Female walrus and young more dependent on Arctic sea ice | Polarjournal
Little walruses stay with their mother for two years and are protected by her. This works better on sea ice than on land, so female walruses follow the sea ice. Thus, with the disappearance of sea ice, not only the cows suffer, but also the calves. Picture: Michael Wenger

Along with polar bears, walruses are the icons of the Arctic. The enormous seals with their protruding tusks live from, with and on the sea ice of the Arctic Ocean. This applies to both groups of walruses, the Atlantic subspecies and its Pacific relative. Until now, it was assumed that female animals primarily lose their birthing space. But a new study shows that Bering Sea walrus cows and their young are also losing their food source as the icy surface recedes.

According to the results of the study, algae that grow and thrive in and on the sea ice are the food base for the mussels that live on the sea floor. From these again, the walruses feed on its dives. And the females and the young more than the bulls. This means that the seals have different hunting and feeding grounds, and therefore the females and pups will be somewhat more disadvantaged than the bulls by the decline in sea ice. The results of the study were published by Chelsea Koch of the University of Maryland and lead author of the paper, along with her colleagues from Alaska and Scotland, in the journal PLoS ONE (Public Library of Science).

Walruses only congregate in their breeding grounds during the winter months. The animals mate in the icy water and the females give birth on the pack ice the following year. Due to the decline, the animals have to move to the mainland, where they are more vulnerable to hunters such as polar bears. Picture: Michael Wenger

As part of their study, the research team looked at specific markers for these algae to find out more about the feeding relationships of Arctic animals in the region. “This study builds on work we’ve done in the Bering and Chukchi Seas to show that these tracers of ice algae and phytoplankton can be used to monitor ecosystem response to disappearing sea ice,” explains Chelsea Koch. It had been known for some time that the region’s walruses were suffering from the decline in sea ice, coming ashore earlier and in greater numbers, and having to stay longer. This, however, forms a problem as these places are further away from productive feeding regions. In their work, the researchers were able to show that the mighty seals usually split up after mating for foraging: While the bulls stay mainly in the northern Bering Sea in summer and move a bit north, the females migrate with their young far into the Chukchi Sea to be close to the pack ice. Only with the sea-ice spreading to the south, they come again into the mating areas in the Bering Sea.

When the walrus bulls have separated from the cows, they seek out haul-outs on the beaches of islands and mainland. But the longer the sea ice is absent, the more crowded the sites become, which in turn can lead to density stress, injury or even death. Picture: Michael Wenger

Using the markers of the algae, Koch and her colleagues were able to show that even in winter, walrus cows in the northern Bering Sea search for mussels more in the regions where sea ice occurs. And in the case of the walruses from the Chukchi Sea, another surprise was revealed: no difference between the sexes, but to the young. These remain longer on the sea ice and take it from there to the mussel search. The fact that the markers are not detectable for a long time in the animals helped the research team to reach this conclusion: “These markers are short-lived in the walrus liver, i.e. a few days or weeks,” Koch explains further. “So we know that this elevated sea ice signature is not an accumulation from the voyages of the last few years.” Thus, this shows that with the sea ice decline that has become particularly evident in the Bering Sea, female and juvenile walruses have even more to lose than just their solid substrate.

Walruses are an important source of food for the local population around the Bering and Chukchi Seas. The tusks are also turned into works of art by craftsmen. But in many countries the import of walrus-ivory is forbidden, which limits the local income possibilities substantially. Picture: Michael Wenger

Chelsea Koch and her team were able to obtain data from the livers of hunted walruses in their study. They were able to count on the help of native Alaskan hunters who are allowed to hunt walrus for their livelihood. For years, the Alaska authorities have been able to use it to create a database to assess the health status of the Pacific walrus population. This is a win-win situation for both sides. This is because walruses are an important source of food for the populations in the Arctic. “Without hunter support of regional community-based harvest monitoring programs, important studies like this would not even be possible,” explains Dr. Raphaela Stimmelmayer of the North Slope Borough Department of Wildlife Management. The work shows once again the importance of collaboration between research and traditional and local knowledge.

Dr Michael Wenger, PolarJournal

Link to the study: Koch CW, Cooper LW, Woodland RJ, Grebmeier JM, Frey KE, et al. (2021) Female Pacific walruses (Odobenus rosmarus divergens) show greater partitioning of sea ice organic carbon than males: Evidence from ice algae trophic markers. PLOS ONE 16(8): e0255686. https://doi.org/10.1371/journal.pone.0255686

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