Four—
Possible Causes of the Decline of Southern Elephant Seal Populations in the Southern Pacific and Southern Indian Oceans

Mark A. Hindell, David J. Slip, and Harry R. Burton

ABSTRACT. There are several characteristics of declining southern elephant seal populations that provide a basis for the formulation and testing of hypotheses designed to explain the decline. Some southern elephant seal populations seem to be declining independently of other major Southern Ocean vertebrate consumers, but only two of the three distinct stocks have exhibited a decline. At Macquarie Island, the adult male and female components of the population seem to be equally affected. Studies conducted in the 1950s and 1960s indicate that elephant seals from Macquarie Island had lower survivorship and slower growth rates and reached sexual maturity later than those from South Georgia. Survival of first-year seals decreased dramatically during the 1960s and led to the almost total failure of the 1965 cohort; the fate of later cohorts has not been determined.

Attempts to explain the recent behavior of southern elephant seal populations have so far concentrated on finding a single cause to account for the characteristics of all populations, rather than a series of independent explanations. Two main hypotheses are advanced, one involving equilibration processes after intense sealing pressure and the other concerned with fluctuations in the ocean environment. However, while there may be a single driving factor influencing the populations of southern elephant seals in the Indian and Pacific Ocean sectors, there may be additional local factors that also regulate the populations. In some cases, the interaction between these effects may obscure understanding of the main driving force. While the hypotheses have not yet been tested, they help in planning future research. Priorities should be the maintenance of censusing programs, long-term and cross-sectional demographic studies, and investigations into several aspects of the biology of first-year seals, particularly diet and foraging ranges.

Southern elephant seals, M. leonina Linn., have been known to science since the time of Linnaeus. For most of that time, however, their contact with man was purely as an exploitable resource, and they were ruthlessly hunted

at all of their major breeding grounds as a source of high-quality oil. Only since the middle of this century have these animals been the focus of serious scientific attention.

The 1950s and 1960s saw major research programs at both the South Georgia and Macquarie Island breeding sites. The result of this research effort was a new understanding of many important aspects of the basic biology of the species, such as reproductive biology (Laws 1956a ), developmental biology (Laws 1953, Bryden 1968a ), behavior and social structure (Laws 1956b ; Carrick, Csordas, and Ingham 1962; Carrick et al. 1962), and population biology (Laws 1960).

A major shift in research perspectives for southern elephant seals occurred in the mid-1980s, with reports of significant declines in several major populations. These reports coincided with an increasing awareness of the value of the Southern Ocean ecosystem both commercially and scientifically. The decline of a major vertebrate predator was regarded as possible cause for concern that other components of the ecosystem were also changing in as yet undetected ways.

This chapter summarizes what is known about the population declines in southern elephant seals at Macquarie Island and at other breeding sites to compare this to trends in other Southern Ocean vertebrate populations and to review the various explanations that have been advanced to explain the decline so far. A new hypothesis is also advanced, and the most fruitful directions for future research are suggested.

Population Trends

Macquarie Island

In 1985, the population at Macquarie Island was found to have decreased at a rate of about 2% per annum (Hindell and Burton 1987), calculated on the assumption that the decline had been operating since the early 1950s. The nature of the census data was such that there was no good a priori reason for assigning the beginning of the decline to any point during the 30-year time series (fig. 4.1). More recent analysis of demographic data collected at Macquarie Island during the 1950s and 1960s suggested that the decline may have started during the early 1960s (Hindell 1991). However, this has little effect on the estimated rate of decline, which remains at about 2% per annum.

There were no apparent differences in the overall rate of decline of adult males and females at Macquarie Island. And as adult male and female elephant seals occupy largely separate regions of the Southern Ocean during their time at sea (Hindell, Burton, and Slip 1991), whatever factor is causing the decline appears to be operating on the younger age classes, possibly before sexual differences in foraging patterns develop.

Fig. 4.1
The peak number of breeding females (diamonds) and males (crosses)
on the isthmus beaches at Macquarie Island between 1950 and 1990.

Kerguelen and South Georgia Stocks

The population decline at Macquarie Island should not be considered in isolation, as similar declines are occurring elsewhere in the Southern Ocean. Study of the behavior of all populations of southern elephant seals will help to understand the situation at Macquarie Island.

