An ocean of possibilities: antarctic petrels’ foraging in the Southern Ocean

Research notes

The Southern Ocean surrounding Antarctica is renowned for its extreme weather – feared and respected by sailors and adventurers alike. Yet the antarctic petrel spends its entire life there. We examined behavioural tactics that allow this species to exploit the everchanging sea-ice habitats.

By: Arnaud Tarroux, Per Fauchald and Øystein Varpe* // Norwegian Institute for Nature Research. Nigel G Yoccoz // UiT The Arctic University of Norway. Sébastien Descamps // Norwegian Polar Institute. *Main affiliation University of Bergen

Foto av en petrell
Antarctic petrel (Thalassoica antarctica) sitting on a rock in its breeding colony at Svarthamaren, Dronning Maud Land (71.9°S, 5.2°E). When foraging at sea, antarctic petrels feed mainly on krill, fish, and squid, which they catch at or just below the surface. Photo: Arnaud Tarroux / Norwegian Polar Institute

The Southern Ocean is a highly dynamic environment. There, like in the Arctic, the sea ice grows and shrinks annually, providing the seasonal pulse that sets the tempo for organisms and antarctic ecosystems. Sea ice is a crucial habitat for wildlife dwelling in polar areas, providing shelter, a platform for rest, sometimes even food, to many species from algae to marine mammals and seabirds. However, the importance of sea-ice habitats may vary among, and within, species.

Individuals within animal populations behave and respond differently to their environment, and this applies to a wide array of behaviours, including searching for food and foraging. Distinct foraging tactics can have different consequences, some being particularly profitable and others less so.

Therefore, the choice of a particular tactic may affect how well an individual will fare and survive, ultimately driving the demography of entire populations. This is why it is central in ecology to understand the extent to which such differences in behaviour occur in wild populations, and what their consequences on individuals are.

The antarctic petrel (Thalassoica antarctica) is one of the few seabird species that spend their entire lives within the boundaries of the Southern Ocean. They breed in large colonies on the Antarctic continent, located up to several hundred kilometres inland. Antarctic petrels share with their cousins the albatrosses a passion for long-haul flights, which allow them to access a large range of oceanic habitats where they can choose to forage. But choosing is not necessarily easy. How do they deal with a highly dynamic, continuously changing environment in their everyday life? How do they decide where to search for food along their foraging trips? And what are the consequences of their decisions?

Foto av snødekt fjell
Svarthamaren seabird colony is situated on the scree-slopes (on the right in this picture) of a nunatak located about 200 km inland on the Antarctic continent. The breeding season lasts from late November to early March. The colony currently hosts fewer than 100 000 breeding pairs of antarctic petrels, less than half as many as in the 1980s. Photo: Arnaud Tarroux / Norwegian Polar Institute.

On the track of Antarctic seabirds

We worked at the Svarthamaren breeding colony, Dronning Maud Land, one of the two largest known colonies for that species. Using miniaturised GPS devices, we tracked the whereabouts of antarctic petrels over three consecutive breeding seasons (2011-12 to 2013-14), during both the incubation and chick-rearing period. In parallel, we collected blood samples from each tracked individual to measure a suite of indicators that gave us information about various aspects of antarctic petrels’ physiological state and body condition when they returned from a foraging trip. Such data about energy acquisition or diet allowed us to assess the physiological costs or benefits associated with their foraging patterns.

Flexibility is key

In order to interpret their behaviour at sea, we analysed the tracks of antarctic petrels using sophisticated movement models. We could thus discriminate between foraging and transiting bouts and characterise the areas where they were preferentially foraging.

The exceptional mobility of antarctic petrels is obvious when mapping individual tracks. For example, while the male bird incubated the egg, one female covered more than 8 400 km during a single trip that lasted slightly less than three weeks.

GPS-tracks showing the movements of antarctic petrels during the two phases of their breeding season. During the incubation period, from late November to mid-January, the male and the female alternate foraging trips and incubation shifts that can last up to several weeks. During the brooding period, once the eggs have hatched, the foraging trips become shorter and thus more frequent, owing to the need to regularly provision the chick with food. (Figure from Tarroux et al 2020)

Our results also highlight large variation in the movements and foraging behaviour of antarctic petrels. Some birds foraged in dense pack ice while others chose to hunt in open waters, sometimes within the same period. Sea ice is an important foraging habitat that antarctic petrels use extensively, but not exclusively. Interestingly, while many individuals foraged relatively close to the sea ice edge (within 50 km), we still observed many individuals foraging much farther from the ice edge, both in open water and in very dense sea ice.

This shows that antarctic petrels are flexible and can forage in a wide array of habitats. This could clearly be advantageous in a dynamic system where ice-covered areas can turn to open water in the course of a day. However, it remains unclear why some petrels choose tactics that differ from those of their congeners.

In the wild South, all choices have consequences

Birds using areas with denser sea ice tended to come back in less good condition that those foraging in open sea. However, this was only the case during the chick-rearing period, implying that breeding antarctic petrels made a trade-off while feeding their chicks. Breeding adults may forage in areas where food is more suitable to their offspring than to themselves, at the cost of their own condition.

Future environmental changes, in particular the forecasted loss of sea ice, might have negative consequences on antarctic petrel populations, for instance affecting the survival of chicks that depend more on ice-associated prey such as krill. Nevertheless, the large individual variability in foraging behaviour that we have observed could help adult antarctic petrels adapt to those changes, by using the foraging tactics that best fit the new environmental conditions.

Proportion of antarctic petrel foraging trips occurring in habitats characterised by various sea-ice concentration (a) and distance to the nearest sea-ice edge (b). (Figure from Tarroux et al 2020)