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.
- From the study 'Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video' (Jewell et al. 2019 Biology Letters),
- Fieldwork with Dyer Island Conservation Trust (DICT),
- Hopkins Marine Station (Stanford University),
- Monterey Bay Aquarium (Project White Shark),
- Harry Butler Institute (Murdoch University),
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.
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 . They patrolled areas the seals travelled through between feeding offshore and going back to rest on the rock . And in response to the sharks’ presence, the seals would move into kelp forest to avoid them . 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
The animal-borne video from the CATS Cam gave an incredible perspective of the white sharks interactions with kelp forest at Dyer Island
Artist impression of our work provided by @Lz_vie who retains all rights to this image
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:
- Jewell OJD, Wcisel MA, Towner AV, Chivell W, van der Merwe L, Bester MN (2014) Core habitat use of an apex predator in a complex marine landscape. Marine Ecology Progress Series 506:231-242
- 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
- Wcisel M, O’Riain MJ, de Vos A, Chivell W (2015) The role of refugia in reducing predation risk for Cape fur seals by white sharks. Behavioral Ecology and Sociobiology 69:127-138
- Jewell OJD, Gleiss AC, Jorgensen SJ, Andrzejaczek S, Moxley JH, Beatty SJ, Wikelski M, Block BA, Chapple TK (2019) Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video. Biology letters 15:20190085