The Prince Edward Islands are two small, uninhabited islands in the subantarctic Indian Ocean that are part of South Africa. The islands are called Marion Island (named after Marc-Joseph Marion du Fresne, 1724-1777) and Prince Edward Island (named after Prince Edward, Duke of Kent and Strathearn, 1767-1820). Marion Island is located in the south of the Indian Ocean, on the path of the Antarctic Circumpolar Current. Halfway between South Africa and Antarctica, Marion Island is the land base for southern elephant seals Mirounga leonina (SES), subantarctic sea lions Arctocephalus tropicalis (SAFS), Antarctic sea lions Arctocephalus gazella (AFS) and killer whales Orcinus orca (KW).
Long-term monitoring of these major predators on Marion Island allows us to measure the response of these animals to environmental change in the region. The large populations of SAFS and AFS that occur together on Marion Island are monitored to assess responses to environmental change. Long-term studies aim to detect changes in diet, food-seeking behavior, and reproductive success (using pup attendance and mass as indices). Sea lions are an abundant predator in the region and population size is increasing for Antarctic sea lions and possibly decreasing for subantarctic sea lions.
These opposing trends in population growth for these two species indicate different factors that impact these populations. Orcas resident on Marion Island could be important predators of seals and other local prey species (for example, KW intensive photographic studies conducted regularly on the island throughout the year have allowed researchers to identify important individuals, the size of the killer whale population and their food search habits. The Marion Island marine mammal program is dedicated to understanding how these populations are changing and how these populations are linked to each other and to the great ocean. Marion Island seal populations were historically exploited for fat collection from the early 1800s to the mid-1900s.
Fat harvesting ceased in the 1930s, when the population number fell too low to be economically viable. Without continuous monitoring of these populations, we would not have an indication of the extent of their recovery, nor a benchmark for future comparisons of population changes. The status of the main predator populations on Marion Island represents the health of the surrounding ocean and is an important indicator of system change throughout the Southern Ocean. The Marion Island Marine Mammal Program is a research program of the Mammal Research Institute, Department of Zoology & Entomology, University of Pretoria.