The Ups and Downs of Shark Movements

SHOW NOTES

Sammy has spent the last decade studying the movements of large, predatory sharks across multiple scales. Her journey into shark science started at the age of seven, when Sammy first decided she wanted to be a marine biologist [8.48]. Every year, her parents would take her and her brother on holiday to a small island off the coast of Perth in Western Australia, where she would spend her time snorkelling. “Our parents made us keep a journal, and I’d write down every fish I saw,” she recalls. Later, as a high school student, Sammy shared her dream of becoming a marine biologist with her career counsellor – specifically, a marine biologist who works with sharks. “And he said to me, as a marine biologist, you’ll be looking through a microscope. You won’t be in sharks. You won’t be on a boat. And was like: I’ll show him,” laughs Sammy. And that she definitely did. In 2018, Sammy graduated from the University of Western Australia with a PhD in shark movement ecology, which involved many hours at sea tagging tiger and sandbar sharks. “I ended up actually dedicating my PhD thesis to him at the end!”

Sammy’s research focuses on the patterns and drivers of animal movement, specifically, the movements of sharks [10.47]. It’s not an easy field of research. Sharks and other marine animals live in a three-dimensional environment – they don’t just move horizontally across the oceans, but also up and down the water column, sometimes diving to depths of over 1,000 metres. “So we need to tie that in, and it gets really complicated really quickly to do that,” says Sammy. “We can’t hold our breath. We can’t survive at those pressures and temperatures. We have to be really innovative when we think about how we can do this.”

Technological and scientific advancements have opened up opportunities for research in this area [14.07]. For example, acoustic tags emit a unique ‘ping’ every 60 to 120 seconds, which are then picked up by listening devices (acoustic receivers or hydrophones) placed in certain locations underwater. Scientists can then download the data from these receivers to see who’s been around. “It’s like if I was walking around and every 60 to 120 seconds, I said, ‘Sammy, Sammy, Sammy’. That’s what the tag is doing. And the receiver is listening, and saying, okay, Sammy was here at this time on this date.” These are great for studying the movements of animals that spend time by the coast, but once that animal moves further offshore – beyond the reach of the receiver – their signal can no longer be picked up. “So, for looking at when they even go broader, when they go offshore, we use pop-up satellite archival tags. We also call them PSATs,” Sammy continues. “These will be attached to an animal, usually for around a year, and they’re recording light, depth, temperature, and have a really accurate internal clock. After they’ve been attached the animal, they’ll pop off to the surface. That’s why they’re called a pop-up tag and relay that information all back to a satellite and then back to us as the end user.” These tags allow scientists like Sammy to track a shark’s movements throughout the course of the year, gaining insight into how they utilise their environment.

Sammy’s PhD used some of those technologies to explore the vertical movements of sharks, specifically tiger, sandbar and oceanic whitetip sharks [22.59]. Later, she co-led one of the largest studies of how elasmobranchs use vertical space in the ocean, which was published in the journal Scientific Advances. Compiling over two decades of data, from over 900 tags and 38 species, the study had some surprising findings. The majority of species chose to spend most of their time between the surface and 200m, but 13 of them frequently dived to much deeper depths of over 1,000 meters. The exact reasons for this behaviour are unknown, but there are several strong theories. “The main hypotheses often seen in the literature, it could be for a foraging opportunity. There could be a lot of fish or squid or something, so they go down and feed down there.” Sammy explains. “It could be to thermoregulate, so it’s cold down there, they might need to cool off and go for a short amount of time down there. It could be to avoid predators, like white sharks could be chased by an orca for instance that can’t go as deep as them, orcas are holding their breath. So white sharks might go down where it’s dark, they avoid a visual predator finding them and it’s cool, so it could be to hide effectively.”

