Tagged seal

Stealthy seals sneak past white sharks using subsurface structures

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.

Video: Raw footage of great white shark and seal decoy

The video above was posted by YouTube user TheMrborto and features what looks like a relatively younger white shark interacting with a seal decoy in Klein Bay, South Africa. It’s unclear from the video what the motivation is behind luring the shark with the decoy.

Whether this is an attempt to study white shark behavior or not, the decoy certainly seems to have captured the interest of the shark in this particular video.

Video: white shark feeding on seal footage from NatGeo’s Great Migrations

WARNING: Video features graphic footage of white sharks feeding on a seal.

National Geographic recently posted the above preview footage from the upcoming November 7 episode of Great Migrations, which features some dramatic footage of white sharks feeding on a seal. The narration plays up the drama a little bit with its foreboding tone and lines like “gruesome collision” and “feast for one and death for another.” However, the footage speaks for itself.

UPDATED: As it turns out, this footage is indeed of an actual natural predation event that occurred at Isla de Guadalupe. Thanks to Andy Murch of ElasmoDiver.com for the information!

It is unclear as to whether what we see in the film was a true predation event or whether a seal carcass was dumped in the water to get the shot. The editing of the clip which features multiple sharks, (at least one of which does not appear in any shot with the seal carcass) and the up-close nature of what appears to be the initial attack on the seal make me wonder if the seal was already dead when the first shark bite occurred. If anybody who worked on this footage could share some insight on the event, your comments would be greatly appreciated. Regardless of whether it was the result of predation or scavenging, the event produced some amazing footage. (see update above)

The episode of Great Migrations premieres on National Geographic Channel on November 7, 2010 at 8PM.

Video: white shark predation on seal off Provincetown

A great white shark predation event on a seal was captured on video by a Massachusetts’ family. According to WPRI, the event occurred 20 yards from shore, off the coast of Provincetown last week. Estimates put the white shark at approximately 15′ (4.5m) in length according to the report.

While the WPRI video report refers to the feeding event as “vicious” and “violent,” it is also noted that it is part of “the cycle of life.” Seals often serve as a natural food source for larger great white sharks.

Sesationalizing a great white shark feeding on a natural prey item

The Tribune of San Luis Obispo serves up some sensationlism in the article, ,13-foot shark slashes seal while boaters watch off Cambria; After fishing and diving off Cambria, North County trio gets a chilling, up-close look at a feeding great white. The story details the account of three divers who witnessed a white shark feeding on a seal off the coast of Cambria, Ca. The divers witnessed the event while aboard a Zodiac. While the article itself remains fairly objective and is well-written, the headline and secondary title take a natural feeding event and turn it into a “chilling” encounter for the witnesses, as the shark “slashes” through the seal. Can’t a white shark just eat a meal without it being likened to a scene from a horror movie? I’m glad my meals aren’t documented by the media, I can see the headline now…

Countless Multi-grain Cheerios savagely slashed in kitchen
Witnesses watch in horror as chilling situation unfolds