Southern elephant seal populations can be grouped into three apparently distinct stocks: Macquarie Island, Iles Kerguelen, and South Georgia. The most recent census data on subpopulations in each of these stocks are listed in table 4.1. The relatively inaccessible nature of many of the rookeries combined with the brevity of the breeding season makes accurate population estimates difficult in some cases, and interpopulation comparisons should be made with care. However, one major trend stands out. All the populations from the Kerguelen stock have been declining at least until the mid-1980s, while the populations from the South Georgia stock have been generally stable or increasing. The populations within the Kerguelen stock may be declining asynchronously with each other and with the Macquarie Island population, but this could be partly due to artifacts of the limited data. Nonetheless, these data suggest that there is something different about the South Georgia seals or about some aspect of their environment.

Other Species

Until recently, elephant seals were not regarded as truly Antarctic seals and were seen as ecologically distinct from the "ice seals." Recent studies on the

foraging ranges of elephant seals (see Slip, Hindell, and Burton, this volume) indicate that many adults spend a considerable proportion of their annual cycle in Antarctic waters. Therefore, it is relevant to compare the population trends of other Antarctic vertebrate consumers with those of elephant seals, and as yearling elephant seals may not move far from their natal islands (Carrick et al. 1962), it is also appropriate to examine population trends of other subantarctic species.

The status of other phocid species in the Southern Ocean is somewhat uncertain. Too little is known about the status of Ross seals, Ommatophoca rossi , and leopard seals, Hydrurga leptonyx , to make an assessment of any population trends at the moment (Erikson and Hanson 1990). There is no evidence of long-term changes in Weddell seal, Leptonychotes weddelli , populations at a number of sites around Antarctica (Testa and Siniff 1987; Green, Wong, and Burton 1992), although cycles in reproductive performance have been recorded (Testa et al. 1990). A. W. Erikson and M. B. Hanson (1990) reported a drop in the numbers of crabeater seals, Lobodon carcinophagus , in the Weddell Sea, but it is uncertain whether this represents a real decline in abundance, a change in distribution, or even sampling artifacts (see Green, Wong, and Burton 1992). Both Antarctic fur seal, Arctocephalus gazella , and sub-Antarctic fur seal, Arctocephalus tropicalis , populations are increasing at all of their breeding sites (e.g., Shaughnessy, Shaughnessy, and Keage 1988; Bengtson et al. 1990).

The baleen whale species that were once heavily harvested in the Southern Ocean all seem to be increasing in numbers (Bengtson 1984; Bryden, Kirkwood, and Slade 1990). Penguins constitute over 90% of the Antarctic avian biomass, but to date there have been no reports of major population declines in this important component of the Antarctic marine ecosystem (Woehler and Johnstone 1991). On the contrary, several species have increased (Laws 1985).

Of all the vertebrate consumers breeding on subantarctic islands, only the wandering albatross has declined in numbers (Hindell and Burton 1987), but this has been attributed to interactions with long-line fishing fleets (Croxall et al. 1990). Giant petrel populations have also been reported as declining in recent years (E. Woehler, pers. comm.), but as these animals rely on elephant seal carrion for a significant part of their diet (Hunter 1983), this decline could simply be a consequence of the elephant seal decline. Some rockhopper penguin, Eudyptes chrysocome , populations have also declined in recent years; the causes for these declines are as yet unknown (Moors 1986).

With the possible exception of crabeater seals and giant petrels and the certain exception of rockhopper penguins and wandering albatross, no other Southern Ocean consumer has exhibited large-scale population de-

clines. Several, such as the fur seals, baleen whales, and some penguin species, are actually increasing. This suggests that whatever is responsible for the decline in some southern elephant seal populations is acting on some aspect of their life history that is unique to these populations.

Demographic Data

There have been three studies of population biology of southern elephant seals. Two were based at South Georgia, one in the 1950s when sealing was still a commercial enterprise (Laws 1960) and the other during the 1970s, some 20 years after sealing had ceased (McCann 1985). Both of these studies were cross-sectional in nature, with life tables derived from age structure data, which were in turn derived from age estimates made from tooth rings. The population at South Georgia is thought to have been stable over the time between the two studies, and the population had largely recovered from the effects of sealing by the late 1970s (McCann and Rothery 1988). Consequently, the South Georgia demographic data from McCann's (1985) study may be regarded as representing a stable, undisturbed population and a valuable source of data for comparison with the declining populations.

A longitudinal study based on 15 cohorts of seals branded at Macquarie Island between 1950 and 1965 (Hindell 1991) revealed several features of their population biology pertinent to understanding the decline in numbers. First-year survival was essentially stable during the 1950s (fig. 4.2). The mean first-year survival for females was 46% and for males 42%. In the early 1960s, first-year survival declined dramatically, with the almost complete failure of the 1965 cohort (survivorship was less than 2%). However, survival of all other age classes appeared to be stable between the 1950s and 1960s. Unfortunately, it was not possible to analyze the success of later cohorts.

