Summary
Weddell seals (Leptonychotes weddelli) are the most southerly breeding of all mammals and breed on the fast-ice around the shores of the Antarctic continent. Weddell seals display a polygynous mating system, with operational sex ratios (OSR) ranging from 3:1 to 8:1. It has been suggested that male Weddell seals compete for females by guarding access to the air/water interface at breathing holes and along tidal cracks. All copulatory behaviour occurs in the water and dominant males appear to maintain under-ice territories during the breeding season around these areas of egress (Bartsch et al. 1992). However, because of technical and logistical constraints, there have been no quantitative studies confirming the behaviour of these seals under the ice during the breeding season. From early November to mid-December 1997 we ran a pilot study at the small breeding colony at Turtle Rock in McMurdo Sound (77.27S, 166.85E). In this study we used a new acoustic system (Vemco Ltd., Nov a Scotia) which allowed us to monitor the movements of multiple animals under the ice in real time. This acoustic system consisted of an array of four hydrophones placed though the fast ice and connected to a central base station where time differences in the arrival of acoustic signals were decoded and logged to a notebook computer. We attached depth modulated acoustic transmitters to seven males and five females for a total of 104 days of deployment (range = 1-26 days/male and 1-10 days/female). The signals from these transmitters allowed us to determine x,y,z coordinates for each animal whilst it was under the ice surface, and from this data we were able to reconstruct the movements of the animals both in relation to each other, and in relation to features of the aquatic environment. Because deployments on males lasted several days we were also able to reconstruct the size of male territories under the ice, use of that territory space, and changes in the size of