Armed with an 8 kg hoop-net in my hands I slowly creep closer – eyes locked on my target. Pleased that the wind direction is in my favour I take care not to trip over the slippery kelp that is rotting on the beach. The cold metal ring from the net stings my hands. My target is a lactating Subantarctic fur seal female (Arctocephalus tropicalis). We want to capture her and her pup to deploy a satellite tracker on the female and to tag, sex and weigh her pup. I chose this specific female because she is lying on the edge of one of the many overlapping harems of breeding individuals crowding this beach. Nico is tasked with keeping his eyes on the pup which is currently suckling from its Mother. He has to catch the pup before it scampers away into the herds of other pups. Wiam is in charge of protecting us from the testosterone-filled, territorial 100+ kg adult males. With teeth like razors and a temperament that will scare off vicious Rottweilers, they are our biggest threat. I am just beyond reach of the female when she notices me and moves to run away. ‘Run’ is not the best description, but contrary to popular belief, they are fast! The male now also sees our attempt to steal his female and charges. Wiam jumps in to jab the male away with his sealer-stick, the pup bolts in the opposite direction with Nico in pursuit. A male from the neighbouring harem attempts to steal this female for his harem, other females run in various directions, bulls from the other nearby harems all become excited and attempt to steal females – in short – chaos ensues! Nico and I keep our eyes on our respective targets. Wiam, armed with two sealer sticks, blocks lunging heads from all angles. I lunge at her with the net and place it over her; Nico manages to steal the pup. We move like a well-practiced act. This capture was successful, but they are not always.
By now you are surely wondering why we are disturbing the already stressed breeding individuals. And why do we risk life (unlikely) and limb (highly likely) to do this? It’s all in the name of science (although I will admit, the adrenaline rush is also quite thrilling). After capture, we swiftly deployed a satellite tracker on the female. This device measures her location at sea and diving depth, as well as sea-temperature and light levels. We put out several of these devices on females of new born pups. The idea is to assess a lactating females’ at-sea foraging behaviour and measure her success by the success of her pup. This is done by studying the pup’s survival, growth and eventual weaning mass. This work forms part of the research conducted by the Marion Island Marine Mammal Programme (Bester et al. 2011). Marion Island forms part of the Prince Edward Islands. It is a subantarctic island owned by South Africa and lies within the Southern Ocean. The nearest landfall is Îles Crozet, c. 950 km to the east and South Africa is c. 2000 km to the northwest.
Subantarctic fur seals, like other fur seals and sea lions, are income breeders. This means females with pups have to constantly travel between the rookery, where the pup is, and her feeding grounds at sea. The pup’s fasting capabilities limit the duration the female can stay away from her pup and the subsequent distance she can travel away from the rookery. Small, new-born pups have very little to no fat reserves – females have to forage close to the island and return after 1/2 days. As the pups grow, females can forage further away. This is also good because as the seasons change from summer to winter, food resources become scarcer and females have to travel further to gain sufficient energy. This trait makes them ideal to study as indicators of prey-availability.
The marine environment is highly dynamic, which means available nutrients, primary producers and the following predators are constantly moving and changing in a 3-dimensional space. Any changes in nutrient availability will be passed up the food chain to the top predators. This process is called bottom-up control. As a result foraging behaviour of top-predators is often used as a proxy for prey density and the health of an ecosystem (e.g. Croxall et al. 2002). There is some contention around the concept of using long-term changes in top predator foraging behaviour as a measure of large-scale environmental change, such as climate change and ocean warming (e.g. Hindell et al. 2003). However, using their at-sea location and diving data we can map areas that females prefer to forage in. If multiple individuals travel to the same areas consistently over multiple seasons, then those are consistently productive areas and of conservation concern. This data is then used to design Marine Protected Areas (MPAs). MPAs play an important role in conserving marine biodiversity, while concurrently ensuring the sustainable use of living marine resources (Lombard et al. 2007). This is particularly useful in areas where there is a lot of conflict between anthropogenic fishery activities and fish-predators. Highly mobile marine predators are vulnerable to strangling in fishing gear, depredation and resource competition (e.g. Augé et al. 2014). An MPA that prohibits fishing in that area will not only protect the fish present but also the predators that forage upon them.
These studies of foraging site fidelity and mapping MPAs is one of the many uses for data derived from animal-borne data loggers. Given that the Southern Ocean is sparsely visited and the sea-ice extent around Antarctica often hinders oceanographic data collection, seals and other marine predators have been used with great success to study one of the largest ecosystems on our planet. However, I think that is a story for next time…
For those of you interested, our female returned 9 times to her pup over the 3 months that she carried her device (fig. X). She spent on average 3 days with her pup and 6 days at sea. The pup successfully weaned in October 2014.
Augé, A., B. Chilvers, A. Moore, and L. Davis. 2013. Importance of studying foraging site fidelity for spatial conservation measures in a mobile predator. Animal Conservation: DOI 10.1111/acv.12056.
Bester, M.N., De Bruyn, P.J.N, Oosthuizen, W.C., Tosh, C.A., McIntyre, T., Reisinger, R.R., Postma, M., Van der Merwe, D.S. & Wege, M. (2011) The Marine Mammal Programme at the Prince Edward Islands: 38 years of research. African Journal of Marine Science, 33,511-521.
Croxall, J.P., Trathan, P.N., Murphy, E.J. (2002) Environmental change and Antarctic seabird populations. Science, 297, 1510-1514.
Hindell MA, Bradshaw CJA, Harcourt RG & Guinet C (2003) Ecosystem monitoring: are seals a potential tool for monitoring change in marine systems? In: Gales NJ, Hindell MA, Kirkwood R (eds) Marine mammals. Fisheries, tourism and management issues volume, Chapter 17. CSIRO Publishing, Melbourne, p 330-343
Lombard, A.T., Reyers, B., Schonegevel, L.Y., Cooper, J. Smith-Adao, L.B., Nel, D.C., Froneman, P.W., Ansorge, I.J., Bester, M.N., Tosh, C.A., Strauss, T., Akkers, T., Gon, O., Leslie, R.W. & Chown, S.L. (2007) Conserving pattern and process in the Southern Ocean: designing a Marine Protected Area for the Prince Edward Islands. Antarctic Science, 19,39-54.