Geographic variation in subantarctic fur seal pup growth: linkages with environmental variability and population density
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, espec...
Published in: | Journal of Mammalogy |
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Main Authors: | , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
Oxford University Press (OUP)
2015
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Subjects: | |
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 |
Summary: | 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 ... |
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