Demography of two sympatric fur seals (Arctocephalus gazella and Arctocephalus tropicalis) : intrinsic and extrinsic determinants of survival

Population ecology is key to understanding a population’s behaviour over time, and is important in the development of population viability analysis. To develop robust and accurate population survival estimates, an understanding of the intrinsic (densityregulating) and extrinsic (environmental) factors determining individual survival rates is needed. Here a unique 26-year capture-mark-recapture study on two sympatric fur seal species, Antarctic fur seal (Arctocephalus gazella) and subantarctic fur seal (Arctocephalus tropicalis) from Macquarie Island, was used to quantify demographic responses of animals to intrinsic and extrinsic factors. Quantifying survival rates requires individuals within a population to be identifiable and monitored throughout their lives. However, A. gazella and A. tropicalis have no easily identifiable natural markings, therefore individuals need to be marked with an individually numbered plastic flipper tag. Despite their utility in demographic studies, flipper tags can be lost during an animal’s lifetime leading to under-estimates of individual survival. Consequently, estimating tag loss is a central component of estimating survival rates; if the tag loss rate is underestimated the survival rate is negatively biased and vice-versa. All seals were tagged with at least two tags, one in each flipper, and some with a subcutaneous radio-frequency identification tag as a form of long-term identification, allowing dependent tag loss rates to be calculated. Using a Bayesian approach these data were analysed to quantify the rate of tag loss. Tag loss was age-dependant, with pups having a higher probability (0.4 (95% credible interval: 0.19, 0.58)) than juveniles (0.08 (0.005, 0.23)) and adults (0.04 (0.008, 0.08)) of losing both tags. There was little evidence for differences in the tag loss rates between the sexes and species. The tag loss estimates from this study provided an opportunity to accurately quantify how survival is affecting population growth rates in this unique population that is growing at a substantially lower rate than other similar colonies. The Macquarie Island fur seal population is unique among other fur seal populations in that three species of fur seal A. gazella, A. tropicalis, and New Zealand fur seal (Arctocephalus forsteri) occur on the island, and 17-30% of the population have been identified as hybrids of these three species. It has been suggested the slow growth rate of the Macquarie Island fur seal population may be in part due to the high rates of hybridisation between the three species. With genetic profiles from mitochondrial DNA and microsatellites to distinguish pure species (n = 682) from hybrids (n = 208) hybrid survival probabilities and population growth rates were then compared within a Bayesian framework with those of pure-breed A. gazella and A. tropicalis to estimate if there was a survival cost associated with hybridisation. Modelling showed hybrids had comparable survival rates to pure-breds of both species throughout all age classes; pups, juveniles and adults. This suggests that the survival cost associated with hybridisation was not a key factor in the population’s slow growth (3.4% for A. tropicalis and 8.6% for A. gazella). Examination of other potential factors which may limit the population growth rate of A. gazella and A. tropicalis found the populations low numbers and high predation risk from New Zealand sea lions (Phocarctos hookeri), in addition to distance from other breeding colonies were key factors in the slow population growth rate. With these extrinsic (tag loss) and intrinsic (hybridisation) sources of survival bias quantified, the effect of environmental variability on survival was then determined for A. gazella and A. tropicalis. Despite occupying a similar at-sea niche (having the same prey and foraging behaviour), A. gazella and A. tropicalis had different survival responses to sea surface temperature, sea level pressure (an indication of the Southern Oscillation Index), and wind speed; which was attributed to differences in life lactation strategy. Adult A. gazella, with a lactation period averaging four-months, had a higher survival probability, than A. tropicalis with a nine to eleven-month lactation period, during periods of high sea level pressure, warm sea surface temperature, and high wind speed. This suggests A. gazella’s ability to migrate and forage away from the breeding colony, for eight months of the year, allows them to exploit more highly productive waters further from Macquarie Island and may confer an advantage. The contrasting survival responses of these sympatric species to wind speed, sea level pressure, and sea surface temperature suggest as the Southern Ocean warms the A. gazella population at Macquarie Island will continue to increase while A. tropicalis, which are listed as endangered within Australia territory, are at risk of population decline over time