The Dorsal Fin – Shark News

Stealthy seals sneak past white sharks using subsurface structures

Nov.23, 2014 at 3:47 pm, under Shark News Stories

The following was shared with us by shark researcher Michelle Jewell…


Stealthy seals use subsurface structures to sneak by white sharks

Written by Michelle Jewell (@ExpatScientist)

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.

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.


Research Conducted by Marine Dynamics a Shark Cage Diving Operator in Gansbaai South Africa

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!

For more information, check out the detailed scientific publication in Behavioral Ecology & Sociobiology.


Big thanks to Michelle for sharing her research and awesome infographic! For more from Michelle check out her Michelle’s Expat Scientist blog.

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Video: Orcas hunt and kill tiger shark off Cocos Island

Sep.10, 2014 at 1:03 pm, under Shark Videos

Vimeo user Edwar Herreño recently added a video documenting six killer whales (Orcinus orca) hunting a tiger shark (Galeocerdo cuvier) off Cocos Island. The orcas appear to use a team effort to huntand appear and then drown the shark in the video. The video description says that the orcas fed on the shark after killing it.

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Discovery’s ‘Shark of Darkness: Wrath of Submarine’ draws criticism for being just awful

Aug.13, 2014 at 2:33 pm, under Opinions in the media

Much like last year’s mockumentary “Megalodon: The Monster Shark Lives”, Discovery Channel’s first episode of this year’s Shark Week has come under fire for it’s fictional account of “Submarine,” a giant man-eating great white shark with roots in a South African urban legend. “Shark of Darkness: Wrath of Submarine” plays out like a documentary, though it’s actually just an account of fictional events, plagued with less-than-convincing acting and special effects, in the eyes of many viewers.

Twitter was lit up with disgruntled viewers on Sunday night when the episode premiered, and numerous online media outlets have since voiced their distaste in Discovery’s decision to peddle out another faux “documentary.”

In Discovery’s defense, the show did contain the following vague disclaimer.

Events have been dramatized, but many believe Submarine exists to this day.

While most viewers realized from the get-go that this is a piece of fiction, others bought into it as a real-life account of a ‘monster shark’ with an appetite for humans. “Submarine” was noted to have an “insatiable taste for human blood,” and had adapted methods to attack humans more efficiently.

Is this really the kind of message Discovery Channel should be sending its viewers about sharks?


You can check some other opinions about Shark of Darkness by following the links below.

Atlantic White Shark Conservancy’s “We Need to Talk about Shark Week.”

Vox’s “Shark Week is once again making things up”

Gawker’s “Shark Week Returns With Its Lies”

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PBS digital short: “What if There Were No Sharks?”

Aug.11, 2014 at 3:36 pm, under Shark Videos

The latest episode of the PBS digital short series “It’s Okay To Be Smart” focuses on the important that sharks play in the environment and ponders the question of what would happen if there were no more sharks. During the video a counter representing an estimated number of sharks killed runs to give the viewer an idea of the rate at which sharks are being killed off. For a relatively quick watch, the video does a decent job at summarizing the value that sharks have in the world.

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Video: great white shark encounter off Ocean City, MD

Aug.05, 2014 at 2:07 pm, under Shark Videos

YouTube user Aaron Caplan documented an encounter with an adult white shark (Carcharodon carcharias) 6 miles off the coast of Ocean City, MD. According to the video description, the shark was estimated at 13′-15′ in length and remained around the boat for approximately an hour. The shark mouthed the boat and engine before eating a chum bag. Caplan and his boatmates fed the shark a yellowfin tuna (Thunnus albacares) carcass, and then it left the area.

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