Abstract:
Recent pup population estimates of sympatric Subantarctic (Arctocephalus tropicalis)
and Antarctic fur seals (A. gazella) at Marion Island are presented. Published
pup population estimates of A. tropicalis (1995 and 2004) with an unpublished total
island count in 2013, and annual counts on subsets of rookeries (2007–2015) were
analyzed using a hierarchical Bayesian model. The pup population declined by 46%
(95% credible interval CI: 43%–48%) between 2004 (mean = 15,260, CI: 14,447–
16,169 pups) and 2013 (mean = 8,312, CI: 7,983–8,697), mirrored by a 58%–60%
decline at rookeries counted annually (2007–2015). Population decline was highest
at high-density west and north coast rookeries, despite negligible change in female
attendance patterns, pup mortality or median pupping date over the previous 25 yr.
A better understanding of foraging behavior and its effects on reproductive success
and survival in this A. tropicalis population is needed before we can attribute population
decline to any external factors. In contrast, total island counts of A. gazella pups
in 2007, 2010, and 2013, suggest that this population is still increasing although
the annual intrinsic rate of population growth decreased from 17.0% (1995–2004,
744 pups) to 4.0% (2010–2013, 1,553 pups). The slowed growth of A. gazella is
likely the result of saturation at the main rookery
