High in the food chain – seals in the Southern Ocean
The Marion Island seal populations have been studied for the last 30 years. Cheryl Tosh and Marthán Bester describe their field research.
Deep in the Southern Ocean, Marion Island is a platform for breeding seals and seabirds such as penguins and albatrosses, with killer whales patrolling the inshore waters. Because many species to have their young on land there is always life in some form at Marion Island throughout the year. The seal species at Marion Island have their young on land and forage at sea. There are three seal species, the southern elephant seal (Mirounga leonina), the Antarctic fur seal (Arctocephalus gazella) and the Subantarctic fur seal (Arctocephalus tropicalis).
The Prince Edward Islands
The Prince Edward Islands are two small sub-Antarctic islands that are part of South Africa – Prince Edward Island and Marion Island. The island group is about 955 nautical miles (1 769 km) south-east of Port Elizabeth on mainland South Africa. Marion Island (46°54′45″S 37°44′37″E), the larger of the two, is 25.03 km long and 16.65 km wide, with an area of 290 km2 and a coastline of some 72 km, most of which is high cliffs. The highest point on Marion Island is Mascarin Peak, reaching 1 242 m above sea level. Boot Rock is about 150 m off the northern coast.
Occurring at the juncture between the African Continental Plate and the Antarctic Plate, Marion Island has been volcanically active for 18 000 years. The first historical eruption was recorded in November 1980 when researchers recorded two new volcanic hills and three lava flows. Researchers observed another small eruption in 2004.
Besides volcanic cones, Marion Island is home to Marion Base, part of the South African National Antarctic Programme. Focusing on biological, environmental, and meteorological research, the base is situated on the island’s northeastern coast.
Studying Marion Island seals
Southern elephant seals have been intensively studied at Marion Island for the last 30 years, under the leadership of Prof. Marthán Bester. Intensive mark-recapture studies followed the comings and goings of elephant seals on the island. As a result of these studies scientists are starting to understand the survival rates of different age and sex classes of the population and the importance of migration and immigration in controlling the population size. All these factors are affected by changes in food resources, which can either be shown by changes in body condition or changes in behaviour.
The behavioural responses of southern elephant seals are studied using state-of-the-art satellite relay data loggers that are attached to the animals. The devices are linked with satellites, and send data about the position of the animal, the water temperature and salinity, and the diving behaviour of the animal, which provides scientists with valuable information about the habitats that these seals are using and how their behaviour changes over time.
Southern elephant seals
Southern elephant seal behaviour shows an enormous amount of individual variation. Some individuals concentrate on feeding in highly productive and stable areas where different marine currents meet – called marine frontal structures. Others feed in short-lasting mesoscale eddies – the unstable areas of the ocean currents that surround the island. Some individuals concentrate on foraging in areas of the ocean that have a particular temperature or current or are at a specific depth. The way that this population spreads its feeding efforts in different parts of the same environment may well be the key to its long term success. Because different individuals can use a variety of habitats they may not compete directly with each other for the same resources. This variation in foraging behaviour will also help the population to withstand changes in their environment.
Recently, researchers at Marion Island have also started to measure changes in body condition of these seals using novel photogrammetrical methods. Dr Nico de Bruyn has developed a protocol using photographs taken with calibrated cameras to build a three dimensional model that shows the volume of elephant seals. Elephant seals are loyal to their birth and breeding sites, so the same animals can be photographed repeatedly in order to build up a time series of three-dimensional models, which may eventually be used to describe body mass changes in response to environmental conditions and behaviour. Researchers have already established that the larger southern elephant seal cows are less likely to breed every year at Marion Island, but in spite of this these cows probably produce more pups than smaller cows over a breeding lifetime.
While the southern elephant seals represent the heavyweights of Marion Island, the fur seal species, the Antarctic and Subantarctic fur seals, represent strength in numbers. Fur seal populations at Marion Island have exploded in the last couple of years, contrary to predictions about climate change affecting food resources negatively. These agile swimmers are often seen torpedoing along the Marion Island coastline until they haul out
at their beach of choice.
Large colonies of seals form during the breeding season, when males aggressively protect small harems of four or five females. Some beaches are densely packed with harems and males are not able to cross another male’s territory once it is established. Any male crossing a territory is seen as a challenger and will be viciously attacked. Such attacks can be fatal. The high drama of the breeding season abates as soon as most of the females have pupped or been mated. After the harems break up the females that have suckled their pups for a few days will leave to forage. The little pups then have a a long wait until their mothers return, during which time they have to fend for themselves. The length of these foraging trips can be an indication of the mother’s foraging success – those that return earlier have found the food they need faster than those who are away for longer. Researchers at Marion Island regularly weigh fur seal pups throughout their suckling period. This growth monitoring, combined with oceanographic indicators, may shed some light into why these animals are so successful at Marion Island.
