The Australian fur seal (Arctocephalus pusillus doriferus) population has displayed a relatively slow rate of recovery since being hunted by commercial sealers during the early 19th century. Despite this, population abundance doubled in the past 2 – 3 decades, indicating that the population growth rate has recently increased. Yet, the factors influencing the population’s dynamics are poorly understood, primarily because basic demographic rates are unknown. / Female age, survival, fecundity, breeding and physiology were studied at Kanowna Island, Bass Strait, Australia, between 2003 – 2006 by conducting censuses and captures (n = 294). Mark-recapture estimates of pup production were used to validate direct pup counts, allowing a 9-year dataset to be used for calculation of the population growth rate (2.2% p.a.) and investigation of environmental influences on reproductive success. Annual pup production (x = 3108) was synchronous, with 90% of births occurring within 28 days of the median birth date of 23 November. Births occurred earlier in years when pup production and female body condition were high and these factors were correlated with local oceanographic indicators, suggesting that reproductive success is constrained by environmentally-mediated nutritional stress. / Pregnancy was assessed by blood plasma progesterone radioimmunoassay and the pupping status of the same females was observed during breeding season. Despite high mid-gestation pregnancy rates (x = 84%), the birth rate was lower than in other fur seals (x = 53%), suggesting that late-term abortion is common. Lactating females were less likely to pup, indicating that nutrition may be insufficient to support concurrent lactation and gestation. / Age and morphometric data were used to construct body growth, age structure and survivorship models. Adult female survival rates were similar to those of other fur seals (x = 88.5%). A life table was constructed and its age-specific survival and fecundity rates used in a Leslie-matrix model to project the population growth rate (2.2% p.a.) and determine the relative influence of each parameter. The abundance of female non-pups was 6 times greater than that of female pups, which is approximately 50% higher than previous conversion factors used to extrapolate population abundance from pup censuses in Australian fur seals. However, if the non-pup sex ratios of other otariids are considered, the pup:population conversion factor is 4.5. / Compared to a study performed before the recent population increase, the modern population displays similar body growth and fecundity rates but higher survival rates. This suggests that recent population growth resulted from a relaxation of hunting and/or predation mortality rather than from increased food availability. The low population growth rate is attributed to a low birth rate associated with nutritional stress, yet despite this, body growth occurs rapidly. Such characteristics are typical of sea lions rather than fur seals, perhaps because Australian fur seals employ the typical sea lion strategy of using benthic foraging to exploit a continental shelf habitat. The effect of ecological niche on population dynamics in the Otariidae is discussed.
| Identifer | oai:union.ndltd.org:ADTP/269972 |
| Date | January 2009 |
| Creators | Gibbens, John Robert |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
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