Another hypothesis is that these species might actually be going deeper to navigate [30.30]. It seems counterintuitive – if you or I got lost, we would be heading up higher to take a look around. And, the deeper you go in the ocean, the darker it is, right? Well, yes, but there are also other cues, like seamounts (essentially underwater mountains that rise up from the seafloor). It is also thought that the earth’s electromagnetic field changes as you go down, which sharks use to navigate. “I think hammerheads, that was one of the original ones that they were thought to dive deep because they have lots of electrodes on their faces. So it’s thought they go down and use the field to navigate and use seamounts,” says Sammy.

An additional finding was that some of the vertical distributions of species overlap [34.16]. It’s a bit like living in a high-rise apartment building, and bumping into other residents on the stairs on your way up or down. The species with the highest overlap were the tiger shark, oceanic manta ray and whale sharks, that not only shared similar patterns of vertical movement, but also overlapped in their horizontal movements as well. This could have interesting implications for interactions between the species: “…for the tiger shark, that’s a predator, so there’s a higher likelihood there’s a predator-prey interaction going on there. Oceanic mantas and whale sharks [are] both filter feeders, probably feeding in some locations on similar prey resources, so they might be competing for the same food source. But whatever it is, they’re not – especially for the tiger shark – they’re not closely related. So whatever this evolutionary strategy was they co-evolved it and it seems to be working for them, which is very cool.”

Knowing where animals are distributed vertically is also important for their conservation [37.41]. For example, different fisheries will be fishing at different depths. Purse seine might be operating between 200-500 meters, whereas deep-sea trawls target much deeper waters. Knowing the vertical distribution of sharks allows us to understand which fishing gears overlap with which species, to inform targeted conservation strategies.

Sammy has also looked at how shark movements might be affected by the lunar cycle [40.19]. Another study, looking at large epipelagic fishes – such as tuna and swordfish, as well as sharks – reviewed 190 papers to see if the lunar cycle was influencing how these animals move in the water column. The short answer is: yes. Swordfish in particular, that are very visual predators, were moving deeper at times when lunar illumination was highest, possibly following prey that were escaping into the depths to avoid them. But the sharks proved to be much more complicated. “Sharks just have so much individual variability; I learned that from tiger sharks. I could never find a pattern. They all do their own thing. Which is kind of annoying from the science part, but now I’m really invested in figuring out what’s driving this individual variability,” says Sammy.

More recently, Sammy’s work has shifted to focus on another visual predator: the great white shark [44.55]. More specifically, Sammy is leading research at a long-term field site off the coast of California, conducting photo ID and deploying acoustic and satellite tags. And this research is lending a fascinating window into the lives of these animals – particularly into individual preferences. For instance, some individuals move around a lot, whereas others like to return to the same sites, year on year. “I haven’t published this yet, but we have one individual that was tagged twice over with a satellite tag, and recorded both offshore migrations. And it was crazy how consistently it did the same thing each year while there are other individuals offshore in completely different places,” Sammy explains. “They kind of have this memory component, like they’ve learned how to do this, which I think is crazy. Like if you chuck me in the middle of a forest, I’m going have no idea how to get home, but they do this thousands of kilometres.”

For Sammy, this is really living the dream – the culmination of her lifelong passion and determination to study sharks. “It’s seeing through a shark’s eyes, that innate intelligence… and again, that kind of challenges a perception of like, especially white sharks have got such a reputation as these like mindless animals, that don’t really think much about where they go. And they’re just hell bent on finding as much food as possible. But you know, they’re much more intelligent than we give them credit for.”

ABOUT OUR GUEST

Sammy grew up in Perth, Western Australia, where she also completed her PhD in 2018 at the University of Western Australia. Her thesis explored the patterns and drivers of vertical movement in predatory pelagic fishes, with a focus on sharks.

She soon after joined Barbara Block’s lab at Stanford University’s Hopkins Marine Station in 2019 as a postdoc, where she now continues to work as a research scientist studying shark (as well as other fish) movement across multiple scales. She also currently leads the lab’s field team in surveying and tagging sub-adult and adult white sharks.

Instagram: @Sammy_Shark and @MontereyBayWhiteSharks

BlueSky: @sammy-shark.bsky.social