Nine months adrift in the Ross Sea
The release period after the breeding season can quickly resemble a sea voyage where the penguins have to choose one route rather than another, move on the drifting ice rather than swim, rest rather than fish… a story of compromise.
Adélie penguins are dependent on sea ice. These walking and swimming birds use drifting patches of pack ice as a means of transport during their nine-month winter migration in the Ross Sea. But the latter is not always favorable, and the penguins need to balance the benefits and costs of this type of navigation. This is the finding of a study published in the Ecological Society of America’s journal : Ecology.
The U.S. research team went to the southern Ross Sea during the summer, at the foot of Mount Erebus and Mount Terror. They fitted loggers to 87 Adélies and recorded 146 trips over three consecutive years to two colonies: one at Crozier Point, east of Ross Island, and the other at Royd Point, west of the island. The first is the largest known colony of Adélie penguins. The former has 300,000 breeding pairs, compared with 2,500 for the latter. On this island, Adélie penguins account for 15% of the global population.
In the Ross Sea gyre
Les manchots quittent la terre ferme après la nidification et partent vers le nord, avant de revenir vers le sud pour la saison suivante. Ils entrent dans le gyre de la mer de Ross, un courant circulaire qui tourne dans le sens des aiguilles d’une montre. La lecture de leur trajectoire indique qu’ils se déplacent sur de plus grandes distances quand ils vont dans la direction des plaques de glace. Ils les utilisent et montent dessus pour atteindre le point le plus au nord au cœur de l’hiver. C’est la plus grande migration connue pour l’espèce, plus de 12 000 km.
Most penguins take the north-westerly route (compass heading) in the direction of the current, when the gyre intensifies from March onwards. However, they can be hampered when the plates drift in the wrong direction or are too slow. So they need to be able to assess the benefit or cost of their craft. The nature of the ice is also food for thought. Penguins walk on ice to move forward. If it’s rough, they go slower.
The southerly winds favour the ascent, but hamper the return southwards. Some take a different route, avoiding the north coast of Victoria Land and heading northeast (compass heading), where settlements have been increasing over the past 20 years. Birds then swim more, expend more energy and need to feed more. But the strategy can be more rewarding.
“Our Adélie penguins are closely tied to sea ice (sea-ice obligates) and our previous research showed that they prefer to stay within high sea-ice concentrations of ~80% but need open water as well for foraging. Too far north and the sea ice concentration may be too low. Too far south and it is too dark and there is too much consolidated sea ice. Penguins may also be foraging under the sea ice, coming up to capture prey trapped under the ice. But they largely are diving into the water column to forage,” explains Dennis Jongsomjit, space ecology researcher and principal investigator. Sea ice also provides shelter from predators, to manage heat loss and to change plumage.
The gyre is particularly prolific for various species of krill, lanternfish and toothfish. The penguins take advantage of this time to replenish the reserves they need to lay their eggs and raise their broods, before returning to their nesting grounds in September and October. “Information is limited here, but the Ross Sea shelf slope seems to be a hotspot due to the upwelling that occurs there as well as the southern boundary of the circumpolar current,” says the researcher.
Ross Island populations have been increasing for at least 25 years. “We have detected a possible slow down in their population growth,” confides the researcher. “Compared to populations found on the Antarctic Peninsula, populations in the Southern Ross Sea have been in good health. But the reasons for why are complex and not completely understood”, he adds.
Fishing and climate
“Both toothfish and Adélie penguins feed on silverfish so fewer toothfish might mean more food for Adélie penguins.” Added to this are changes in wind patterns, which lead to stable or increased sea ice in the Ross Sea region. “As climate change alters sea ice patterns,” adds Dennis Jongsomjit, “it could impact the energetic costs of migration, breeding success, and population dynamics of Adélie penguins, as well as the overall ecology within one of the most pristine ecosystems on Earth.”
Camille Lin, PolarJournal
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