A demographic comparison of two elephant seal populations

Abstract

The demography of two elephant seal populations was examined and compared. This was done to investigate the reasons for the observed decreases in populations at Marion and Macquarie islands. While a well-established demographic programme had been in place at Marion Island since the 1980's (see Pistorius et al. (l999a) for a review) one had to be established at Macquarie Island. A long-term demographic programme was initiated at Macquarie Island in 1993 and hot brands were used to mark seals. Hot-iron branding was a rapid and reliable method of permanently identifying elephant seals that did not prejudice survival and did not appear to cause undue stress (in the short-term). Neither branding nor handling showed any long-term effects as measured by survival after one year. From the inter-island comparison of survivorship, age at first breeding and wean mass I concluded that the observed decreases in elephant seal numbers between the I 950s and 1990s in the Pacific and Indian Ocean sectors were driven by resource limitation in the Southern Ocean. A conglomerate of factors including local predation by killer whales and intra-specific resource competition was postulated as a cause for the inter-island (regional) differences in population trends. Presently it appears that per capita more resources are available to the Marion Island population than are available to the Macquarie Island population. The vital rates that had the greatest impact (elasticity) on fitness (population growth) for all populations i.e. Marion Island, Macquarie Island and South Georgia, were, in order of importance: (I) juvenile survival, (2) adult survival, (3) adult fecundity and (4) juvenile fecundity. At Marion Island juvenile and adult survival contributed equally to the fitness of the population while at Macquarie Island and at South Georgia Island juvenile survival was more important than adult survival in determining population fitness. The global population of elephant seals in 200 I was estimated at approximately 738 772 which represents an increase of I I % from the last world estimate (664 000). It seems clear from the evidence presented and reviewed here that the present changes in seal populations, unlike the period of direct exploitation in the 19th and 20th centuries, are neither a consequence of direct human interactions nor present-day commercial activities. While the significance of inter-specific competition between elephant seals and other Antarctic predators remain largely unknown or quantified, it would seem prudent that these relationships be studied. This is because resource competitors (toothed whales) have been increasing in number since the cessation of commercial whaling. Even though elephant seals are considered the most studied of all pinnipeds (Ling&Bryden, 1992) much remains to be learnt. Foremost are to gain a clear understanding of the in situ, diet of elephant seals and to assess the role pathogens play in the regulation of seal populations. In addition to these studies it is also important that the current long-term monitoring programmes at Marion and Macquarie islands continue, as they provide valuable base line information on the fate of elephant seal populations.