There were several significant differences between the Macquarie Island population parameters and those found in the stable South Georgia population. First-year survival of the Macquarie Island elephant seals during the 1950s was substantially lower than the 60% for both sexes estimated for South Georgia (McCann 1985). Survival in all other age classes was also lower at Macquarie Island than at South Georgia (fig. 4.3), although the differences in methodology make quantitative comparisons difficult. Seals from the South Georgia stock have higher growth rates, both preweaning and postweaning, than those from Macquarie Island (Bryden 1968b ). The slower growth rates of the Macquarie Island seals may be responsible for the 12-month delay in the average age at first breeding compared with South Georgia (Carrick, Csordas, and Ingham 1962).

Fig. 4.2
Survival to age = 1 year for males (dashed lines, open circles) and females
(solid lines, solid circles) in 13 cohorts between 1951
and 1965 (from Hindell 1991).

Possible Explanations

An acceptable explanation of population trends in the southern elephant seal will need to account for the known characteristics of the declines. While it is possible that the declines in different populations may have independent causes, it is worthwhile to attempt to find a single explanation that will account for the decrease of all elephant seal populations.

Southern elephant seal declines are characterized by the following:

1. Both the adult male and adult female components of the Macquarie Island population appear to have declined at the same rate.

2. Only populations within the Macquarie and Kerguelen stocks have declined, while those from the South Georgia stock are probably stable. The timing and rates of the declines may also differ between subpopulations.

3. Southern elephant seal populations appear to have declined independently of other mammalian or avian species in the Antarctic or subantarctic sectors of the Southern Ocean.

4. Growth rates were lower at Macquarie Island than at South Georgia

Fig. 4.3
Post-year 1 survival curves for  (A)  females from Macquarie Island (showing 95%
confidence limits in dotted lines) and South Georgia, and  (B)  males from
Macquarie Island (showing 95% confidence limits in dotted lines)
and South Georgia (from Hindell 1991).

during the 1950s and 1960s. Age at first breeding was also one year later at Macquarie Island than at South Georgia.

5. Survivorship in all age classes was lower in the Macquarie Island population than in the South Georgia population.

6. There was a marked decline in the first-year survival of Macquarie Island animals between 1960 and 1965 which resulted in the almost complete failure of the 1965 cohort; post-year-one survival did not appear to change at this time (Hindell 1991).

A number of explanations have been advanced to explain these observations with varying degrees of success. The hypotheses can be loosely divided into three groups: intrinsic factors, predation, and the availability of food.

Intrinsic Factors

The most likely intrinsic factor that could influence elephant seal population size is some kind of density-dependent mechanism. Both northern and southern elephant seals can exhibit density-dependent pup mortality, with pup mortality increasing on very crowded beaches (Reiter, Stinson, and Le Boeuf 1978; van Aarde 1980b ). Although it is possible that this mechanism might regulate elephant seal populations, it fails to explain many of the known characteristics of the decline. For example, it cannot account for both the apparent stability of the South Georgia population and the concurrent decline at Macquarie and Kergeulen. Also, as the overcrowding of harems results in increases in preweaning mortality, this mechanism does not account for the sharp decline in first-year survival during the 1960s (the animals used in that study were all branded after they were weaned).

It has been suggested that mass mortalities caused by disease, particularly parasite infestations, may be an important factor in population regulation in marine mammals (Harwood and Hall 1990). However, there is no evidence of widespread disease in southern elephant seal breeding aggregations.

Predation

Killer whales, Orcinus orca , are undoubtedly a major cause of juvenile mortality in southern elephant seals (Condy, van Aarde, and Bester 1978). Increases in the number of killer whales preying on newly weaned elephant seals might produce the changes in juvenile mortality described above and may also help explain the lower adult survival at Macquarie Island. However, there is no evidence for increased abundance of killer whales, and it would not satisfactorily explain the differences in growth rates and attainment of age of first breeding between the Macquarie and South Georgia populations.

Availability of Food

This category can be divided into three subgroups: competition, equilibration processes, and changes in the ocean environment.

