Cow Movements at Sea

The three cows that were tracked ranged over entirely different areas (fig. 5.2). None were followed for the total time at sea (67, 69, and 78 days, respectively), but two cows appeared to be returning to the island when recordings ceased. The cows spent a large proportion of their time in reasonably well-defined areas at, or near, the limit of their feeding range (between 1,100 and 1,400 km) (Bester and Pansegrouw 1992). The most circumscribed foraging area (that of the 4-year-old cow) lay south of the Antarctic Polar Front at 54°–57°S latitude and 25°–29°E longitude in cold surface water (minimum temperature of –1.7°C). The two other cows moved north to widely separated (by 15°–20° longitude) areas between approximately 40°–45°S latitude (fig. 5.2). The northwest-bound cow encountered warmer surface water (maximum temperature of 14.1°C) consistent with the mean position of the Subtropical Convergence at 41°40¢ S latitude which is recognizable at the sea surface by a mean decrease in temperature from 17.9°C to 10.6°C (Lutjeharms, Walters, and Allanson 1985). The northeast-moving cow appeared to remain in water between 5.3°C to 7.3°C at the Subantarctic Front (central surface temperature 7°C; Lutjeharms 1985). The cows therefore ranged widely without overlap in their feeding areas, the choice of which might have been influenced by biological enhancement at oceanic frontal systems (see Lutjeharms, Walters, and Allanson 1985). It is likely, however, that the small sample size has identified only a small part of the actual feeding range of elephant seal cows from Marion Island during the postbreeding period (Bester and Pansegrouw 1992), since three of five postbreeding cows from Macquarie Island foraged in deep oceanic waters off the Antarctic coast (Hindell, Burton, and Slip 1991).

Fig. 5.2
At-sea movements of 3 southern elephant seal cows from Marion Island,
during their postbreeding pelagic period. The mean position of the
Antarctic Polar Front is shown (dashed line).