Several major breeding populations of southern elephant seals have exhibited substantial declines over the last three decades. Many explanations for these declines have been advanced, but to date none have satisfactorily accounted for all of the observed changes. Most of the hypotheses are hampered by the lack of data concerning, first the processes involved in elephant seal population regulation and secondly, what the animals are doing during the extended marine phase of their life history. This study investigates these poorly understood aspects of southern elephant seal biology.
Mark-resight data from 13 cohorts of southern elephant seals from Macquarie Island were used to study several aspects of the population dynamics of the population n during the 1950s and 1960s. First year survival was stable during the 1950s at about 46% for females and 42% for males. This declined during the early 1960s to a low of only 2% for both sexes in 1965. Post-year one survival did not differ between the decades. Comparisons with a stable population at South Georgia indicated that survival in all ages classes was lower in the Macquarie Island population. Mean age at first breeding was on average one year later at Macquarie Island than at South Georgia. It is proposed that the decline in elephant seal numbers in several populations is due to those populations returning to pre-sealing levels after they had risen to abnormally high levels with the end of commercial exploitation early this century.
Water temperature and depth of dive data collected by Time Depth Temperature recorders were used to estimate the location of elephant seals during their time at sea. Sea surface temperatures indicated that all four males and five of ten females swam south into antarctic waters, while five females remained in warmer subantarctic waters. Water temperature/depth profiles revealed that males and females in antarctic waters were in separate regions, with the males in colder water than the females. Comparisons with oceanographic data suggest that the areas frequented by the males lie over the Antarctic Continental Shelf. The females that used antarctic waters were in areas adjacent to and just north of the areas used by the males; those females that used the subantarctic waters remained near the Antarctic Polar Front. There were no apparent seasonal differences in the areas used by either males or females.
Over 50,000 individual dives were recorded by the Time Depth Temperature recorders. These dives were analyzed with respect to gender, time of the year and the approximate geographic location of the seal. Six dive types could be distinguished on the basis of parameters such as the time spent at the maximum depth of the dive and the general form of the dive. The possible functions of each dive type are discussed and they are proposed to be "rest" dives, "travel" dives, "surface" dives, general "non-foraging" dives, "pelagic foraging" dives and "benthic foraging" dives. The seals spent 90% of their time at sea submerged. Most surface intervals were less than 10 minutes. Twenty to 30% of the time is spent on various types of "non-foraging" dives. Most females performed only "pelagic foraging" dives; all males performed both "pelagic" and "benthic foraging" dives. All "benthic" dives occurred over the Antarctic Continental Shelf and were 400- 500m deep. "Pelagic foraging" dives occurred both over the continental shelf and in oceanic waters, ranging in depth from 200m to 1200m. Many of these dives also exhibited marked diurnal variations in depth, although the magnitude of the variation depended on the latitude at which the dives were made. The seals spent between 10-20 minutes at the bottom of each "foraging" dive, where they usually displayed small "wiggles" which were perhaps an indication of active foraging behaviour. The potential prey types used by the seals are discussed in the light of these behaviours.
The longest dive recorded in this study was 120 minutes, twice as long as any previously recorded dive duration for a marine mammal. All of the seals performed some dives that were longer than the theoretical aerobic dive limit for that seal. Forty four percent of all dives made by post-moult females exceeded the calculated limit compared to 7% for post-breeding females and less than 1% for adult males. The extended dives displayed characteristics that were predominantly those of "foraging" dives, although some were apparently "rest" dives. Dives longer than the calculated aerobic limits often occurred in bouts The longest bout consisted of 63 consecutive dives and lasted two days. Post-moult females performed longer bouts of extended dives than post-breeding females. Extended surface periods (greater than 30 minutes) were not related to the occurrence of extended dives or bouts of extended dives. The possible physiological mechanisms to enable such prolonged continuous dives are discussed. Southern elephant seals may increase the aerobic capacity of dives by lowering their metabolism to approximately 40% of resting metabolic rate on long dives. Some animals appear to use physiological strategies that allow them to prolong the time available to them at the bottom of a dive, while others use alternate strategies that limit the time available at the bottom of their dives.
The results of the mark/resight study are discussed with respect to other mammalian population dynamics studies and the possibility of a food mediated density dependent population regulating mechanism in southern elephant seals is discussed. The results of the diving studies are used to discuss the place of the southern elephant seal in the antarctic marine ecosystem, and to test some of the predictions of the "overshoot" hypothesis advanced to explain the decline in numbers of southern elephant seals over much of its range. The deep diving foraging behaviour of the seals shows that they have few direct mammalian or avian competitors for food resources and that they are likely to exploit prey that have slow population recovery rates; both predictions made by the "overshoot" hypothesis. However, several lines of evidence, primarily the high rate of first year mortality, indicate that it is resource utilization of yearling animals that now needs to be investigated to advance understanding of the declines in populations of southern elephant seals.