The estimation of baseline population data is an important ecological question, but there is often little quantitative information to support historic population estimates. In these cases, the use of ad hoc or opportunistically-collected historical data may help develop estimates of baseline ecosystem conditions. Throughout the 19th century, South Georgia served as the epicenter for sealing and whaling activities in the Atlantic Sector of the Southern Ocean. During this time, Antarctic fur seals (Arctocephalus gazella) – an economically and ecologically valuable species – were virtually eliminated from the island. Since the cessation of fur sealing approximately 100 years ago, populations have rebounded remarkably; fur seals are now so common that concern has shifted from their conservation to the environmental damage they create at high densities. However, we do not know if the population has recovered to, or perhaps exceeded, pre-exploitation levels. To model the impact of the fur sealing industry in South Georgia, we used ship logs to recreate a time series of sealing activity and developed an index of harvesting effort spanning 123 years, detailing 205 individual sealing voyages. Using this data, we developed an age-structured, density dependent model of the harvest and population dynamics of the Antarctic fur seal.
Results/Conclusions
Using an Approximate Bayesian Computation framework, published demographic data and harvest data from ships logs, we obtained baseline pre-exploitation population estimates. Additionally, yearly estimates of survival and reproduction for this population throughout the harvesting period were generated. We find that the use of Approximate Bayesian Computation methods to simulate the dynamics of historical populations allows for an estimation of baseline ecological conditions and fur seal populations, but rely strongly on demographic rates. On South Georgia, the population recovery and potential expansion of Antarctic fur seals on South Georgia has important ecosystem implications. First, it is hypothesized that the fur seal population may be exploiting the niche left vacant following the decimation of southern whale populations in the region, and could be inhibiting the recovery of some species of baleen whale. Second, seals are often considered ecosystem engineers, known to trample vegetation which is critical breeding habitat in sub-polar ecosystems. This is particularly important in South Georgia, as a multi-million dollar restoration program has just been completed.