Aspects of the ecology of Weddell seals Leptonychotes weddellii along the Mawson Coastline, East Antarctica
Creators: name: Pike, RJ
Publisher: name: University Of Tasmania
Year: 2023

description: This study investigated the diet and patterns in occurrence of haul-out sites of Weddell seals along the Mawson coast of East Antarctica. Seats of Weddell seals (n = 303) were collected from 139 seal haul-out sites along 250 km of coast between Fold Island (Kemp Land coast - 66.83¬¨‚àûS, 58.79¬¨‚àûE) and Auster Islands (MacRobertson Land coast - 67¬¨‚àû25'S, 63¬¨‚àû50'E) during 1998 to 2000. Identification of species-specific hard parts from the seats found eighteen species of fish, two species of octopod, two species of squid, and four species of crustaceans. Randomisation analyses were used to determine associations between abundance of primary prey species and year, season, geographic location and water depth at the sample collection site. No significant interannual variation was found, but the diet varied seasonally, as the squid Psychroteuthis glacialis featured more in the diet in the early summer months compared to winter. Larger beaks of this squid were found in seats collected from ice over deeper waters. The diet of Weddell seals along the Mawson coast is therefore more diverse than reported for other parts of Antarctica and the diet also varies seasonally and with changing bathymetry. The diversity in diet may be attributed to the complex seafloor topography along the coast, allowing seals to forage within a wide range of habitats at different depths. For the investigation into patterns in occurrence of haul-out sites, three study areas were established in 2000 at different locations along the Mawson coast. One study area (Macey) had a high concentration of icebergs and islands, another area (Mawson) had islands near the coast and fewer icebergs, and the third area (Colbeck) had very few icebergs and no islands. Within each study area, there were two transects (1500m wide, 1500m apart) extending 20km north over the sea ice from the coast. Each study area was surveyed three times - in winter (late July), late winter (mid September) and spring (midOctober). Non-parametric tests were used to examine variation in density of haul-out sites and density of seals with respect to distance to coast, between areas and between survey periods. During the 3 surveys, 349 seals were observed amongst 165 sites across all 3 areas. Friedman tests found no significant association of number of seal sites or number of seals with distance to coast. Kruskall-Wallis tests found significant variation between the three areas within each survey period; Macey consistently had more sites than the other two areas. Friedman tests tested for temporal variation within each area, however no significant changes in number of seal sites or number of individual seals were detected. Although not formally tested due to small sample sizes, there appeared to be no. spatial separation of sexes. Weddell seal holes were also used by emperor penguins in the Macey and Colbeck areas and by crabeater seals in the Macey area. This study implies that haul-out sites are not randomly distributed at the local scale (with differences shown between different areas along the coast, ie, regional scale) and that number of sites and number of seals hauled out on ice or seen in holes increases from winter into the breeding season. The results suggest that density of seal sites in the fast ice areas off the MacRobertson Land coast is affected by environmental factors such as bathymetry and presence of icebergs. This study suggests that local bathymetric features have an important influence on the biology of Weddell seals in the Mawson area. Changing bathymetry is associated with variation in diet. Water depth can also influence grounding of icebergs and ocean currents that affect the physical structure and cracking of the sea ice, enabling Weddell seals to access the ice surface to breathe and to haul-out for resting and pupping.