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Agents of seal: stealthy seals use subsurface structures to sneak by sharks

michelleMichelle Jewell is a Zoologist specialized in predator/prey behaivour and the Scientific Communicator for EDNA Interactive.  She has spent the past 4 years studying the behaviour of white sharks and Cape fur seals at Geyser Rock, ‘Shark Alley’, South Africa.  

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:

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.

Also, check out our previous study about how white sharks move around and utilize this unique habitat.

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.