Shark Movements, Food Webs, and Flower Gardens
SHOW NOTES
Before getting into his PhD research, Brett recalls one of his most unforgettable ocean experiences: his first great hammerhead encounter while interning at the Bimini Biological Field Station [4.25]. He and a colleague had headed out in a small skiff, with the purpose of baiting for hammerhead sharks to show an excited group of visiting researchers. But the pair weren’t hopeful; it was out of season for great hammerheads at the time, and none had been sighted recently. To pass the time, they took turns freediving down to put bait in a milk crate they’d placed on the sandy bottom. Two hours later, Brett was finning back down to the crate – bait in hand – when his buddy alerted him to the presence of something unexpected: “Out of the murky depths, there’s this bigger shadow moving towards me… It’s literally a great hammerhead, a full-on adult, 13-foot, huge!” While at first intimidating, the experience left him in awe, and cemented Brett’s fascination with sharks.
But sharks weren’t always an interest of Brett’s. Rather, he was more drawn to the complex interactions between predators and prey – a curiosity sparked by a David Attenborough documentary called The Trials of Life [08.59]. “I was probably like four or five years old, just sitting on the ground in front of the TV, watching this episode of how orcas interact with sea lions at Patagonia, where they purposely beach themselves to grab their prey…And I was like, what is this? This is crazy. This is wild,” he recalls. Over time, this curiosity grew into a passion for understanding marine food webs and how sharks — in all their diversity — fit into them (11:17).
Now a PhD student at Texas A&M University in Galveston, Brett studies both movement ecology and trophic ecology of sharks [12.00]. Movement ecology, in a very basic sense, is the idea of trying to understand where sharks go over space and time, and why. The drivers of these movements can be many and varied, from finding food to finding a mate, or shifts in temperature. But when it comes to sharks, the exact details remain largely unknown. “Even though sharks seem to be these charismatic megafauna, I feel ironically, we still don’t know a lot about them,” says Brett. “…when you sit back and you look at the literature, especially compared to bony fish, we don’t know a lot of just simple questions about even some of the more popular species, or super abundant species.” This includes the sandbar shark, one of Brett’s study species.
Trophic ecology, meanwhile, zooms in on one of the potential driving factors of movement: foraging, or finding food [14.30]. Understanding what sharks eat, and how they fit into the food web, is also important. “A lot of the larger shark species we talk about are at the highest trophic level [at the top of the food chain]… but not all sharks are there. Some are lower down and feed the higher-level sharks”, explains Brett. Understanding this, he argues, is key to gauging how sharks influence ecosystem health — both directly through predation and indirectly by altering prey behaviour.
These are the questions Brett is trying to answer with his PhD, which focuses on the sharks of the Flower Garden Banks National Marine Sanctuary [18.00]. Located in the Gulf of Mexico, approximately 160 kilometres off the coast of Texas and Louisiana, it is a remote and isolated oasis with “probably one of the healthiest coral reef ecosystems… and it’s the highest latitudinally in the western Atlantic.” Named after its iconic salt domes, and the extensive reef-building corals that live on top of them (which, to local fishermen, looked like flower beds lying beneath the surface), Flower Garden Banks is the only National Marine Sanctuary in the Gulf of Mexico. The sanctuary’s coral cover — often exceeding 60% in places — supports blacktip, bull, dusky, and whale sharks, along with the scalloped hammerheads, silky and sandbar sharks Brett studies most closely. That’s not to mention the vast diversity of other fishes, corals, invertebrates, cetaceans and sea turtles. Yet despite its richness, the site is little-known: “A lot of people don’t even realize that it’s there. I didn’t know that it was there until I applied for this program, and this is me being a student spending so much time studying marine science in the US. And I think it’s sad actually, that we have this incredibly beautiful bio-rich area in the Gulf of Mexico that isn’t really well known.”
As the conversation turns deeper into Brett’s PhD research, he explains how the sanctuary is not an isolated patch of reef but part of a wider network of natural banks and artificial structures, like decommissioned oil and gas platforms [24.11]. These sites are just as important as the coral gardens themselves: “There are a lot of other natural banks that aren’t within the parameters of the sanctuary. They’re not protected, but they’re also being utilized by these [same species]. Not only the natural banks, but these artificial reefs as well, these oil and gas platforms.” For juvenile silky sharks, in particular, these structures may provide refuge from larger predators, creating an unexpected link between industrial infrastructure and shark survival.