Competition

Southern elephant seals could be competing with other Southern Ocean vertebrate consumers for basic food resources, particularly as a number of these species are currently increasing in number. However, adult elephant seals forage on deep-dwelling benthic and pelagic prey, which are generally unavailable to other mammal or bird species (Hindell, Slip, and Burton 1991), so the chances of direct competition must be limited. As we know little about the prey of first-year elephant seals, it is possible that this age class does share a prey species with one of the other increasing species, but recent data from northern elephant seals suggest that juveniles of 2 years are capable of undertaking the same dive patterns as adults (Le Boeuf, this volume). If young animals from the southern species can also make deep dives, they will equally be able to avoid direct competition with shallower-diving species. This hypothesis fails to account for the sudden decline in first-year survival observed in the 1960s, as it is difficult to understand how this could be caused by competition with other populations that were increasing at a steady rate.

It has also been suggested that elephant seals have had their resource base reduced by the recent introduction of commercial fishing operations in various regions of the Southern Ocean. M. Pascal (1986) suggested that the decline in elephant seal numbers at Iles Kerguelen may be linked to the very intense fishing activity in the area which occurred concurrently with the decline. However, there is little or no commercial fishing for deep water fishes in the areas identified as principal foraging grounds for elephant seals from Macquarie Island (Hindell, Burton, and Slip 1991; Slip, Hindell, and Burton, this volume).

Equilibration Processes

One explanation consistent with the known characteristics of the decline is the population "overshoot" hypothesis. This hypothesis states that the current decline in elephant seal numbers at Macquarie Island (and the other populations) is a direct consequence of the heavy exploitation of elephant seals during the eighteenth and early nineteenth century. The sealing industry reduced the elephant seal population at Macquarie Island to far below the original level and continued to suppress it for almost one hundred years (Hindell and Burton 1988a ). During this time, prey species may have increased so that when commercial sealing ended in the early part of this century, an abundant resource was available to the elephant seals, which enabled an increase in the population such that it overshot the presealing population size. Thus, the population

decline described since then may be the result of subsequent overexploitation of the food resource and a decrease toward equilibrium.

This hypothesis accounts for many of the observed characteristics of the decline. The South Georgia population may not have demonstrated an overshoot and decline because it was exploited in a managed fashion (at least for this century), while the differences in timing and rates of the declines in the other populations could be explained by different timing and extents of sealing pressure. The reduced growth rates and lower adult survival at Macquarie Island during the 1950s may have been a result of the population approaching the maximum carrying capacity of an inflated food resource. As the food resources became inadequate, the population began to decrease, principally driven by an increase in juvenile mortality as documented for the early 1960s.

The observation that both the male and female components of the population declined at a similar rate further supports the notion that the decline is a consequence of juvenile mortality. As adult male and female elephant seals use different foraging grounds, it seems most likely that the factor causing the decline has acted on the age classes in which pronounced sexual differences in morphology and behavior have not yet developed.

If the populations in the Kerguelen and Macquarie stocks declined synchronously, then the overshoot hypothesis would be substantially weakened. The hypothesis predicts that the timing and rate of the decline depends on the severity and duration of the sealing at each island (or group of islands where a common food source is used). It is unlikely that these would result in synchronous declines at a large number of populations over a considerable geographic range. Unfortunately, the time series data on abundance are inadequate to solve this problem.

Another problem with the overshoot hypothesis is that it may not account for declines in small populations, like Marion Island or Campbell Island, that may never have been heavily exploited. The hypothesis will only apply if (a) these populations share common feeding grounds with other, exploited populations and are thus exposed to the same "boom and bust" responses of the prey species or (b) such small populations are only splinter groups of the larger populations, perhaps formed when the exploited populations were approaching their maximum predecline densities.

Changes in the Ocean Environment

Another possible explanation is that the decline is due to changes in the ocean environment that have had an impact on the food species used by southern elephant seals (Burton 1986). Recent studies on global climate change have brought an increased awareness of large-scale environmental patterns and correlations between climatic fluctuations and population changes. For example, air pressure dis-

tribution over the Southern Hemisphere is influenced irregularly by the Southern Oscillation, which results in El Niño off Peru (van Loon and Shea 1988), and a scarcity of krill and a drop in breeding success of some land-based predators around South Georgia was observed in years that followed a strong El Niño Southern Oscillation (ENSO) event (Croxall et al. 1988; Priddle et al. 1988). Anomalies in air pressure at sea level affect the generation and movements of cyclones and anticyclones in the area between the subtropical ridge and the Antarctic continent (van Loon and Shea 1988), and these atmospheric processes force changes in the physical environment of the Southern Ocean, with subsequent effects on the biota (Sahrhage 1988).

