Maximum Depths and Durations

There was considerable individual variation in mean depth of dive, ranging from 269 ± 159 m to 589 ± 175 m, and in mean duration of dive, ranging from 16.0 ± 7.9 minutes to 36.9 ± 11.5 minutes (table 14.1).

The maximum dive depth recorded was 1,430 m by a postbreeding female (table 14.1). Although dives to such depths are relatively uncommon, constituting less than 1% of all dives, most seals undertook some very deep dives; 87% of females and 63% of males recorded dives of over 900 m in depth. This difference between the sexes might be a result of males foraging in shallower waters than females. The very deep dives were type 6 dives, generally a simple deep spike. These can occur in the middle of a set of type 1 or type 4 dives and are generally single deep dives. However, deep dives of over 1,000 m can occasionally occur in bouts of 3 to 10 dives over several hours. The functions of these deep dives are unclear, although possible explanations include avoidance of predators, exploration, or chasing prey.

Most seals undertook at least some dives with duration time greater than 50 minutes, and the longest dive duration recorded was 120 minutes by a postbreeding female (table 14.1). The longest male dive was 88.5 minutes. A full description of the 120-minute dive is given in Hindell et al. (1992). Long duration dives were generally type 6 composite dives. Their function might have been, for example, to escape an attack by a predator following a bout of foraging. These long dives do not appear in consistent patterns, occurring at any time during the course of a bout of more moderate dives. The only other phocid known to dive for as long or as deep is the northern elephant seal (Le Boeuf et al. 1988, 1989).

Given the similarities in morphology and life cycles of the two species of elephant seal, it is hardly surprising that there are many similarities in diving behavior between the southern elephant seal and its northern cogenitor. Adult southern elephant seals show a general pattern of continuous, prolonged deep diving. On leaving Macquarie Island, type 4 or traveling dives were common, interspersed with some type 1 pelagic foraging dives and occasional type 3 resting dives. After two to three weeks, the animals appeared to reach their foraging grounds, and females settled into a pattern

of type 1 pelagic foraging dives for most of the day, with a few hours of type 3 resting dives occurring in the early morning. The type 1 pelagic foraging dives became shallower at night, probably following the vertical migration of prey species. After males reached their foraging grounds, the general diving pattern was composed of up to 21 hours each day of type 2 benthic foraging, broken by a few hours of type 3 resting dives in the early hours of

the morning. As the males and females returned to Macquarie Island, the most common dives were type 1 pelagic foraging dives and type 4 traveling dives.