Researchers are able to monitor where these animals are foraging using satellite relay data technology. We can find out what they have beeing eating by analysing fish ear bones (called otoliths), squid (cephalopod) beaks and other remains sampled from droppings (scats) collected on the beaches. The otoliths and cephalopod beaks are specific to different species.
Orcas – mammalian predators in the sea
While the seals of Marion Island rely on food from the sea, they form an important link in the food chain for yet another large mammalian predator being studied at Marion Island.
Marion Island’s killer whale (Orcinus orca) population feeds on a wide range of prey species that haul out on the island to have their young. The killer whales have been seen hunting penguins, fur seals and even elephant seals. These versatile predators are capable of adapting their hunting strategies to the available prey. They may ambush penguins by hiding out of sight while a lone killer whale chases the penguins in the direction of the waiting pod. They also lay siege to seal colony beaches, waiting for elephant and fur seals to leave their beaches. These abundant food resources mean that killer whales are always around Marion Island in the summer. Individuals and family groups have been identified over years of research and intensive observations. It appears that the same individuals are present at Marion Island for large parts of the year. During winter the whales leave the island, and satellite telemetry is used to find out where these animals are going during the winter months.
Every year, as part of the annual overwintering team, a team of biologists braves gale force winds, ice pellet storms and torrential rains to monitor, weigh and observe southern elephant seals, fur seals and killer whales.
A typical day for a seal researcher includes waking up as the sun rises, looking out of the window to check the weather, having breakfast, looking out of the window to check the weather, packing your backpack and pulling on your gumboots and looking out of the window to check the weather. Eventually it is time to step out of the door – whatever the weather. On most days, researchers will walk along the Marion Island coastline checking every beach for southern elephant seals. Every elephant seal is then checked for a flipper tag and the colour and number is noted in order to identify individuals. This often involves traversing rocky beaches in bad weather, using rope ladders to climb down cliffs and wading through penguin colonies splattered with dropping in order to get to elephant seals.
Once a month, seal researchers brave needle sharp fur seal teeth while weighing 100 fur seal pups. The first weighing just after the pups are born is easy, with most pups weighing in at around 3 kg. Towards the end of the suckling period, the healthy, agile survivors weigh in at between 10 – 15kg. As if sprinting over treacherous terrain in gumboots to catch pups is not enough, researchers will have weighed over a ton of fur seal pups by the end of the day. Some researchers have also received nips from concerned fur seal mothers in recognition of their efforts on the beaches.
Southern elephant seals and fur seals have satellite transmitters fitted. But researchers have to wait for good weather before they can do this – not something that happens very often.
Satellite transmitters can only be deployed when the seals’ skins are dry. Southern elephant seals are immobilised using specialised drugs, which means that all researchers have to do a wildlife immobilisation course before going to the island. The drugs are given using a spinal needle, a length of drip tubing and a syringe. Only when the seal is immobilised does the deployment team approach the animal with the device and the epoxy resin. They then stick the device onto the seal while monitoring its breathing. They wait until the seal has recovered fully and is able to move independently.
Catching a fur seal in a hoop net requires stealth and finesse. The scientist spots a fur seal mother from a distance while she is basking in the sun, goes into stealth mode, holding the hoop net at the ready for last 10 m of the ambush. The fur seal mother looks up, sees the scientist and runs away… attempt aborted! All the fur seals on the beach are now alerted to the intruder and some flee to the sea. The scientist now has to try at another beach. Eventually, after perhaps three or four attempts, the scientist manages to catch a fur seal mother unawares, deploys a transmitter, weighs her and releases her – and there are still five more transmitters that have to be deployed…
The Marine Mammal Research Programme at Marion Island would not be in existence without the dedication and determination of researchers prepared to leave their families and homes in order to conduct research for 13 months on a remote island in the middle of the Southern Ocean. These dedicated scientists are responsible for collecting data that provide valuable insights in the biology of seals and killer whales in the Southern Ocean. Seal research on Marion Island is not for the faint hearted, but every scientist that has had the privilege to be involved with the programme will tell you that the rewards are endless – a real life-changing experience. A beautiful sunset, an elephant seal giving birth, a fur seal porpoising through the water or watching killer whales resting for two hours almost within touching distance make it all worthwhile.
Cheryl Tosh currently studies the foraging behaviour of Marion Island’s southern elephant seals. She spent 13 months on Marion Island as a seal researcher and is a post-doctoral fellow at the University of Pretoria. Prof. Marthán Bester heads up the Marion Island Marine Mammal Programme. His research is focused on the biology of marine mammals at Marion and Gough Islands.