A man wipes his brow as he drops a receiver into the Gulf of Mexico, grunting with effort as it descends into the blue. The sun is high as he motors his boat to the next spot on his list, where “x” marks the spot for where he will lower the next scientific instrument below the choppy waves. Somewhere above, below or beside it, a shark glides past, unbeknowst to the scientist… yet. He might never see this animal himself with his own two eyes; in fact, the likelihood is that he will not know of its existence around him until the receiver records the coded signal from a tiny transmitter inside its body. Gradually, as time crawls on, individual detections become movements, movements become pathways and pathways begin to reveal something much bigger: how sharks actually experience a marine protected area.

That is what researchers have been doing around Flower Garden Banks National Marine Sanctuary (also known as the “FGBNMS”), an offshore protected area along the continental shelf of the northwestern Gulf of Mexico. The sanctuary encompasses coral reefs, deeper mesophotic habitats and other offshore banks that provide habitat for reef fishes and larger predators including sharks, tunas and billfishes. In 2021, the sanctuary expanded to include 14 additional banks, making it even more important to understand how highly mobile animals use this increasingly connected seascape. Because there is one fundamental problem with protecting an animal that moves through the ocean: a marine protected area has boundaries but a shark does not.

Between August 2022 and October 2025, a research team led by Dr. Brett Sweezy of the Shark Biology and Fisheries Lab at Texas A&M University at Galveston used acoustic telemetry to follow silky sharks ( Carcharhinus falciformis ) and sandbar sharks ( Carcharhinus plumbeus ) through this underwater network. The sharks were fitted with acoustic transmitters that emitted individual signals, which were picked up by receivers spread across 17 sanctuary banks, four banks outside the sanctuary and 14 artificial reefs, including oil and gas platforms, fish aggregation devices and shipwrecks. The team detected 24 of 34 tagged silky sharks and 24 of 29 tagged sandbar sharks. What they found was that these two closely related predators were not using the network in quite the same way.

The silky sharks were all juveniles, averaging about 5.3 feet (1.16 metres) long, and their movements changed with the seasons. Natural banks were particularly important during winter and spring, when their connectivity was highest. During summer, however, their use of natural banks dropped sharply, while artificial reefs remained comparatively consistent throughout the year. The pattern suggests that these young sharks may shift between different types of habitat as environmental conditions and resources change. Sandbar sharks, on the other hand, told a different story. The tracked animals were predominantly adult females, averaging about 6.6 feet (2 metres) long, and their connectivity patterns remained comparatively consistent across seasons. They also appeared to operate on a much larger geographic scale. While only 9% of detected silky sharks were recorded outside the acoustic network, 46% of detected sandbar sharks were detected elsewhere.

“I had my expectations, but was still a little surprised at both,” said Sweezy. “It was difficult to make assumptions about the sandbar shark because they are ‘generally’ coastal, so there can be assumptions that individuals captured near the sanctuary may be due to foraging behaviors. The tagged sandbars did not seem to hang around sites for extended periods of time compared to the silky sharks, and they also covered much greater distances across the array.” The silky sharks presented their own surprise. “Silky sharks are interesting because previous studies have frequently reported them near oil and gas platforms, but this study provides more detail on use of the sanctuary banks in addition to the platforms, which may also be driven by foraging preferences. These silky shark movements also raise questions about why habitat use varies between seasons.”

Eleven sandbar sharks were detected at 34 external receiver stations spanning the Gulf of Mexico and the western North Atlantic. Their movements took them to the Florida Keys, along the South Carolina and Georgia coasts, into Chesapeake Bay and North Carolina and as far north as Massachusetts. One shark, SB-07, accumulated a minimum documented travel distance of approximately 6,548 kilometres. Another, SB-11, completed a 522-day round trip between the FGBNMS network and Florida Keys National Marine Sanctuary . SB-32 also traveled between the two regions. From a human perspective, those locations can look like separate places scattered across a map, but for these sharks? They may just be different parts of the same connected seascape. But the researchers uncovered another unexpected pattern when they looked more closely at where the silky and sandbar sharks overlapped: the two species were detected at 15 of the same receiver sites, representing 43% of the network, yet they were rarely there at the same time. Their average Simple Ratio Index of temporal co-occurrence was just 0.002, and all 15 shared sites fell below the study’s threshold for meaningful temporal overlap. At Stetson Bank, for example, both species were detected on only nine days out of 321 days of detection.

But why would two sharks use the same places but apparently avoid being there together? The researchers suggest that their different sizes and life stages may help explain the pattern. Smaller juvenile silky sharks could face greater predation risk from larger sharks, including adult sandbar sharks, which are known to consume other elasmobranchs like sharks and rays. Avoiding one another in time could therefore allow these predators to share habitat without constantly competing or exposing smaller sharks to unnecessary risk. These findings also complicate how we think about artificial reefs — at the network level, neither species showed a simple preference for natural banks over artificial structures. Juvenile silky sharks repeatedly moved between sanctuary banks and artificial reefs, including oil and gas platforms (structures that can support concentrations of reef-associated fishes, potentially providing foraging opportunities for mobile predators). Oil and gas platforms do not exist primarily for wildlife, so what happens when a structure that has become part of an animal’s movement network is removed? Once they become established features of an ecosystem, their ecological role can become difficult to ignore. Decisions about decommissioning therefore have consequences that may extend beyond the platform itself, particularly for species using multiple habitats as interconnected movement nodes.

As one can see, there is a lot more questions than answers due to this recent publication. But for the scientists, it’s an exciting thread of what is sure to be an intricate ecological tapestry. All one needs to do is pull at the thread to see what answers lie at the end.

If a shark’s life takes it across a network of habitats rather than staying within one protected area, how do we make sure conservation efforts protect the places it actually needs? The answer may not be to make every protected area enormous. Instead, research like this suggests that conservation planning needs to recognize the connections between places. A sanctuary can protect an important piece of habitat, but the animals using it may depend on many other pieces of the seascape too.