they reach their diving limits and where these limits lie is now to be investigated. Picture: Michael Wenger

Ross seals have the largest eyes of all Antarctic seals measured against their bodies. But little is known about their diving habits. With their project, Professor Mcintyre and Mia Wege now want to find out where the physiological limits of the animals lie and whether they reach them during their dives. If they do, the animals may become less flexible to changes in Antarctic waters. Overall, the project leaders hope to learn as much as possible about the mysterious Ross seal and its environment before the animals perhaps disappear never to be seen again.

Dr Michael Wenger, PolarJournal

Link to the study: Wege M, Bornemann H, Blix AS, NordØy ES, Biddle L and Bester MN (2021) Distribution and Habitat Suitability of Ross Seals in a Warming Ocean. Front. Mar. Sci. 8:659430. doi: 10.3389/fmars.2021.659430

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