Tagged shark research

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.

White shark dorsal fin study – abridged version (infographic)

In case you don’t have time to read through the Dyer Island Conservation Trust press release regarding the study of white shark (Carcharodon carcharias) estimates based on dorsal fin identification, Marine Dynamics has provided a handy infographic that summarizes the findings of the study.

Over 20,000 photographs of great white sharks, taken between 2007 and 2012, were used in the study. Dorsal fin recognition was used to identify 532 individual white sharks off of Gansbaai, South Africa. Using this data, researchers extrapolated that Gansbaai’s total estimated white shark population is between 808 and 1008.

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

Recent study on global shark mortality rates

A recently published study on global shark catch and mortality rates by researchers at Dalhousie University in Halifax, Canada has been popping up in the mainstream news media. Many news agencies are reporting that the gist of the study is that “100 million sharks are killed every year.”

However, the report goes on to say that number of sharks killed annually could fall into the range of 63 million to 273 million. The study also estimates that between 6.4% and 7.9% of the global shark population are killed each year. This range exceeds an estimated rebound rate of 4.9% for many species of sharks, based on life history data of 62 different species of sharks. If these estimates are correct and the mortality rate exceeds the rebound rate for a particular species, a population decline in that particular species would be inevitable.

For more information, check out the full journal article, “Global catches, exploitation rates, and rebuilding options for sharks” by Worm et al.

Great white shark tracking data surprises researchers

Tracking data from SPOT tags report the whereabouts of tagged white sharks.
Tracking data from tagged great white sharks has been surprising some researchers.

The travels of great white sharks (Carcharodon carcharias) equipped with the SPOT tags in both the Atlantic and Pacific Ocean have surprised researchers over the past month or so.

According to OCEARCH’s Global Shark Tracker, an adult female white shark (nicknamed “Mary Lee”) left Cape Cod waters last September and headed south, as researchers had expected. By the beginning January, Mary Lee had made her way to the waters off northern Florida, which was inline with the theory that Atlantic white sharks spend their time in waters off the southern states of the east coast of the United States during the winter months and then head north during the summer months when water temperatures rise.
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Video: How to Track a Great White Shark and Why

South Africa’s Dyer Island Conservation Trust has put together a “mini-documentary” explaining the steps that go into tracking great white sharks and the motivation behind the effort to track the species. If you’ve ever wanted to know more about what goes into the process of tagging white sharks, this video is great place to start. The video also does a good job of approaching shark conservation with a positive and upbeat approach.

To learn more about their marine conservation and research programs check out the Dyer Island Conservation Trust website.