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I’ve spent more than a decade working with and researching white sharks; inevitably there’s always something new to learn and just every so often that something new really takes your breath away...
One such time was when we’d been satellite tracking a large female shark in South Africa for a year or so when her tag stopped transmitting. Satellite tags often get algal growth or biofouling on them and never work again. But then, months later and out of the blue, the tag pinged in, and the shark was almost 4,000 miles away off the Chagos Archipelago (British Indian Ocean Territory), somewhere we had no idea a South African white shark would travel to. Over the next few weeks and months, we watched as she turned south and headed our way until we saw her in person again in Gansbaai.
Now seeing a white shark in person might not sound like everyone’s cup of tea, but in Gansbaai, working with Marine Dynamics and The Dyer Island Conservation Trust, it was a daily occurrence in my life as we took tourists out for cage dives and got to know the sharks quite well. We even named hundreds of them and became the first research group to get a decent grasp of how many were in the local population, which was around 900 individuals1. But being a scientist, I was always curious to know more about them: why were they here? And what exactly did they get up to when our boats headed home for the night? And so, I spent many days and nights tracking them to find out. The principle was quite simple: stick an acoustic tag or ‘pinger’ on the shark's back, get a hydrophone in the water, listen to the pings and follow the shark. After that, you pretty much have to go exactly where the shark goes and try and figure out what it’s doing – and that’s pretty much exactly what I did throughout my Masters!
What we found was that, during the winter, the white sharks would hang out close to the seal colony in Gansbaai, a little piece of rock off Dyer Island called Geyser Rock. On this rock were between 50 and 60,000 Cape fur seals, and these guys were the big draw for white sharks in the winter time. Like the shark we tracked to and from the Chagos, sharks would come from many miles (sometimes thousands) all the way here to feed on the seals at just the right time when the ~6-month-old pups were first starting to get the hang of swimming.
The sharks would stay close to the islands, or adjacent kelp forest, all day long and only swim away from the islands during the night – something very different to the way white sharks behaved at other seal colonies in South Africa2. At the same time, a separate study found that the seals were adjusting their behaviour in response to the sharks’ presence and using the kelp forests as a refuge from them3. We knew we had discovered a really interesting and unique predator-prey interaction, but both of these studies were ultimately limited to surface-based observations. What was going on below the water remained unseen. That was until we were approached by a group of scientists from Stanford University who wanted to test some new animal-borne video camera and motion sensor tags on the white sharks in Gansbaai. This, I realised, would be our chance to fill that missing link in our research and see exactly what those sharks were up to close to the kelp forests all day.
It’s not easy getting an animal-borne video tag onto the fin of a white shark, let alone getting it to stay there long enough for the shark to do something interesting! The trick to getting the tags on the animals is to use a piece of bait and some chum (mashed up fish bits and oils) to entice a shark to come near, then, while it’s distracted looking at the bait, quickly lean over the side of the boat and use a pole to clamp the tag to the shark's fin. It’s not all that complex but definitely not an easy task either! After that, the clamp needs to hold the camera view at a steady forward-facing angle and remain attached for a predetermined number of hours or days. Once the time is up, a corrodible link will release the tag, which floats to the surface, and we then have a VHF antenna we can use to hear and locate it or hope nice people find them on the beach and call our number!
After all that, once we have the tag in hand and are safely back at base, we can start the download. If nothing’s gone wrong, we’ll have usable data. And if we’re lucky, something interesting too. The trick to getting something interesting… well, that comes down to good luck as much as good judgment! You can’t make the shark do anything. Instead you just hope it doesn’t start swimming to the Chagos with your equipment immediately after you put it on! Indeed, in the first season, the team from the US arrived after the sharks had already left the island for the season and were aggregating close to the shoreline in their traditional summer location. Anyone from Gansbaai can tell you this means two things: poor visibility and no seals. We wouldn’t be finding out what they were doing close to the kelp after all. But, we persevered. We tagged quite a number of the animals and captured breaching from a white sharks’ eye perspective as well as establishing the proof of the concept for the studies that were to follow4.
Eventually, we got the timing just right. We began an expedition early in the winter season, the time of year I’d do a lot of tagging in my Master’s. The sharks were by the islands (several of the same individuals I knew quite well) and the water visibility was pretty clear most days. We got several tags out and when we checked the video… we finally got what we were waiting for!
From the early deployments, we could see kelp in more and more of the footage; the sharks weren’t just on the periphery of it, they were actually cutting into the forest and making contact with both stipes and fronds. Then on our fifth deployment, we started seeing seals too. The first seal darted straight past the shark, which turned to react, but the seal was long gone. Then, about 45 minutes later and while the shark was swimming through a dense patch of kelp forest, groups of seals appeared in the footage. They were in groups of up to four or five and were most definitely aware the shark was there. Several hunkered to the seabed, and some blew bubbles at the shark as it swam near while others darted through the kelp lines. The shark swam around this area for several minutes, and we saw the seals and bubbles again and again until, eventually, the shark picked up its activity and went straight through the kelp line after them. The next moment in the footage was incredible; the shark continued swimming actively and then at one moment approached the surface fronds where at least three seals can be seen taking refuge and successfully avoiding the shark just as they were hypothesised to3.
Eventually, this shark swam so far into the kelp forest that it pulled its tag off, meaning we will never know if it grabbed one of those seals in the kelp that day. This was just one of several sharks we saw making these movements into dense kelp. The motion sensors on the tags work a bit like a Fitbit and allow us to rebuild the movements of the sharks in three-dimensions. We were able to confirm that the sharks moved with greater activity and turned more frequently within kelp forest5. Though we were unable to get a predation captured on film, it is likely they were occurring here during the time we were out looking for them – and will have been occurring for many years – they are just incredibly difficult to see! I guess, ultimately, it shouldn’t be such a shock that the sharks do this; we’ve figured out that the Cape fur seals use this habitat when the white sharks are around. It makes sense that a predator as in-tune with its environment as a white shark has figured that out too. But it’s still a great demonstration of what we can learn about wildlife with new technology. In terms of the potential of what we can learn by placing more of these tags on white sharks, this is just the tip of the iceberg. We’ve already deployed them on distinct populations off the North East Pacific and North West Atlantic too, and this is only the first chapter of my PhD! Watch this space for more research from our team...
References
1. Towner et al. (2013) Gauging the threat: the first population estimate for white sharks in South Africa using photo identification and automated software. PLOS ONE
2. Jewell et al. (2014) Core habitat use of an apex predator in a complex marine environment. Marine Ecology Progress Series
3. Wcisel et al. (2015) The role of refugia in reducing predation risk for Cape fur seals by white sharks. Behavioural Ecology and Sociobiology
4. Chapple et al. (2015) Tracking sharks without teeth: a non-invasive rigid tag attachment for large predatory sharks
5. Jewell et al. (2019) Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video. Biology Letters
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