The foraging grounds of adult southern elephant seals from the Macquarie stock lie off the coast of the Antarctic continent between longitude 135°E and 165°W (see Slip, Hindell, and Burton, this volume), and elephant seals from the Kerguelen stock have been regularly sighted ashore between 38°E and 110°E (Bester 1988; Gales and Burton 1989). Fluctuations in air pressure, affecting the physical environment of this area of the Southern Ocean might influence distribution and abundance of prey of these stocks. ENSO events are poorly correlated with longer-term climatic variability across the western margins of the Australian continent (Allen, Beck, and Mitchell 1990), and there appears to be no relationship between ENSO events and elephant seal numbers at Macquarie Island. However, climatic fluctuations in this area and the southern Indian Ocean appear to be a consequence of other forcings (Allen and Haylock 1993).

Analyses of mean sea level pressure data since 1957 from the wider Indian Ocean region, including coastal Antarctic stations, show that weather patterns in southwestern Australia are associated with fluctuations in the continental anticyclone and the long-wave trough to the south-southeast of Australia. This high latitude trough has shown strong persistence near the Antarctic station of Casey (110.5°W) in winter (June, July, August) and has fluctuated only in intensity, deepening to pressures below 985 hPa between 1966–1971 and 1975–1989 and becoming shallow with pressures above 985 hPa between 1957–1965 and 1972–1974 (Allan and Haylock 1993). This permanent low pressure region centered north of Casey has broadened since the early 1960s to include the Vestfold Hills on the shores of Pridz Bay, a known foraging area for seals of the Kerguelen stock (Bester 1988; Gales and Burton 1989). The deepening of this trough between 1966 and 1971 corresponds to the increase in first-year mortality at Macquarie Island.

Thus, although the causative factors are unknown, the fluctuations in this trough may have forced or have been associated with changes in the ocean environment that resulted in reduced food resources for elephant seal populations of the Macquarie and Kerguelen stocks and led to their

decline. The South Georgia population may have been unaffected by such changes and remained stable because variation in environmental conditions differ from one area of the Southern Ocean to another (Sahrhage 1988). In the absence of hard data, this is very speculative.

Predictions from These Hypotheses

Of the possible explanations presented above, two (the equilibration and the ocean environment hypotheses) are consistent with what we know of the decline of elephant seals at Macquarie Island and provide a possible explanation of the decline of other elephant seal populations. There is an obvious need for more research into many aspects of elephant seal population biology and ecology.

One approach to deciding future research directions is to test the various predictions arising from the alternative hypotheses and perhaps eliminate them. Not all have testable predictions, but the last two can be tested. This offers a framework for further research.

Predictions from the Equilibration Hypothesis

The equilibration or overshoot hypothesis predicts that elephant seals have a very simple predator-prey relationship and that they utilize a resource not exploited by other major predators. This is partly supported by recent studies of adult foraging behavior (see above).

As first-year survival decreased rapidly in advance of adult survival, some factor limited first-year survival. This suggests that first-year animals will be fundamentally different from adults either in their diet, their foraging grounds, or their ability to find and capture prey in competition with adult seals.

The hypothesis also predicts that the population should eventually stabilize at around the presealing population size. As this was estimated to be about 90,000 to 100,000 seals for Macquarie Island (Hindell and Burton 1988a ) and the present population size is approximately 90,000 seals, the decline should slow down or stop in the near future. However, the equilibration process may be expected to continue with a series of "undershoots" and "overshoots" of diminishing magnitude.

The seals would show an increase in growth rate and adult body size coincidental with the stabilization of population size. This would also be accompanied by decreases in the age of first breeding and increases in both adult and first-year survival.

As northern elephant seals are very similar in many ways to their southern congener, and they were also subjected to intense sealing pressure in the last century, they may exhibit population trends similar to southern elephant seals. At present, northern elephant seal populations are increas-

ing rapidly (Stewart et al., this volume). If the equilibration hypothesis holds and the populations are left completely undisturbed, they should eventually level off and then decline as they also fully exploit their major food species.

Predictions from the Environmental Change Hypothesis

The central premise of this hypothesis is that the Southern Ocean presents elephant seals with an environment where their specific food resources are patchily distributed in both space and time, and this patchiness is influenced by ENSO events. Accessibility to these patches directly affects survival of first-year animals (and thus regulates population size) and can change over short periods (less than a decade).

Thus, if it is valid, first-year survivorship should change markedly over quite short time intervals, certainly within a decade and possibly at even shorter intervals; they should be nondirectional, responding directly to variations in the marine environment. In contrast, while the equilibration hypothesis also predicts changes in first-year survival, it predicts them to be directional with a trend for increasing survival as the population returns to its original level.

