Pack-ice seals

Crabeater seals spar on the on ice
Crabeater seals (Photo: Wayne Papps)

The Antarctic pack-ice seals program (APIS)

The APIS program was an international, multi-disciplinary research initiative aimed at improving our understanding of the ecological role of pack-ice seals.

A key goal of the APIS program (which ran from the mid 1990s to mid 2000s) has been to make accurate estimates of the circumpolar distribution and abundance of the four species, as well as better information on their

  • population structure
  • dietary preferences
  • movements
  • diving behaviour

Australia's main contribution to the APIS program has been to make a significant contribution to the circumpolar survey of seal distribution and abundance. Working with five other nations (Germany, Norway, South Africa, United Kingdom and United States), the survey component was completed in 1999/2000 and data analysis is ongoing. Australia's data analysis has been completed. Read the summary report [PDF] of the APIS program.

Why study pack-ice seals?

Seals are top-level predators in the Southern Ocean ecosystem. Being large and abundant with a widespread distribution, they are important consumers of animals lower in the food chain such as krill, fish and squid.

Studies of pack-ice seals help in understanding the ecosystem and in managing the effects of human activities, such as fishing for krill and fish. As consumers of animals harvested by humans, it is important to understand the role of seals as predators, including how much food they require, to ensure that harvesting of their food supply is not detrimental to the population.

Unlike other marine mammals such as whales, seals spend time hauled out of the water onto ice. Consequently seals are more visible than most other life in the Southern Ocean, and so along with other visible species such as penguins they provide a feasible means of monitoring ecosystem structure and function over large scales.

Studying the more accessible predators can provide insights into their prey, which are much much more difficult to study.

How do we count seals?

The critical pieces of information required to understand the role of a species in the ecosystem are how many individuals there are, what they eat, and how much they eat. While the APIS program gathered information on all of these, the greatest uncertainty in current information, and hence the highest priority, is knowing the number of animals.

Counting seal populations in pack-ice is not as easy as it sounds, and there are many logistical and technical problems to address. Given the size of the area they inhabit, and the fact that many will be underwater at any time, it is obviously not possible to count all animals. The real 'science' is not in the counting, but in estimating how many are not counted.

Even in summer when the pack-ice is at its minimum extent, it still covers up to 4 million sq km of sea around the continent. Because it is impossible to search all of this huge area, we count seals along sample strips searched from ships and helicopters. Provided these sample strips are representative of the whole area, we can then extrapolate from the sampled areas to the entire area.

The searches made from ships and aircraft are for seals hauled out on the ice. All marine mammals spend a considerable amount of time in the water, so just counting seals hauled out without regard to those in the water would under-estimate their total numbers. To account for seals in the water, we catch a sample of seals and attach electronic instruments to the fur of their backs; the instruments fall off the seals later when they moult in the summer. While attached the instruments record how much time they spend on the ice and in the water, allowing us to correct the counts of seals on the ice for those in the water.

One final way that seals may not be counted is for observers to simply miss them, even though they were in view. Counting seals on a background of bright ice and dark water from a helicopter travelling at 80 knots is very difficult and no-one can guarantee a perfect count. To overcome this problem scientific and technical staff at the Australian Antarctic Division developed sophisticated survey equipment and techniques whereby two observers count the same area independently of each other; provided they do this in a specific way we can count the number of individual seals seen by the two observers, but more importantly also estimate how many they each missed.

Once all these issues have been addressed it is possible to estimate the total number of seals in the area surveyed, and with knowledge of their diet and energy requirements, how much of each prey type they consume.

What will be the conservation and management outcomes?

In combination with other work being carried out on predators and prey in the Southern Ocean, the pack-ice seal studies will contribute to the setting of harvest quotas that are safe for both the harvested species and the predators dependent on them.

This page was last modified on 12 August 2010.