Core habitats of white sharks at Dyer Island

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.

A white shark cruises through turquoise waters near Dyer Island

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 [1], or maybe on how Hidden Markov Models revealed sex-specific and individual hunting strategies of white sharks in Gansbaai [2]. 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 [3].

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.

Maps of Mossel Bay (top) and Gansbaai (A & B) with a zoom in to the Dyer Island and Geyser Rock island system on a rough sea day. The maps are from my Master's and early tracking papers, I'm happy for people to reuse them but please add an acknowledgment or reference!

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.

Zane was a sizable male shark with a distinctive lack of tail and an old mark from a satellite tag. It’s not known how he got the tail injury but he’s carried it since at least he was a sub-adult having been just over 3 meters when he was tagged in 2004 [4].

Download the paper (Jewell et al. 2014) here:

Download

Now we had to start tracking the sharks. My initial hypothesis had been that we’d see some similar crepuscular seal hunting patterns to Mossel Bay. Still, our tracking inshore as part of colleague Alison Towner’s Master’s/PhD projects had already seen the large sharks inshore, tended to stay inshore most of the time. It was clear the foraging pattern close to the rocks in Shark Alley was also very different.

I set out with a team of interns and headed to Shark Alley one morning in early April, right at the beginning of the winter season and laid a short anchor in a tuft of kelp right in the mouth of Shark Alley, a perfect spot to find Zane. With this kind of tagging research, you can’t be too selective, as there could be anywhere between 50 and 100 sharks around Dyer Island during peak seasons back then. It’s very much a case of you have to take what you’re given.

But this time we got lucky.

Well, they do say the harder you practice… but there he was. Zane in all his glory. I estimated he would have been about 4.5 meters with a full tail, so marked him down at 4.2 total length. It’s since been confirmed we were always a little under in our estimates of shark size and he was probably closer to 5 meters. That’s a big shark! I got the tag on him, and we could begin tracking the first of the adult sharks at Dyer Island.

We worked in teams around the clock throughout the better weather days of the next month. Myself, Alison, Nick Jones, Dicky Chivell, David Edwards and Michelle Wcisel were the principle skippers and trackers, but we also had many enthusiastic interns join us to learn the techniques and provide extra hands on the exhausting shifts.

The most important thing to remember with active tracking is, if you’re not out at sea tracking the animal, you’re not getting any data. And that meant long days and nights on the water.

I had become a night tracking expert during my time in Mossel Bay, but it was an additional challenge being further offshore in remote and potentially dangerous waters. There are plenty of shallow pinnacles, kelp and reef to worry about, and I also had to learn to drive a tractor and retrieve the boat by myself. Sometimes we’d come back to shore at 2, 3 am, and Kleinbaai Harbour is a boat launching slipway only. But we persevered. It soon became clear we had some incredibly unique patterns occurring.

Zane would use the same small area all day every day. We call this type of behaviour Area Restricted Search (ARS), and you can read more about it in our HMM paper [2]. But Zane’s was extreme, sometimes less than 100 m² in size and sometimes with as much as a 98 % overlap in one day’s area use to the next. He made repeated circuits between the kelp on one side of Shark Alley, to the thermoregulating seals on the other. Time and time again.

If he could grab a seal without them seeing him coming, he would pull it away from the shallow areas and eat it, before repeating the circuit. If the seals saw him coming, they would ‘mob’ him away. A behaviour where the seals’ group together and porpoise high out of the water behind the shark. This may be a mix between aggressively herding a predator away or a form of ‘stotting’ whereby a prey shows off to a predator that it’s one – been seen – and two – been seen by a fit and healthy individual that will be difficult to catch.

Either way, it was enough to put Zane off for a while, and he would head back to the kelp and try again a few minutes later. We saw several other sharks making similar moves on the seals during this time.

During a rough day when the seas were too big to launch our research vessel, Nick, Dicky and I were part of a cage dive trip right in this spot. We saw Zane and over ten other large sharks using this area. None were particularly interested in our cage diving and went right back over to the seals after checking us out for a few minutes.

Zane’s daytime movements were never far from the seals, while at night he would move to distant reefs or travel significant distances out to sea before returning the following morning.

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 legendary white shark Slashfin's core area (defined by a MKDE) at Dyer Island, South Africa

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.

References