If changes in weather patterns and ocean circulation have a direct effect on the distribution and abundance of elephant seal food resources (particularly those of first-year seals), then the seals would be expected to exhibit changes in feeding strategies and foraging grounds over quite short time spans, as they attempt to adjust to changes in density and location of prey species. The hypothesis further predicts, as does the equilibration hypothesis, that first-year animals will have different feeding strategies and foraging grounds than older seals.

Directions for Future Research

There are three main lines of investigation that should be pursued to test the predictions from these hypotheses and to promote understanding of the underlying reasons for the observed changes in elephant seal numbers. With most species of large mammals, juvenile survival is a major component of the process of population regulation. The single most striking aspect of the population dynamics of the Macquarie Island elephant seals was the decline in first-year survival that occurred in the early 1960s. Changes in population size since that time may have been largely due to this major perturbation in the recruitment process, although it is unknown whether reduced first-year survival was sustained after 1965. Both the equilibration and the environmental change hypotheses rely heavily on the premise that first-year elephant seals are different from adult seals, particularly with respect to diet and foraging areas or to their ability to exploit them. First-year

animals are also the component of the population that we know least about, and so they are an obvious target for research efforts. Foraging behavior and foraging areas can be studied and contrasted with those of adults, using time-depth and geolocation recorders. Although a considerable number of recorders will inevitably be lost through natural mortality, making it difficult and expensive to obtain acceptable sample sizes, this remains the best method for collecting the relevant data. It may also be possible to study diet directly as these young seals often haul out on sub-Antarctic islands during the winter months (Hindell and Burton 1988b ).

Another priority is the establishment of demographic studies. There are two possible approaches. The first is to renew long-term mark-resighting programs similar to the one at Macquarie Island in the 1950s or the one that recently began at Marion Island (Bester and Wilkinson, this volume). Although these studies may not produce valuable data for a number of years, long-term demographic information is fundamental to testing several of the predictions outlined above and is also central to management decisions that may need to be made in the future. The second approach is to conduct cross-sectional age structure studies similar to those from South Georgia. These have the advantage of supplying some types of demographic data, such as estimates of age-specific survival, quickly. It may be possible to obtain age estimates from incisors painlessly removed under anesthetic (Arnbom et al. 1992), which will greatly increase the practicality of such an approach.

Another research priority is the maintenance of simple censusing programs. In many ways, this should be seen as the primary research priority, as it can be performed cheaply and easily at a large number of locations. Data on any future population fluctuations will be required to test predictions from both the equilibration and environmental change hypotheses and provide ongoing information on the status of the species throughout its range.

References

Allen, R. J., K. Beck, and W. M. Mitchell. 1990. Sea level and rainfall correlations in Australia: Tropical links. Journal of Climate 3: 838–846.

Allen, R. J., and M. Haylock. 1993. Circulation features associated with the winter rainfall decrease in southwestern Australia: Implications for greenhouse and natural variability studies. Journal of Climate 6: 1356–1367.

Arnbom, T. A., N. J. Lunn, I. L. Boyd, and T. Barton. 1992. Ageing live Antarctic fur seals and southern elephant seals. Marine Mammal Science 8: 37–43.

Barrat, A., and J. L. Mougin. 1978. L'elephant de mer Mirounga leonina de L'ile de la Possession, archipel Crozet (46°25¢ S, 51°45¢ E). Mammalia 42: 143–174.

Bengtson, J. L. 1984. Review of Antarctic Marine Fauna. Selected Papers Presented to the Scientific Committee of CCAMLR 1982–1984 , 1–226.

Bengtson, J. L., L. M. Ferm, T. J. Harkonen, and B. S. Stewart. 1990. Abundance of Antarctic fur seals in the South Shetland Islands, Antarctica, during the 1986–87 austral summer. In Antarctic Ecosystems: Ecological Change and Conservation , ed. K. R. Kerry and G. Hempel, 265–270. Berlin and Heidelberg: Springer Verlag.

Bester, M. N. 1988. Marking and monitoring studies of the Kerguelen stock of southern elephant seals Mirounga leonina and their bearing on biological research in the Vestfold Hills. Hydrobiologica 165: 269–277.

Bryden, M. M. 1968a . Development and growth of the southern elephant seal (Mirounga leonina Linn.). Papers and Proceedings of the Royal Society of Tasmania 102: 25–30.

———. 1968b. Control of growth in two populations of elephant seals. Nature 217: 1106–1108.

