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

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.

Interannual variation in the post‐weaning and juvenile survival of subantarctic fur seals: influence of pup sex, growth rate and oceanographic conditions

“…3). This result is consistent with that found during the rearing period (Chambellant et al . 2003), and confirms that the growth and survival of males are affected more strongly by environmental conditions than that of females in polygynous and sexually dimorphic mammals (Clutton‐Brock, Albon & Guiness 1985; Rose et al .…”

“…2003) and the factors that could influence this demographic parameter (Pomeroy et al . 1999; Chambellant et al . 2003).…”

“…We expect to observe such a relationship in otariid seals, which should confer some selective advantages in the heavier‐at‐weaning individuals. For example, as individual birth weights of subantarctic fur seal ( Arctocephalus tropicalis Gray) pups are correlated positively with their probability of early preweaning survival (Chambellant et al . 2003), it seems likely that higher weaning mass could also enhance their probability of post‐weaning survival.…”

Foraging ecology of subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island: seasonal changes in relation to maternal characteristics and pup growth

“…Furthermore, the greater mean seasonal trip duration (see Fig. 2) compared to that previously reported in the same colony (Georges & Guinet 2000a is in accordance with the finding of a long-term trend in decreasing pup growth performances throughout the decade (see Chambellant et al 2003), suggesting that the subantarctic fur seal population on Amsterdam Island may be reaching its carrying capacity (Chambellant et al 2003).…”

“…Mother-pup pairs were selected at random from this group of known-age pups, between mid-April and end of July 1999, and from mid-January to mid-March 2002, respectively. During the 1999 and 2002 breeding seasons, we observed similar patterns of seasonal sea surface temperature (SST) changes (see 'Results'), and the pup growth rates were also similar (Chambellant et al 2003). We therefore considered the 1999 and 2002 sampling periods as a single pup-rearing period.…”