What is a ‘core habitat’ and why would a shark have one?
I’ve spent a large chunk of my career listening to pings from tags on sharks and following them around bays, headlands and islands shadowing their daily movements. And they’re not random, not for white sharks in any case. We’ve spent many hours sitting in one spot with the shark barely moving from an area before it suddenly decides it wants to be somewhere else and moves almost in a straight line in that direction. This may be a specific reef it likes to hang out or a seal colony full of prey it would like to eat. But repeated visits to these particular areas, that can sometimes be extremely small, make them ‘core’ areas or hotspots and they play crucial roles in the shark’s daily life.
So how do we get to here? Well firstly, if you’re keen on learning about the specifics in how we tag and track sharks and the different techniques available, I’ve got you covered, but you’ll need to take a detour to our page on ‘Tagging and tracking sharks: the how and why’.
For this study, we used the ‘active acoustic telemetry’ method, and in short, that means we placed an external acoustic tag on a free-swimming white shark and followed the pings of the shark up and down the coast – in this case – off Gansbaai, South Africa.
The study was the main basis of my Master’s thesis at the University of Pretoria and built on previous research I’d undertaken with Oceans Research in Mossel Bay and lead onto further research published in Functional Ecology.
One day I’ll write a post on how we calculated the home ranges of sharks in Mossel Bay , or maybe on how Hidden Markov Models revealed sex-specific and individual hunting strategies of white sharks in Gansbaai . Although, until then you’ll have to follow the references to those studies!
Anyway, what we were trying to learn from this study was how the shark movements in Gansbaai, particularly around Dyer Island and Geyser Rock, compared to the previously mentioned study in Mossel Bay. To do this, we were tagging and tracking white sharks close to the seal colony on Geyser Rock.
Now there’ a few differences between the two areas. Mossel Bay, around 250 km to the east of Gansbaai, is relatively protected and has a seal colony tucked right in the corner of the bay close to the shoreline. The sharks visit this seal colony, aptly named ‘Seal Island’ predictably at dawn and dusk throughout winter months .
The island is so close to the shore that if you book a room at the nearby hotel at the right time of year and watch the seals leaving in the morning, you can actually watch white sharks hunting them from your balcony! You’ll need a bit of luck, good conditions and a sharp pair of eyes, which I guess I must have had back then because I witnessed several interactions from there during my time in Mossel Bay.
I’ve put a couple of maps below so you can see the two settings for the studies.
Mossel Bay gains a lot of protection from the Point headland, making it ideal for active tracking white sharks. It’s also on the Indian Ocean side of the two oceans, meaning the water’s a few degrees warmer.
Gansbaai, on the other hand, is far more exposed, Danger Point offers a little protection to Kleinbaai Harbour (where I used to live), but after that, you’re pretty much exposed to the South Atlantic. And the seas get big. Very big.
You also get Dyer Island and Geyser Rock, roughly 10 km from shore. There are multiple sea bird colonies on Dyer Island, including the endangered African Penguin, you can learn more about them here. But they aren’t the reason white sharks come to the area; there are 50-60,000 Cape fur seals on Geyser Rock just across a narrow stretch of water called Shark Alley.
That’s a lot of seals!
By the time we got to tagging and tracking the sharks of Dyer Island, I had already worked a couple of season’s cage diving with Marine Dynamics, who sponsored this research. I had become familiar with the way certain sharks would patrol extremely close to Geyser Rock in Shark Alley, grabbing seals as they thermoregulate in the shallow waters.
One of the sharks I knew used this technique quite often was an adult male we called ‘Zane.’ Zane was one of the easier sharks to identify at Dyer Island and appeared each winter without fail. His most distinguishing feature was his almost complete lack of an upper caudal lobe (tail fin). After that, you just had to look to his dorsal fin and check for the black marks of a previous satellite tag he had carried from a research project almost a decade ago.
I have to thank Grant Tuckett, skipper for White Shark Projects, for this initial tip-off during my first Dyer Island season. I already had access to the database of shark fin ID’s from that satellite tracking project, and within minutes of checking, I found his tagging date, from Dyer Island in 2003.