Bryden, M. M., G. P. Kirkwood, and R. W. Slade. 1990. Humpback whales, Area V. An increase in numbers off Australia's east coast. In Antarctic Ecosystems: Ecological Change and Conservation , ed. K. R. Kerry and G. Hempel, 271–277. Berlin and Heidelberg: Springer Verlag.

Burton, H. R. 1986. A substantial decline in numbers of the southern elephant seal at Heard Island. Tasmanian Naturalist 86: 4–8.

Carrick, R., S. E. Csordas, and S. E. Ingham. 1962. Studies on the southern elephant seal, Mirounga leonina (L.). IV. Breeding and development. CSIRO Wildlife Research 7: 161–197.

Carrick, R., S. E. Csordas, S. E. Ingham, and K. Keith. 1962. Studies on the southern elephant seal, Mirounga leonina (L.). III. The annual cycle in relation to age and sex. CSIRO Wildlife Research 7: 119–160.

Condy, P. R., R. J. van Aarde, and M. N. Bester. 1978. The seasonal occurrence and behaviour of killer whales Orcinus orca , at Marion Island. Journal of Zoology, London 184: 449–464.

Croxall, J. P., T. S. McCann, P. A. Prince, and P. Rothery. 1988. Reproductive performance of seabirds and seals at South Georgia and Signy Island, South Orkney Islands, 1976–1987: Implications for Southern Ocean monitoring studies. In Antarctic Ocean and Resources Variability , ed. D. Sahrhage, 261–285. Berlin and Heidelberg: Springer Verlag.

Croxall, J. P., P. Rothery, S. P. C. Pickering, and P. A. Prince. 1990. Reproductive performance, recruitment and survival of wandering albatrosses Diomedea exulans at Bird Island, South Georgia. Journal of Animal Ecology 59: 775–796.

Erickson, A. W., and M. B. Hanson. 1990. Continental estimates and population trends of Antarctic seals. In Antarctic Ecosystems: Ecological Change and Conservation , ed. K. R. Kerry and G. Hempel, 253–264. Berlin and Heidelberg: Springer Verlag.

Gales, N. J., and H. R. Burton. 1989. The past and present status of the southern elephant seal (Mirounga leonina Linn.) in greater Antarctica. Mammalia 53: 35–47.

Green, K., V. Wong, and H. R. Burton. 1992. A population decline in Weddell seals: Real or sampling artifact? CSIRO Wildlife Research 19: 59–64.

Guinet, C., P. Jouventin, and H. Weimerskirch. 1992. Population changes, movements of southern elephant seals on Crozet and Kerguelen archipelagos in the last decades. Polar Biology 12: 349–356.

Harwood, J., and A. Hall. 1990. Mass mortality in marine mammals: Its implications for population dynamics and genetics. Trends in Evolution and Ecology 5: 254–257.

Hindell, M. A. 1991. Some life history parameters of a declining population of southern elephant seals, Mirounga leonina. Journal of Animal Ecology 60: 119–134.

Hindell, M. A., and H. R. Burton. 1987. Past and present status of the southern elephant seal (Mirounga leonina ) at Macquarie Island. Journal of Zoology, London 213: 365–380.

———. 1988a . The history of the elephant seal industry at Macquarie Island and estimates of the pre-sealing numbers. Papers and Proceedings of the Royal Society of Tasmania 122: 159–176.

———. 1988b . Seasonal haul-out patterns of the southern elephant seal (Mirounga leonina L.) at Macquarie Island. Journal of Mammalogy 69: 81–88.

Hindell, M. A., H. R. Burton, and D. J. Slip. 1991. Foraging areas of southern elephant seals, Mirounga leonina , as inferred from water temperature data. Australian Journal of Marine and Freshwater Research 42: 115–128.

Hindell, M. A., D. J. Slip, and H. R. Burton. 1991. Diving behaviour of adult male and female southern elephant seals. Australian Journal of Zoology 39: 595–619.

Hunter, S. 1983. The food and feeding ecology of the giant petrels Macronectes halli and M. giaganteus at South Georgia. Journal of Zoology, London 200: 521–538.

Laws, R. M. 1953. The elephant seal (Mirounga leonina Linn.). I. Growth and age. Falkland Islands Dependencies Survey, Scientific Reports 8: 1–61.

———. 1956a . The elephant seal (Mirounga leonina Linn.). II. General, social and reproductive behaviour. Falkland Islands Dependencies Survey, Scientific Reports 13: 1–88.

