Cryptic habitat use of white sharks in kelp forest

Using fin mounted cameras to witness great white shark behavior from the animal's perspective.

Receiving a video diary from a shark

We’ve been putting biologgers on white sharks for a while now, defining their core habitats at Dyer Island even longer, and tagging and tracking them – well in my case at least – for more than a decade. We learn new things about them all the time; they may make incredible migrations, they may recover from severe injuries, or return to the same site year after year. But it’s only when we put biologgers on them, particularly biologgers with cameras in-built, that we get to pry into the secret lives of sharks. And that’s precisely what we did in this study.

Our team headed to Dyer Island and Geyser Rock for our second South African expedition in May 2014

Say you’ve got a penpal, and you’d like to know what this friend’s been up to, but they have no way of getting a hold of a pen or pad of paper. You’d have to give them what they need, get them to write down what happens to them and then get it back off them the next time you meet or have them leave it somewhere you can find.

In a roundabout way, that’s what we’re doing when we place biologgers on white sharks. We go to a spot we know we’re likely to bump into them, give them one of our diaries to fill in for us and then have them drop them off (or more accurately pop them up) at sea so we can retrieve them. You can read more about how we use biologgers and the different versions we use here.

A white shark with a CATS Cam attached at Dyer Island in our 2014 expedition

By the time we got a chance to use these diaries on the white sharks at Dyer Island, we’d already learned some important things about them. We knew they spent most of their days in small areas close to the seal colony on Geyser Rock [1]. They patrolled areas the seals travelled through between feeding offshore and going back to rest on the rock [2]. And in response to the sharks’ presence, the seals would move into kelp forest to avoid them [3]. What we didn’t know is how these interactions played out beneath the surface.

The way to find that out? Well to give them some of those diaries for a start…

But what exactly are the sharks ‘writing’ in these diaries for us? Well in the case of this particular study, we used Customised Animal Tracking Solutions (CATS) Cam tags, and these give us a datalog from an accelerometer (a bit like a 3D motion sensor or Fitbit), a magnetometer (a 3D compass), a depth sensor and a camera, plus a few other sensors we didn’t use in this particular study.

The accelerometer tells us how much the sharks move, the magnetometer tells us the direction of the movement and the depth tells us where they are in the water column. Combined with the camera, we can see where they are and what they’re doing. We can also work out things like how active they are, how many tail beats they make and how much they turn. Put them all together, and you get something that looks a bit like this:

A deduced (dead) reckoned plot created from accelerometer, magnetometer data combined with video data from a CATS Cam deployment at Dyer Island

Stills from the CATS Cam footage of a white shark at Dyer Island swimming through kelp forest and interacting with Cape fur seals (credit Jewell et al. [4])

By using the techniques described and shown above, we were able to determine that white sharks not only swim through kelp but they did so regularly and repeatedly. Not only this, but they also interacted with the Cape fur seals inside the kelp canopy. We could actually see Cape fur seals hugging the seafloor in groups and blowing bubbles at the white sharks when they saw them.

These diaries gave us an incredibly detailed account of the interactions between white sharks and Cape fur seals in this system, one that we just could not have seen without the new technology. Yet we could never have reached our conclusions with the diary data alone. The previous studies confirmed the spatial patterns of both the seals and the sharks centred around the kelp forest for extensive durations during daylight hours.

We knew the seals were using the kelp forest as a refuge. It seems the sharks have figured that out too. The kelp forests at Dyer Island provides a predictable supply of Cape fur seals through the winter months and the white sharks there are more than capable of swimming into the forests to find them!

The animal-borne video from the CATS Cam gave an incredible perspective of the white sharks interactions with kelp forest at Dyer Island

Download the paper (Jewell et al. 2019) here:

Download

One of the things I loved most about this study was the attention it got from the media and general public, I got to talk sharks and kelp to so many people across the world, and I still have people reply ‘that was you?’ when I bring up the findings in conversation.

The study’s results were translated into Swedish, German and Norwegian among others, it appeared on the BBC, in Forbes and in The New York Times, I appeared live on CBSN, the Latest and The Naked Scientists, and I really knew it had gone big when Perth Now put it in a headline alongside a story about Harry and Megan! A summary of a lot of the media coverage can be found on the studies Altmetric page here.

But it was the engagement with people that was the most rewarding, a diver from Tasmania got in touch to say he had been diving in kelp forest almost 15 years ago when a great white swam through the canopy and circled the divers a few times. I found some artwork on a thread on Facebook and got in touch with the artist, Lisa Smith whose from Canada, and has let me repost it here. We’ve since become good friends.

Finally, my good friend and long term colleague Ed (who also made this site) and I were chatting about what we could do for a media release. In the past, we’ve done infographics, videos and even a press conference. This time we wanted to try something different, and Ed had this crazy idea that he wanted to make a digital sculpture of a metal white shark swimming through some kelp.

Knowing Ed, and knowing that he’s ideas work, I just let him run with it! One of the results is below, but if you want to see more and read about what he did, please visit his website by clicking here.

Artist impression of our work provided by @Lz_vie who retains all rights to this image

Ed's kelp shark sculpture in postcard form for the press release of this study. I highly recommend you take a look at his site (link in text above) to check out how he made this and many other shark designs for media use.

One final note I’ll leave you with, some people I’ve spoken to since the study was released seemed concerned about diving in a kelp forest in case a white shark swims there. White sharks can swim just about anywhere in the ocean. It’s just what they do. But encountering one is extremely rare, and bites are rarer still. The reason these sharks were going into this particular kelp forest was that there were 50-60,000 Cape fur seals on the island adjacent to it and they attracted the largest aggregation of white sharks ever described.

If you get the chance to dive in a South African kelp forest you should absolutely take it – it’s an incredible experience!

If you’d like to see a video linked presentation of me discussing this research during the Covid-19 lockdown (complete with appropriate home haircut) you can find one below:

References