Geographic variation in subantarctic fur seal pup growth: linkages with environmental variability and population density

• Published in: Journal of Mammalogy
• Main Authors: Oosthuizen, W. Chris, de Bruyn, P. J. Nico, Wege, Mia, Bester, Marthán N.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2015
• Subjects: Nature and Landscape Conservation; Genetics; Animal Science and Zoology; Ecology; Ecology, Evolution, Behavior and Systematics; Gough; Amsterdam Island; Marion Island
• Online Access: http://dx.doi.org/10.1093/jmammal/gyv181; http://academic.oup.com/jmammal/article-pdf/97/2/347/38268357/jmammal_97_2_347.pdf

Abstract Marine predator populations are sensitive to temporal variation in prey availability, but prey dynamics are often difficult to quantify. Long-term measures of offspring growth is a useful performance attribute to gauge the potential demographic direction for such predator populations, especially where other metrics (e.g., population size estimates) are lacking. Subantarctic fur seal ( Arctocephalus tropicalis ) females are central place foragers during a protracted lactation period, and their foraging success determines the growth and vitality of their offspring. Using data spanning over 2 decades, we assessed geographic and temporal variation in growth rates and weaning mass of subantarctic fur seal pups at 2 of the species’ principal populations (Gough and Marion islands) and identified environmental conditions that may, through assumed bottom-up mechanisms, affect body mass at weaning. While Marion Island pups grew at an average rate of between 0.040 and 0.067kg/day early in lactation (comparable to conspecific growth at Amsterdam Island), the mean growth rate at Gough Island (approximately 0.030kg/day) was lower than the growth rate represented by the bottom 5% of the body mass distribution at Marion Island. Notwithstanding substantial interannual variability, we found support for a negative trend in weaning mass at both populations, suggesting a rise in limiting factors that is hypothesized to relate to concurrent local population size increases. Weaning mass tended to be higher when sea surface temperatures were warmer (with a stronger positive effect at Gough Island) and during positive phases of the Southern Oscillation Index (La Niña events), with a stronger positive effect in males. Given the low weaning mass of Gough Island fur seal pups, continued population growth here seems unlikely. While density-dependent regulation appears to have increased in strength at Marion Island, terminating rapid population growth, current weaning weights remain above the physiological limits of growth in ...