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Population declines and possible causes
The large seasonal concentrations of southern elephant seals on islands, especially during the breeding period, resulted in the development of a large sealing industry at all the sites (McCann 1980, Hindell & Burton 1988a). By the early part of the last century these sealing operations had so depleted the populations that they were reduced to uneconomic levels through most of their rnnge (Busch 1985, Hindell & Burton 1988a). Sealing ceased in the I early 1900’s, except at South Georgia where a well organised annual catch continued until 1964 (McCann 1980), and numbers at sites other than South Georgia were thought to have recovered to their original levels by the 1950’s (Carrick et al. 1962a). Since then however, there have been reports of declining populations at islands forming the Kerguelen and Macquarie Island stocks (Harrat & Mougin 1978, ~ndy 1978, Van Aarde 1980a, Skinner & Van Aarde 1983, Burton 1986, Hindell & Burton 1987, Taylor & Taylor 1989), while those of the South Georgia stock, or more specifically South Geo~gia itself, have remained stable (McCann & Rothery 1988). Recent reports (SCAR 1991) also suggest a stabilisation of the Kerguelen population between 1984 and 1989.
Reproductive success and maternal investment
One aspect of the southern elephant seal’s life cycle where a weakness may contribute to the decline is that of reproduction. The hypothesis put forward by Skinner & ·van Aarde (1983) suggesting that the numbers of bulls at Marion I~land were insufficient to fertilise all the breeding cows, focused attention on the process of pup production in southern elephant seals at Marion Island. The successful production, and rearing, of offspring to independence provides an indication of the evolutionary value of an individual female (Wittenberger 1981). How successful the female is, has been seen in other seals to be influenced by such variables as her age, size and social status (Reiter, Panken & Le Boeuf 1981, Mc~nn 1982, Huber 1987, Ribic 1988). The contribution of the female to future generations can also be influenced by what sex offspring she produces. Variance in reproductive success among males in polygynous species is usually greater than amongst females (see Clutton-Brock 1988). In both southern elephant seals and the congeneric northern elephant seal, M angustirostr~, a male that is successful in mating for just one season can father more offspring than a female can in her entire lifetime (Le Boeuf & Reiter 1988), and thus can produce more grandoffspring for its own mother than a female could. However, few males will ever get the chance to breed, while virtually all females that reach maturity will produce offspring (Le Boeuf & Reiter 1988). Theoretically there is then a conflict in which sex to produce.
CHAPTER 1 : GENERAL INTRODUCTION
Life history
Identification of stocks
Population declines and possible causes
Reproductive success and maternal investment
Objectives of the study
CHAPTER 2: S’I’UDY AREA
Climate
Topography
Beach topography
Trypot Beach
CHAPTER 3 : THE TIMING OF EVENTS WITHIN THE REPRODUCTIVE CYCLE OF SOUTHERN ELEPHANT SEALS AT MARION ISLAND
Introduction
Methods
Results
Discussion
CHAPTER 4: POPULATION BIOLOGY
Introduction
Methods
Results
Discussion
CHAPTER 5: FACTORS AFFECTING REPRODUCTIVE SUCCESS
Introduction
Methods
Results
Discussion
CHAPTER 6 : FACTORS AFFECTING SEX OF OFFSPRING AND IT’S CONSEQUENCES Introduction
Methods
Results
Discussion
CHAPTER 7 : CONCLUSIONS
SUMMARY
OPSOMMING
REFERENCES
