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Shark Sleuth

For marine scientist Jenn Caselle, few places are as ideal for studying sharks as Palmyra Atoll.

“We are using acoustic telemetry to understand how these predators are moving and utilizing habitat around the atoll.”

Dr. Jenn Caselle
Marine ecologist, UC Santa Barbara 

For Dr. Jenn Caselle, a marine ecologist with the University of California at Santa Barbara, few places are as ideal for studying sharks as Palmyra Atoll.

“I’ve been conducting research at Palmyra for more than a decade, and every time I go down there I have to reset my baseline,” she says. “There are just so many sharks.”

Caselle’s colleague, University of Hawaii scientist Dr. Alan Friedlander, makes a similar mental adjustment. “Every time I jump in the water at Palmyra, I am reminded that I am not at the top of the food chain,” he says.

Caselle vividly recalls a recent research dive off a rarely-visited section of Palmyra’s north coast. “We were quickly surrounded by more sharks that I could count,” she says. “I spend a lot of time in the water around sharks but in that instance, I have to admit, I experienced a moment of real trepidation.”

And yet it is Palmyra’s robust shark population that draws Caselle and many other top marine scientists to the atoll. Palmyra is the world’s most natural marine laboratory, and one of the few places in the world where scientists can study sharks in their natural densities. Its coral reefs contain complete food webs dominated by sharks and other top predators, enabling scientists to study everything from sharks to microbes.

“We know that Palmyra is a healthy system and that sharks are a key part of that,” says Caselle. “But the exact linkages and how apex predators like sharks exert their influence…we don’t have the answers yet. We know that corals seem to be healthier in systems with complete food chains. So our research is trying to answer that much bigger question.”

Caselle and other scientists from the Palmyra Atoll Research Consortium are working to further understanding of the connections between apex predators and ecosystem health. Caselle works collaboratively with marine scientists from the University of Hawaii, the University of California at Berkeley, Cal State Long Beach, the Florida Museum of Natural History and The Nature Conservancy.

On most trips, a good portion of their time is spent tagging sharks. Working the lagoons and reefs, they hook a shark, pull it alongside the boat, and within minutes release it back into the ocean with a small acoustic transmitter inserted inside. Each transmitter has its own code and broadcasts the movements of the shark, which are recorded by an extensive system of receivers located around the atoll and throughout the lagoons.

“Most of what we’ve done to date is to look at the movement patterns of many of Palmyra’s top predators, including blacktip and gray reef sharks,” Caselle says. “We are using acoustic telemetry to understand how these predators are moving and utilizing habitat around the atoll.”

The goal, she says, is to inform shark conservation efforts by gaining a better understanding of how sharks interact with each other when they are restored to natural densities, and how they interact with their prey.

“We can apply this work to the design of marine protected areas by knowing what sort of space sharks need,” she says. “As shark conservation efforts begin to succeed globally, and shark populations increase in certain areas, we will be able to tell people how sharks are likely to behave and what their effects are on marine animals lower on the food chain.”

In time, they hope their work will also help explain the relationship between healthy shark populations and healthy coral reefs; how sharks—their movements, their behavior and the role they play in marine ecosystems—are crucial to coral reef restoration efforts around the world.
 

 

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