Adelie penguin population dynamics: 18 years in a colony
Antarctica’s population of Adélie penguins (Pygoscelis adeliae), which numbers approximately 2.5 million breeding pairs, closely reflects underlying changes in the lower levels of the food web and the ice environment on which they are dependent. For example, their notable decrease on the Antarctic Peninsula is thought to be a direct response to a reduction in sea ice as a consequence of climate change. Across their distribution of ice free breeding sites along the Antarctic coastline and offshore islands, their population trends vary, with some populations decreasing, some remaining stable and others increasing.
Although Adélie penguins have a varied diet that includes fish, squid, amphipods and jellyfish, they eat large quantities of Antarctic krill – which is the subject of a major Antarctic fishery. This penchant for krill and their dependence on the sea ice environment makes Adélie penguins an important ‘indicator’ species for the CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources) Ecosystem Monitoring Program (CEMP).
The Australian Antarctic Division has played a key role in CEMP since it was established in the mid-1980s, when nations were encouraged to monitor indicator species to detect potential negative impacts from the fishing industry. Back then, CCAMLR recognised that monitoring would need to distinguish between fisheries impacts and change due to ‘natural’ environmental variability, but climate change was not then seen as a potential confounding influence. Now there is much discussion within CCAMLR about how to distinguish between climate change and fisheries impacts; an incredibly difficult task.
Béchervaise Island, near Mawson station, was established as a CEMP site in 1989 and the data collected from this site forms the basis of Australia’s contribution to the CEMP. Although a krill fishery was operating in this area then, it has subsequently concentrated in the South Atlantic, and there has been no krill fishery off East Antarctica since the early 1990s. This has provided an opportunity to examine variability in CEMP parameters in the absence of fisheries impacts, to provide baseline data and the ability to explain the cause of natural variation, particularly the links with the environment. We now have up to 18 years of consistently collected data with which to assess population trends and the underlying processes that influence population dynamics. Here we describe results for two important factors contributing to population change: penguin reproductive success and penguin survival.
For some years we were aware that the ability of parents to feed their offspring was related to the extent of ice around a breeding colony. However, it wasn't apparent what type of ice was important (fast ice* or sea ice, for example) or where or when the presence of ice mattered. To address this we examined annual penguin reproductive success in relation to the ice environment obtained from satellite images. Through a series of statistical analyses we found that reproductive success is clearly related to the amount of ice present, particularly the area immediately adjacent to the breeding site (Figure 1). Extensive fast ice during the ‘guard stage’ of the chick rearing period, when chicks require at least one parent to remain at the nest with them, meant that few chicks survived. Additionally, having some sea ice further away from the colony, but still within their foraging range, improved breeding success when fast ice close to the breeding colony was limited.
Reproductive success was low in years with extensive fast ice, in part because of the forced traverse across the ice to reach open water to forage. It was clear that the presence of extensive fast ice increased the duration of foraging trips, thereby reducing the frequency of feeding chicks, and ultimately leading to their demise. But there were other factors at work too. Many foraging trips in some years weren’t long enough to reach the fast ice edge, indicating that penguins were able to forage within the fast ice area in some years but not in others. Furthermore, extensive fast and sea ice, or the oceanographic processes driving the timing of sea ice break-out, may also be associated with reduced prey availability near Béchervaise Island.
Adélie penguin survival
In stark contrast to the relative ease with which we can examine the summer-based activities of reproduction by deploying field biologists to measure population parameters, measuring mortality directly is near-impossible. To make matters worse, most mortality is thought to occur during the inter-breeding period when Béchervaise Island penguins travel up to 1500 km away from their breeding site. This nine month period also sees a dramatic change in the marine environment, as the sea ice transforms from its minimal extent in March through to its maximal extent in September.
Estimating survival requires the detection of individuals over multiple years through a ‘mark-recapture study’. Each year at Béchervaise Island up to 300 chicks are tagged with micro-chips and their presence at the island in subsequent years is determined by manual tag readers and an automated monitoring system. Through these detection methods we can generate a series of detection histories for every bird that has been tagged over the last 18 years and use them to estimate penguin survival. Our results indicate that young penguins have highly variable survival over their first winter, while survival of the older birds is more consistent through time (Figure 2).
Understanding the processes driving Adélie penguin survival is challenging. Statistical analyses suggest a link between penguin survival and the ice environment at their presumed winter foraging grounds, some 1500 km away from their breeding site (Figure 3). This influence was most apparent during the deep austral winter when the sea ice changes rapidly. For fledgling penguins, too much ice was detrimental to their survival, whereas for the older birds either too much or too little sea ice was detrimental. While survival of the older birds was strongly associated with the environment, there was a large amount of variability in the survival of the younger birds that we are yet to explain. Understanding the underlying mechanisms which result in this association between penguin survival and the environment requires further examination of the specific interactions between penguins and their ice environment through satellite tracking and finer resolution satellite imagery.
What does this mean for CEMP?
Although there has been no krill fishery in the Mawson region in recent years, notifications to CCAMLR suggest that the krill fishery is likely to expand in tonnage and to new areas. Monitoring and assessment procedures need to be available or in place for this eventuality. The impacts of climate change in the Southern Ocean will be complex, but the sea ice environment is likely to be affected. From our work it is clear that this, in turn, is likely to impact directly and indirectly on Adélie penguins. Interpreting the causes of future changes will always be difficult and subject to some doubt. Knowing the nature and extent of the linkages between Adélie penguin reproduction and survival and important environmental features such as sea ice, will improve CCAMLR’s ability to make the correct interpretation and from that, take the appropriate management actions, if the krill fishery resumes in east Antarctica.
Expectations of how climate change is likely to affect different localities around the Antarctic continent need to be considered along with our understanding of the interaction between penguins and the ice environment, to determine likely population change in the future and to detect change as it occurs. This is best done through long-term studies such as the Béchervaise Island monitoring program, in conjunction with detailed studies on the foraging locations of predators and environmental conditions at those locations. We intend to continue the long-term monitoring of Adélie penguins at Béchervaise Island and expand the regular monitoring of a selected suite of parameters for populations in the Australian Antarctic Territory through surveys. New technologies will allow us to determine their broader status and trends and to relate this to changes in the environment. Future tracking studies are also planned to determine where and under what environmental conditions the penguins are foraging during the austral summer and winter months.
LOUISE EMMERSON and COLIN SOUTHWELL
Southern Ocean Ecosystems program, AAD
* Fast ice is ice attached to the continent.