Infographic: How seals use subsurface structures to sneak by white sharks

Written by Michelle Jewell.  Follow her on twitter: @ExpatScientist

If you have ever avoided parking on a risky-looking street or taken a different route between classes to avoid a bully, your behaviour has been altered due to the perceived presence of ‘predators’.  In the wild, prey animals also change their behaviour when they think predators are around, and these altered anti-predator behaviours can often influence other species, and then influence more species, and eventually change the entire ecosystem.  This is an example of indirect effects predators have on ecosystems.

My research has focused on these principles of predator/prey interaction in the ocean, and a great place to study oceanic predators and their prey are Cape fur seal colonies in South Africa.  Every summer in the southern hemisphere (November), Cape fur seals give birth to thousands of pups.  For example, the seal colony Geyser Rock has a population of about 60,000 Cape fur seals and every year they give birth to 10,000 seal pups!  By winter (April – September) these 6 month old ‘young-of-the-year’ seals begin to venture off the colonies and swim offshore with the adults to the fishing grounds.  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 closely around seal colonies every winter to catch leaving/returning seals.  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 can see 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.

Dyer Island Geyser Rock Shark Alley South Africa Seal Island False Bay

A) The Western Cape of South Africa with B) Seal Island, False Bay and C) Dyer Island & Geyser Rock.

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…

Agents of seal cape fur white shark research dyer EDNA Science

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.

Have a question?  Leave it in the comments and I will answer!

Advertisements

8 thoughts on “Infographic: How seals use subsurface structures to sneak by white sharks

    • Hi Sarah! I saw very few (7 to be exact) during the study period. Compare this with Seal Island, False Bay where they can see +30 in a single morning! I also have to consider that there could be shark-on-seal predations happening under the water, but usually these attacks leave enough blood and red bits on the surface to still detect them. Even with predations happening, you rarely see the main event. Usually your tip off that a predation has occurred is a load of gulls picking up seal bits from the surface of the water. — Michelle

  1. Mike says:

    Very interesting article and very good pictures, so it is easy to understand.

    I have got some questions:

    Why do the sharks leave in the summer and where do the sharks go in the summer?

    I have read, that sharks usually follow the whales, but I think in South Africa are not big colonies of whales as for example in Australia or in Hawaii?

    How did you find the special behaviour of the seals out? Have you tagged some seals? Or have you watched the seals every day.

    What do you think, how many sharks are living in/coming to South Africa?

    I have read some month ago, that the abundance of the Great White Shark is rising in the USA?
    What about the abundance in South Africa?

    Thank you very much in advance.
    with kind regards
    Mike

    • Hello Mike! Great questions!

      Why do the sharks leave in summer and where do they go?
      Cape fur seals breed and moult during the summer time, so there is very little movement to/from the colony. Also, by the end of winter, the surviving pups have figured out how to avoid white sharks, so sharks during this time of year have very few chances to catch seals. For the summer, some sharks head inshore, other sharks go far offshore towards the Antarctic, and other sharks go east to Mozambique before coming back to the seal colonies in time for the next winter. You can actually see where our sharks have been and where they are going with the OCEARCH shark tracker. Why they go to these place and why some go one way while others go elsewhere is being studied as I type!

      Sharks follow whales?
      There are large populations of Southern right whales in western and southern South Africa every winter through to summer. Southern right whales come to South Africa from the Antarctic to mate and give birth. There’s little evidence that white sharks follow whales in South Africa since whales arrive in winter when white sharks are already hunting at the seal colonies, and then whales are still there when most of the white sharks take off or go inshore.

      How did you study the behaviour of seals?
      Hours and hours and hours of watching them. Tagging seals is very trick work (they are hard to catch, and then once you tag them they like to destroy your tags on the rocks) but I would love to put some critter cams on them to see how they use the kelp/reefs.

      How many sharks are living in South Africa?
      I actually co-authored a study on this! In Gansbaai, we counted 532 sharks over 5 years (which gives you a population estimate of 808-1008), but unfortunately, we do know that several of these sharks have been killed. We made an infographic for that study too.

      Is the population of white sharks rising in SA?
      We don’t really know. There wasn’t a very accurate population estimate of white sharks before they were protected in SA to use as a baseline, so the studies coming out now with population estimates will now be the baseline. We will only know 20-30 years from now if the population is going up or down. South Africa does have an extensive shark net system on the east coast though which targets white sharks, and this net system catches ~25 white sharks every year.

      Whew! Thanks Mike! 🙂
      Michelle

  2. Mike says:

    Hi Michelle,
    thank you very much for your informative answer. You really make your sience understandable for the public. Your research is very interesting, so I have got some additional questions and would be glad, if you will answer me again:
    Why do the seals on Seal Island avoid swimming at sunrise? Do sharks prefer hunting at sunrise?

    Why it is an advantage for the seals to swim in groups?
    I think, seals are not able to protect each other. So it shouldn`t be a bigger risk for a shark to attack a group of seals? And it could be more difficult for the seals to hide in a group?

    And why do the seals leave Seal Island in just one corridor? This could make it much easier to find them for the sharks?
    Thank you.
    Mike

    • Hello Mike!

      Thank you for your compliments! Scientific communication is a passion of mine. As for your questions…

      Why do the seals on Seal Island avoid swimming at sunrise? Do sharks prefer hunting at sunrise?
      White sharks at Seal Island primarily hunt seals during the pre-dawn/dawn hours. During these low light levels, sharks swimming at depth are able to see seals at the surface, but the light is not strong enough to penetrate deep into the water, meaning that seals cannot see sharks. It’s the perfect time to ambush!

      Why it is an advantage for the seals to swim in groups?
      Predators single out and target individuals, so if you form a group that’s constantly mixing, it becomes very difficult for that predator to single you out. There has been great research at Seal Island on these principles of “selfish herd” and “domains of danger”, which is available here: http://beheco.oxfordjournals.org/content/early/2012/10/09/beheco.ars153.full

      And why do the seals leave Seal Island in just one corridor?
      It’s the fastest path away from the island directly towards the fishing grounds offshore. Taking a longer route increases the time you’re under risk. Also, by going with the majority, you are also reducing your individual risk. For a hypothetical example, say every hour 100 seals move through the south corridor but 10 get eaten, and at the same time 4 seals move through the east corridor but 1 gets eaten. While 10 is greater than 1, if you break it up into a percentages, 10% of seals in the south corridor were eaten and 25% of seals in the east corridor were eaten. Which would you rather chance??

      So seals at Seal Island don’t benefit from changing their movement patterns like seals at Geyser Rock do (there is no additional cover), which is why seals at Seal Island don’t change their movement patterns when sharks are around.

  3. Mike says:

    Hi Michelle,
    I just have another question about white sharks and would be happy, if you answer me. I read in a newspaper that white sharks only attack people in waters with temperatures below 75°F. Is this true? I tried to find more information about the preferred temperatures of the GWS in books, but I haven’t found anything.
    Thank you very much in advance.
    With kind regards.
    Mike

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s