Seal hunting with the Great White Shark
Researchers record the behaviour of these threatened hunters with underwater cameras and motion sensors
The great white shark is one of the most fascinating marine animals on earth. Although many films and books portray it as an insatiable predator, in fact virtually nothing is known about its hunting behaviour. An international team of scientists, including researchers from the Max Planck Institute for Ornithology in Radolfzell, have now observed white sharks hunting seal off the coast of South Africa. The movement patterns recorded using cameras and sensors show that, contrary to previous assumptions, the animals venture into kelp forests and hunt for seals there. A knowledge of how white sharks react to their prey and what role their environment plays should help prevent future accidents involving humans.
Until now, scientific observation of sharks hunting prey animals such as seals or sea otters took place mostly at the surface of the water - anything taking place under water remained hidden. With the help of new underwater cameras and GPS transmitters however, it is now possible to gather detailed information about the behaviour of hunters and prey, as well as the influence of their habitat.
Researchers from California and Radolfzell have been studying the great white sharks in the marine reserve around Dyer Island off the coast of South Africa. In order to fit the sharks with cameras and transmitters, they used bait to lure the animals, which measure up to five metres long, to their boat. While the sharks were busy with the bait, the researchers used rods to attach small clip-on cameras to the dorsal fin. Using these cameras and the integrated motion sensors, scientists can record the habitats through which the sharks are travelling, and where they hunt. After three days, the cameras release from the dorsal fin and float to the surface, where they can be collected and evaluated.
Hunting in kelp forests
There are a large number of kelp forests in the waters around Dyer Island, and the sharks there show a different hunting behaviour compared to others of the same species living in areas without kelp forests: they hunt not only at twilight, as is usual, but also in daylight. They also appear less frequently at the surface of the water.
In the past, researchers have assumed that the underwater forests form a barrier to sharks, providing a refuge for seals. However, an analysis of the underwater videos paints a rather different picture: the sharks off Dyer Island swim quite happily into the kelp forests to hunt for Cape fur seals. “It could be that the sharks have a variety of individual hunting strategies, or perhaps they have developed locally varying hunting traditions. If this is the case, our results would be the first evidence of local traditions in fish,” explains Martin Wikelski, Director at the Max Planck Institute for Ornithology. The researchers now want to track the sharks throughout their lifetime to learn more about the animals' behaviour.
Tracking marine animals
The small cameras and sensors have been specially developed as part of the Icarus project, led by Martin Wikelski. Scientists taking part in the Icarus project aim to track migrating animals around the globe. They can then use the data gathered to study the spread of animal diseases, for example, or even climate change. Using Icarus, researchers can also record fish behaviour, such as salmon migration, locating the cameras in the sea once they have released from the fish, and analysing the captured video. “Our success using these cameras on the sharks shows that we can also use Icarus to study the migrations of marine animals,” says Wikelski.
Like other ecosystems the oceans loose biodiversity at an alarming rate. Although it is not known exactly how many white sharks there are worldwide, the animals are classified as endangered. Their numbers are only recovering slowly, as animals only become sexually mature some years after birth. Although white sharks cannot be fished commercially, the killing of individual animals to prevent attacks is permitted in some countries. “In order to protect the sharks, it is therefore important to better understand their behaviour, so as to avoid unwanted encounters between sharks and people as far as possible," says Wikelski.