To uncover these patterns, Brett has been using different technologies to tag and track sharks [26.00]. One is acoustic tags, which Brett explains as “like the hot pass system or easy pass system on a highway, when you are passing on a toll road and the badge that you put on the inside of your windshield talks to a sensor when you go under the toll bridge. It’s exactly how a tag works with an acoustic receiver that we place out in the water. If a shark that is tagged with a receiver swims within range of this acoustic receiver, it will pick up your shark’s ID and say, Hi, this shark was here at this time and date.” Acoustic tags were used for all three species in Brett’s study (sandbar, silky, and scalloped hammerheads). But for scalloped hammerheads, Brett also chose satellite tags, which would be attached to the shark’s dorsal fin and would only transmit a signal when the shark was at the surface. Given scalloped hammerheads spend decent amounts of time at the surface, and were likely the ones who would spend more time outside the sanctuary – and out of range of Brett’s receivers – they were a good candidate.
The data so far have revealed contrasting stories [35.40]. Silky sharks cluster near the artificial reefs by day but head out across natural banks by night. “Our maps show this high-frequency clustering really close to artificial reefs… at night they leave to forage, and then when the sun comes up, they return”. Sandbar sharks, by contrast, avoid the shallower coral reefs, preferring deeper banks and shelf drop-offs where they can hunt species like red snapper and large jacks in the open. “If you try to think about why a sandbar shark wouldn’t be at this a beautiful complex coral reef environment, it’s because the habitat is super complex,” Brett explains. “Like East Flower Garden has coral coverage higher than 60%…that just means there’s so many different places for fish to hide and get away from predators. So, you have this large two meter plus adult shark trying to come in and get a small little like reef fish. It probably isn’t worth their time and energy.”
And sometimes, the tags reveal epic journeys [40.40]. Brett was thrilled when sandbars he tagged in the Flower Gardens appeared nearly 800 miles away in the Florida Keys. One even travelled from the Flower Gardens to the Keys and then back to the Mississippi Delta. “Now we have this big biannual movement cycle of these individuals…knowing that they’re coming back to this, you know, small, unique oasis in the Gulf of Mexico is super fascinating.” This information is also vitally important to informing future conservation strategies, ensuring that protection is sufficient outside the boundaries of the sanctuary itself.
Beyond movement, Brett also wanted to understand the sharks’ diet and roles within the food web. But offshore logistics are tricky — ice melts, freezers fail, and samples are lost [45.35]. So, he trialled a novel method: dried blood spot analysis. “You put small drops of blood on a cellulose card. Once it dries, you fold it up, put it in a bag with a desiccant, and it can stay at room temperature for a year or more. Which is insane”. The approach could make shark diet studies more accessible worldwide. “Can this methodology be applied to anybody, anywhere, at any time? That’s what I’m trying to test,” explains Brett. It also has the added bonus of being less invasive, and thus reducing the animal’s stress, in comparison to other sampling methods.
With a year left of his PhD research, Brett is looking forward to making sense of all his data, and making recommendations based on his findings that will help towards the better protection of the sharks of Flower Garden Banks: “[We’re] looking at the interactions of how these species not only move on a bank, like over space and time, but move between the different banks and not only between the banks, but leave the sanctuary and come back. And that’s a really important at the end of the day for trying to understand how effective is management at protecting these species? Especially from commercial fishing pressures.”
You can read more about Brett’s project here.
ABOUT OUR GUEST
Brett Sweezey
As a child, Brett remembers sitting on the floor wide-eyed, watching BBC Earth’s The Trials of Life series and the episode ‘Hunting and Escaping’, where a pod of orcas deliberately beached themselves on the shoreline of Patagonia to capture a lone sea lion. His interest and excitement in marine predators naturally led him to the fascinating world of shark ecology. After receiving an undergraduate degree in biology from Old Dominion University in Virginia, USA, Brett volunteered at the Bimini Biological Field Station in The Bahamas, where he was introduced to the technological advances of acoustic transmitters and satellite tags. He became interested in the techniques for tagging animals and how even the simplest tags could be used to collect movement, depth and water temperature data.
Brett’s fascination for utilising electronic tags led him to graduate school at the University of New England, where he used acoustic transmitters and satellite tags to estimate discard mortality by analysing the movement (or lack thereof) of Atlantic cod and dusky sharks, respectively. However, his thought process relating to movement ecology shifted as he became curious about how the distributions of predator populations could be associated with foraging behaviour.
Brett is now pursuing his PhD in marine biology at Texas A&M University at Galveston to estimate the habitat use and ecological connectivity of scalloped hammerhead, silky and sandbar sharks at the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico. He plans to combine electronic tagging techniques with stable isotope analysis to estimate the movement patterns and trophic ecology of shark species in this region. These data can be used to inform future ecosystem-based fisheries management strategies at this remote and isolated marine sanctuary.
Instagram, Threads and TikTok: @saltwatersweezey
https://www.tamug.edu/wellsfisherieslab/people.html
