ANARE News: Tracking Pack-ice Seals

Pack-ice seals are among the dominant top predators of the Antarctic marine ecosystem. The Crabeater seal is by far the most abundant of the pack-ice seals, and is thought to now be the largest single consumer of krill. This was not always so; indirect evidence suggests that crabeater seal populations have increased dramatically following the decline in whale populations due to their exploitation, and the subsequent freeing up of krill stock as a food resource for other species.

Better knowledge of the distribution and abundance of pack-ice seals, particularly the Crabeater seal, is essential for understanding the significant predator-prey interactions in the Antarctic marine ecosystem. Such information is required for developing predator-prey models to examine the effects of possible management strategies for krill fishing.

The Antarctic Pack-Ice Seals program (APIS), developed by the SCAR Group of Specialists on Seals, is a multi-national project aimed at quantifying the role of pack-ice seals in the Antarctic marine ecosystem. The major element of the APIS project is a circumpolar survey of pack-ice seal abundance and distribution.

Previous attempts to estimate circumpolar pack-ice seal populations have been undertaken by individual nations. The success of these attempts has been hinder-ed by the enormous extent of pack-ice and the substantial logistical problems in obtaining a representative sample across the pack-ice. Estimates from these surveys vary dramatically - from 12 million to 75 million - and this uncertainty is too large for the development of satisfactory predator-prey models. The APIS project aims to provide improved estimates of pack-ice seal abundance by involving several nations in co-ordinated surveys and employing improved survey methods and better resources over those used in previous attempts.

Australia's contribution to APIS will be to survey abundance and distribution in a major sector of the Australian Antarctic waters. There are two basic elements to the survey work:

The circumpolar survey is planned to occur in the 1998/99 season. Before then, information on haul-out behaviour must be obtained, and there are many logistical and methodological problems to overcome to ensure the survey is successful. These behavioural and developmental studies began in the spring of 1994, and continued on the recent spring voyage of the 1995/96 season. The behavioural studies involve attachment of satellite-linked time-depth recorders to a number of seals. To do this seals must be caught and anaesthetised to allow attachment of a recorder, which is glued to their back. The recorders are shed when the seals moult in late summer. The recorders accumulate data on haul-out, dive depth and movement, and relay the data back to Kingston via satellite. We successfully deployed four recorders in 1994, and a further two in the most recent voyage. We plan to deploy 25 recorders over the four seasons prior to the circumpolar survey.

The survey developmental work involved the testing of a sophisticated automatic data logging system for aerial survey from the long-range Sikorsky helicopters, and the trialling of survey techniques from the Aurora Australis. Most previous surveys have been flawed in assuming that an observer can see all seals on the ice within several hundred metres of a ship or helicopter; it is now known that some seals are missed even at short distances from the observer (hence previous estimates may be too low), and our survey techniques must take this problem of "sightability" into account.

The elusive nature of pack-ice seals, the inaccessibility of their pack-ice habitat, and the vagaries of Antarctic weather, make this project an ambitious and challenging one. The current phase of developmental and behavioural work is essential to identify and address the problems before the circumpolar survey occurs. The benefit from the survey will be a significant increase in our understanding of not only the pack-ice seals themselves, but also of the structure and function of the Southern Ocean ecosystem.