———. 1956b . The elephant seal (Mirounga leonina Linn.). III. The physiology of reproduction. Falkland Islands Dependencies Survey, Scientific Reports 15: 1–66.

———. 1960. The southern elephant seal (Mirounga leonina Linn.) at South Georgia. Norsk Hvalfangst-Tidende 49: 466–476, 520–542.

———. 1985. Ecology of the Southern Ocean. American Scientist 73: 26–40.

McCann, T. S. 1985. Size, status and demography of southern elephant seal Mirounga leonina populations. In Sea Mammals of South Latitudes: Proceedings of a Symposium of the 52d ANZAAS Congress in Sydney —May 1982 , ed. L. K. Ling and M. M. Bryden, 1–17. Northfield: South Australian Museum.

McCann, T. S., and P. Rothery. 1988. Population size and status of the southern elephant seal (Mirounga leonina ) at South Georgia, 1951–1985. Polar Biology 8: 305–309.

Moors, P. J. 1986. Decline in numbers of Rockhopper penguins at Campbell Island. Polar Record 23: 69–73.

Pascal, M. 1986. Numerical changes in the population of elephant seals (Mirounga leonina L.) in the Kerguelen Archipelago during the past 30 years. In Marine Mammal Fishery Interactions , ed. J. R. Beddington, R. J. H. Beverton, and D. M. Lavigne, 170–186. London: George Allen and Unwin.

Priddle, J., J. P. Croxall, I. Everson, R. B. Heywood, E. J. Murphy, P. A. Prince, and C. B. Sear. 1988. Large-scale fluctuations in distribution and abundance of krill—A discussion of possible causes. In Antarctic Ocean and Resources Variability , ed. D. Sahrhage, 261–285. Berlin and Heidelberg: Springer Verlag.

Reiter, J., N. L. Stinson, and B. J. Le Boeuf. 1978. Northern elephant seal development: The transition from weaning to nutritional independence. Behavioral Ecology and Sociobiology 3: 337–367.

Sahrhage, D. 1988. Some indications for environmental and krill resources variability in the Southern Ocean. In Antarctic Ocean and Resources Variability , ed. D. Sahrhage, 33–40. Berlin and Heidelberg: Springer Verlag.

SCAR. 1991. Report of the workshop on southern elephant seals, Monterey, California, May 22–23, 1991.

Shaughnessy, P. D., G. L. Shaughnessy, and P. L. Keage. 1988. Fur-seals at Heard Island: Recovery from past exploitation? In Marine Mammals of Australasia—Field Biology and Captive Management , ed. M. L. Augee, 71–77. Sydney: Royal Zoological Society of New South Wales.

Taylor, R. H., and G. A. Taylor. 1989. Re-assessment of the status of southern elephant seals (Mirounga leonina ) in New Zealand. New Zealand Journal of Marine and Freshwater Research 23: 201–213.

Testa, J. W., G. Oehlert, D. G. Ainley, J. L. Bengtson, D. B. Siniff, R. M. Laws, and D. Rounsevell. 1990. Temporal variability in Antarctic marine ecosystems: Periodic fluctuations in the phocid seals. Canadian Journal of Fisheries and Aquatic Science 48: 631–639.

Testa, J. W., and D. B. Siniff. 1987. Population dynamics of Weddell seals (Leptonychotes weddelli ) in McMurdo Sound, Antarctica. Ecological Monographs 57: 149–165.

van Aarde, R. J. 1980a . Fluctuations in the population of southern elephant seals Mirounga leonina at Kerguelen Island. South African Journal of Zoology 15: 99–106.

———. 1980b . Harem structure of the southern elephant seal Mirounga leonina at Kerguelen Island. Review Ecologie (Terre Vie) 34: 31–44.

van Loon, H., and D. J. Shea. 1988. A survey of atmospheric elements at the ocean's surface south of 40°S. In Antarctic Ocean and Resources Variability , ed. D. Sahrhage, 3–20. Berlin and Heidelberg: Springer Verlag.

Vergani, D. F., M. N. Lewis, and Z. B. Stranganelli. 1987. Observation on haul-out patterns and trends of the breeding populations of southern elephant seals at Península Valdes (Patagonia) and Stranger Point (King George Island). SCCAMLR-VI/BG/ 36: 1–9.

Woehler, E. J., and Johnstone, G. W. 1991. Status and conservation of the seabirds of the Australian Antarctic Territory. In Seabird Status and Conservation: A Supplement , ed. J. P. Croxall, 279–308. Technical Publication No. 11, International Council for Bird Preservation, Cambridge, England.