Long-term evaluation of pup growth and preweaning survival rates in subantarctic fur seals, Arctocephalus tropicalis, on Amsterdam Island

Authors: Chambellant, Magaly; Beauplet, Gwénaël; Guinet, Christophe; Georges, Jean-Yves

Source: Canadian Journal of Zoology, Volume 81, Number 7, July 2003 , pp. 1222-1232(11)

Publisher: NRC Research Press

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Abstract:

This study is the first to investigate pup preweaning growth and survival rates over seven consecutive breeding seasons in subantarctic fur seals, Arctocephalus tropicalis, on Amsterdam Island, southern Indian Ocean. Growth and survival were studied in relation to year and pup sex, birth date, birth mass, and growth rate at 60 days of age. The pup growth rate decreased over the 7-year study period and was the lowest ever found in otariids, which suggests that lactating females experience constant low food availability. Male and female pups grew and survived at similar rates. Pups that were heavier at birth grew faster and exhibited better early survival (i.e., the first 2 months of life) than pups that were lighter at birth. However, no such relationship was detected for late survival (i.e., from 2 months to weaning) in this long-lactating species. No relationship was found between pup growth rate, pup survival rate, and sea-surface temperature (SST) gradient during the study period, especially during the later years of good trophic conditions (i.e., a high SST gradient). Such dissociations suggest that variation in food availability may not be the only factor influencing pup performance until weaning. We therefore propose that the subantarctic fur seal population is reaching its carrying capacity and that a density-dependent effect is occurring on Amsterdam Island.

Les taux de croissance et de survie pré-sevrage des petits de l'otarie à fourrure subantarctique, Arctocephalus tropicalis, ont été étudiés pour la première fois durant sept saisons de reproduction consécutives sur l'île Amsterdam. La croissance et la survie ont été mises en relation avec l'année, le sexe, la date de la naissance, la masse à la naissance et le taux de croissance à 60 jours. Durant ces sept années, le taux de croissance des petits a diminué et les valeurs obtenues sont les plus basses rapportées chez les Otaridés, ce qui laisse croire à une disponibilité alimentaire constamment faible pour les femelles nourricières. Mâles et femelles grandissent et survivent à un taux similaire. Les otaries plus lourdes à la naissance grandissent plus vite et survivent mieux que les plus légères au cours des deux premiers mois de leur vie. Cependant, chez cette espèce à allaitement prolongé, la masse à la naissance n'a aucun effet sur la survie de deux mois jusqu'au sevrage. Aucune relation n'a été trouvée durant l'étude entre taux de croissance, taux de survie et gradients de SST (température de surface de la mer), notamment lors des dernières années de bonnes conditions trophiques (fort gradient de SST). Ces dissociations nous amènent à conclure que cette population d'otaries à fourrure subantarctique atteint sa capacité limite et qu'un phénomène de densité-dépendance se produit sur l'île Amsterdam.

Document Type: Research Article

Publication date: July 1, 2003

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