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  • Shark cage diving in Gansbaai, South Africa with Marine Dynamics. Experience the exceptional and come face to face with a great white shark! 

  • The exact world record white shark is a contested issue, but chances are it is between 6-7m. In Gansbaai, the largest white shark ever caught was at Danger Point and measured up to 5.9m.

  • If you see a white shark in the water don’t panic. Chances are high that the shark has already detected you and isn’t interested. White shark attacks are normally associated with poor visibility, so avoid murky conditions.

  • White sharks have a unique system called a “counter current heat exchange”, which keeps their body  tempreture +/- 7C above the surrounding water temperature. 

  • All sharks have an incredibly unique system on the tip of their nose called the “ampillae of Lorenzini”. These are small pores filled with a gel that transmits the electrical currents in the water to the shark’s brain so that it can assess its environment.

  • White sharks give birth to live young (not eggs), and they give birth to 6-8 pups at one time. Pups are usually between 1.0-1.5m in length and are born with teeth.

  • Body language has been a well documented form of shark communication and has identified body arching, jaw gaping, and other postures as specific social tactics.

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INFOGRAPHIC - Stealthy seals use refugia to sneak by white sharks

Monday, November 17, 2014 |  0 Comment Tags: Cape Fur Seals, Dyer Island Conservation Trust, michelle jewell, predator prey , white sharks,

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This post was written by former Dyer Island Conservation Trust researcher Michelle (Wcisel) Jewell, follow her on Twitter here!

Predators are highly influential in ecosystems because of the many top-down effects they can have. The most obvious and direct way predators influence an ecosystem is by eating and reducing the number of prey animals in the system, but another equally important way is the indirect influence they have on the behaviour of prey animals.

If you have avoided parking on a risky-looking street, taken a different route between classes to avoid a bully, or abandoned a forest hike because of snapping twigs in the distance, you have been indirectly affected by perceived ‘predators’. In the wild, prey animals will also change their behaviour when they perceive that predators are around, and these altered behaviours often influence other species, ultimately shaping the ecosystem.

A great example of this happened in Yellowstone National Park when wolves were reintroduced after being absent for nearly 70 years. Naturally, elk population numbers declined since wolves ate the elk, but many researchers were surprised when the actual forest began to change shape. Elk learned that the best way to avoid wolves was to forage in the middle of open fields so that they could spot approaching wolves. This meant that elk were no longer eating plants near the forest edge, which meant that new budding plants on the forest edge could grow fully without being overly grazed by care-free large populations of elk. Previously eradicated species of cottonwood trees began to reappear in Yellowstone thanks to the indirect effects of the wolves being present and changing the behaviour of the elk.

My research has focused on these same principles of predator/prey interactions in the ocean, and a great place to study oceanic predators and their prey are Cape fur seal colonies in South Africa. Every summer (November), Cape fur seals give birth to thousands of pups, and by winter (April – September) these ‘young-of-the-year’ seals begin to venture off their islands to swim offshore to the fishing grounds with the adults. These young-of-the-years are typically slow, plump from months of a mostly fat milk diet, and – most importantly – naïve. White sharks take advantage of this naivety and aggregate around seal colonies every winter. Young-of-the-year pups are forced to learn how to avoid sharks quickly or suffer some rather permanent consequences. This means that during a full year, every seal colony goes through a period of high white shark presence (winter) and very low to no white shark presence (summer). Therefore, we are able to study how seals act ‘normally’ during the summer when there are no/very few sharks and how they change their behaviour in the winter to avoid white sharks.

Also, there are many different kinds of seal colony islands along the coast, which lets us ask more questions about how seals use their environment to avoid sharks. I conducted my study at the Dyer Island/Geyser Rock system, which is home to ‘Shark Alley’ as well as many shallow reefs, kelp forests, and shipwrecks. About 100km to the east is another seal colony called Seal Island, which is a world-famous spot to see white sharks predate on seals, but this island system lacks the abundant nearby structures/reefs/kelp forests that are present at Geyser Rock. By looking at these two different kinds of islands, we can also examine how structures – or ‘refugia’ – may alter how seals avoid white sharks at Geyser Rock from how seals avoid white sharks at Seal Island.

And here’s what we found…

Cape fur seal white shark research EDNA science Michelle Jewell Dyer Island Conservation Trust infographic

Do all of these structures and anti-predatory tactics of Cape fur seals change white shark movements around Geyser Rock? Most definitely! Check out that study (and infographic!) here.

This project was funded by the Dyer Island Conservation Trust and supported by Marine Dynamics shark tours, Volkswagen South Africa, and OCEARCH. The infographic was designed by EDNA Science. You can read the detailed scientific publication on Behavioral Ecology & Sociobiology by clicking here.

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