Cape fur seals at Geyser Rock. The narrow channel between Geyser Rock and Dyer Island, Shark Alley, can be quite treacherous for Cape fur seals during the winter time.
We collected more and more data at Dyer Island and Geyser Rock. The following season I even managed to tag another legendary shark of the area Slashfin, who like Zane, had been visiting Dyer Island for more than a decade. He’s most recognisable by an old injury to his fin, and also like Zane, we saw almost every winter without fail.
Rather than perform the movements close to the seals in Shark Alley, Slashfin used an area between the offshore side of Geyser Rock and breaking pinnacle known locally as ‘Wilfred’s Klip’ (Wilfred’s Rock). Again, his movements were close to the seals during the day and further offshore at night. The only time we saw the sharks stayed reasonably close to Geyser Rock after dark was if there was strong moonlight, during which both Zane and Slashfin remained just offshore of Geyser Rock, between the seal’s colony and their offshore foraging grounds.
Finally, it was time to analyse this data. The first step was to define how movement patterns matched up between the individuals and to those sharks tracked in Mossel Bay. We used rates of movement (how much the animal moved between tracking positions), a linearity index (how straight the tracking positions were), and the distance of each tracking position from Geyser Rock.
These confirmed the sharks were using the island system (where they had access to seals) in a much more tortuous manner during the day than the night. Completely different from what was found in Mossel Bay.
Next, we had to define the activity areas or ‘home range’ of white sharks at Dyer Island. This wasn’t easy because, by nature, sharks can’t go onto land, and with all the rocky outcrops and island ridges, there was a fair amount of no-go areas about. We used biased-random bridges (BRB) and a movement-based kernel density estimate (MKDE) to account for this. We were effectively creating home range estimates in movement corridors, around the barriers caused by the heterogeneous structures of the environment.
Now that’s a mouthful and yet contains none of the details for how we did it, so in lay terms: we defined the core areas the sharks used in this complex area.
The large sharks had clearly defined hotspots. They were predictably found within them, even during the years before and after this study. But we also had a small shark who did the same, making limited movements between a spot in the reefs and the cage diving area marked as the Geldsteen on the map.
The only shark that roamed a comparably larger activity area was a medium-sized shark, a young-adult male, who moved between all the other shark’s core areas without really settling on one to use over and over. All of these areas were smaller than those found in Mossel Bay though. It seems the white sharks of Dyer Island can fulfil most of their needs without travelling far from the seals.
I have to thank Simon Benhamou, who wrote the MKDE package, for helping troubleshoot our estimates. I was a complete novice at coding when I started this project and his advice invaluable to me completing my Master’s on time, as was that of my colleagues at the Mammal Research Institute, University of Pretoria. For more details on the methods used, both my Masters Thesis and our papers can be found in the publications section of my Academic Profile Page. The Supplementary Material provides specific details on the tagging, BRB and MKDE methods.
- Jewell OJD, Johnson RL, Gennari E, Bester MN (2013) Fine scale movements and activity areas of white sharks (Carcharodon carcharias) in Mossel Bay, South Africa. Environmental Biology of Fishes 96:881-894
- Towner AV, Leos‐Barajas V, Langrock R, Schick RS, Smale MJ, Kaschke T, Jewell OJD, Papastamatiou YP (2016) Sex‐specific and individual preferences for hunting strategies in white sharks. Functional Ecology 30:1397-1407
- Johnson R, Bester MN, Dudley SFJ, Oosthuizen WH, Meÿer M, Hancke L, Gennari E (2009) Coastal swimming patterns of white sharks (Carcharodon carcharias) at Mossel Bay, South Africa. Environmental Biology of Fishes 85:189-200
- Jewell OJD, Wcisel MA, Gennari E, Towner AV, Bester MN, Johnson RL, Singh S (2011) Effects of smart position only (SPOT) tag deployment on white sharks Carcharodon carcharias in South Africa. PLoS One 6:e27242