Eléphant de mer Austral

No recent comprehensive estimate of Southern Elephant Seal abundance is available throughout the entire distribution. The worldwide population was estimated to be 650,000 in the mid 1990s (SCAR EGS 2008). Four distinct populations have been identified in the Southern Ocean (Gales et al. 1989, Hoelzel et al. 1993, Slade et al. 1998). While the movement of breeding individuals between these populations is rare, it does occur (Fabiani et al. 2003, Reisinger and Bester 2010). These populations include subpopulations at or close to Argentina (Peninsula Valdés and the Falkland Islands), in the Atlantic sector (South Georgia, South Orkney Islands, South Shetland Islands, Bouvetøya and Gough Island), in the Indian sector (Iles Kerguelen, Iles Crozet, Heard Island and the Prince Edward Islands) and in the Pacific sector (Macquarie Island, Campbell Island and Antipodes Island). A further four historical populations are extinct, likely due to exploitation: St. Helena Island, Tristan da Cunha, Islas Juan Fernández and the Bass Strait (Bester 1980, Carrick and Ingham 1962, Wace and Holdgate 1976). A subpopulation from Victoria Land on the Antarctic coast became extinct a few hundred years prior to human discovery (Hall et al. 2006).

While Southern Elephant Seals breed at 14 islands or island groups, five of these are responsible for 99% of pup production. More than 50% of pup production takes place at South Georgia (Boyd et al. 1996, M. Fedak pers. comm. in SCAR EGS 2008), with Isle Kerguelen responsible for some 21% (Authier et al. 2011) and the Peninsula Valdes, Heard Island and Macquarie Island each responsible for greater than 5% (Lewis et al. 1998, Slip and Burton 1999, van den Hoff et al. 2014).

The Peninsula Valdes/Falkland Island colonies have increased in recent times (Lewis et al. 1998, Galimberti et al. 2001, SCAR EGS 2008). Colonies in the Atlantic sector of the Southern Ocean are also growing or stable (Boyd et al. 1996, Galimberti et al. 2001, SCAR EGS 2008, Gil-Delgado et al. 2013). In contrast, those in the Indian Ocean (Guinet et al. 1999, Slip and Burton 1999, Pistorius et al. 2004, Bester and Hofmeyr 2005, de Bruyn 2009, Pistorius et al. 2011) and Pacific Ocean (van den Hoff et al. 2007, 2014) sectors have been either stable or decreasing to the mid 1990s. However, recent cessations of decrease, and increases, have been recorded at island groups in the Indian Ocean sector (McMahon et al. 2009, Authier et al. 2011, Pistorius et al. 2011). This is not true for the Macquarie Island subpopulation, the only substantial population within the Pacific sector (van den Hoff et al. 2014). Numbers of adult females in this subpopulation declined at a mean annual rate of -0.8% between 1988 and 2011, and continued to do so through 2014.

The causes of these declines are debated. They are not completely understood and may differ between populations but evidence indicates that the low survival of adult females (Pistorius et al. 2004, de Bruyn 2009, Pistorius et al. 2011, van den Hoff et al. 2014) or of pups (McMahon and Burton 2005, McMahon et al. 2005ab) is important. Survival of both adult females (Pistorius et al. 2011, van den Hoff et al. 2014) and pups (McMahon and Burton 2005, McMahon et al. 2005b) varies interannually with climatic conditions. The low survival of adult females may be due to the effects of the limitation of food on an age and sex class that is responsible for successfully weaning offspring, and migrating to distant foraging grounds to recover from terrestrial fasts (Pistorius et al. 2011, van den Hoff et al. 2014) whereas the low survival of pups may be due to low weaning mass and also reduced food availability to naïve, and therefore vulnerable, animals (McMahon et al. 2003, 2005a).

Generation length has been calculated at 9.5 years (Pacifici et al. 2013). Population change over three generations from 1982–2009 is inferred to have been stable or positive (SCAR EGS 2008, Authier 2011, Pistorius et al. 2011, van den Hoff 2014).