Dr Colin Southwell, Australian Antarctic DivisionThe Antarctic Division is currently undertaking a major survey of pack-ice seals in the Australian Waters as part of a multi-national project aimed at determining the significance of pack-ice seals in the Southern Ocean ecosystem.For more information, email: seals@antdiv.gov.au
Pack-ice seals (Crabeater, Leopard and Ross seals) live entirely in the pack-ice zone. They spend much of their time in the water feeding, but also haul out onto the ice to rest, to breed, and to moult. The survey involves sampling areas of the pack-ice by flying over them in a long-range helicopter and counting the number of seals in defined strips to either side of the aircraft. This allows the number of seals hauled out on the ice to be estimated.
Survey work is planned to occur at the time of day when most seals are hauled and hence available for counting. However, not all seals haul out at the same time, even at peak haul out times, so to convert estimates of the number of seals hauled out to the number in the Southern Ocean it is necessary to estimate the proportion of seals hauled out at any one time.
To do this seals are caught while hauled out on the pack-ice and satellite- linked dive recorders are attached to them. These instruments record information on the proportion of time hauled out by the seals, which is relayed back to the Division via the satellite. The following diagram shows the results from one such seal for the breeding season (spring) of 1994. This shows that the seal spent nearly all of its time in the water at night, and about 80% of its time hauled out on the ice in the day between 9am and 3pm. From this information helicopter surveys will be planned to occur in the middle of the day when haul out is at its peak. Also, to calculate the total number of seals in the Southern Ocean counts from the helicopter will need to be increased by around 25% to allow for seals being under the ice and unavailable for counting.
A seal was captured on the 20 September 1994 in the outer pack-ice at latitude 63° S, and a satellite-linked transmitter glued to its back. The transmitter recorded the location of the seal for the next three months before it fell off the animal. For the first two months the seal remained in the outer pack, moving continuously in a non-directional manner. Over the third month the seal moved apparently purposefully in a south-easterly direction, coinciding with the retreat of the pack-ice with approaching summer. The seal moved at an average rate of movement of 40 kilometres per day over the three months.