The shark took my fish: Foster Scholar Grace Casselberry investigates depredation in Florida Keys National Marine Sanctuary

By Yaamini Venkataraman

October 2019

It’s a bright, sunny day in Florida Keys National Marine Sanctuary — perfect for a fishing trip. As you sit on your boat, enjoying the slight rocking from the waves, you feel a tug on your line. You wrestle with an Atlantic tarpon, a large, shiny silver fish that many anglers flock to this national marine sanctuary to catch. Just as you’re about to bring the fish out of the water, a hammerhead shark swoops in and eats your prize. As you sit there mourning the loss of your tarpon, a little spark of optimism grows when you realize there is a scientist working on this exact issue. She is Dr. Nancy Foster Scholar Grace Casselberry, and she wants to prevent hammerhead sharks from eating your catch.

grace holding a small shark on a boat — Foster Scholar Grace Casselberry holds a blacktip shark. She became interested in shark science during college when she read scientific papers about animal tracking, and is now a member of the Gills Club’s Science Team. The program, run by the Atlantic White Shark Conservancy, connects young girls interested in marine science with shark science researchers like Casselberry. Photo © Dana M. Bethea

For her Ph.D. in marine science and technology at the University of Massachusetts Amherst, Casselberry is studying this phenomenon in Florida Keys National Marine Sanctuary. Her project involves tracking hammerhead sharks to figure out when and where they interact with Atlantic tarpon. This information will indicate where hammerheads take advantage of tarpon fishing in the national marine sanctuary, and help resource managers and fishing guides reduce this interaction.

Like much of the National Marine Sanctuary System, Florida Keys National Marine Sanctuary is a multi-use space. The sanctuary protects the ecosystem and its inhabitants, but visitors can still swim, dive, boat, and fish. Anglers are one group of adventurers that flock to the Florida Keys, many to fish for Atlantic tarpon. Here, they compete to see who can land such an iconic fish. This species is catch-and-release: once a tarpon is measured and photographed, it’s gently placed back in the water.

Recently, fishing guides in the Keys have reported more instances in which hammerhead sharks are snapping up Atlantic tarpon while they’re on the line at fishing hotspots. This phenomenon is called depredation.

“I have seen hammerheads move the tarpon around, just cruising outside the schools like a border collie herds sheep. It’s amazing to watch,” says Captain Kevin Grubb of Bahia Honda Fishing Charters. According to Casselberry, her research was sparked by the anglers themselves: discussions with these fishing guides led to an “organic growth of predator-prey research.”

fishermen attempting to reel in a tarpon — An Atlantic tarpon jumps out of the water as an angler attempts to reel it in. Photo courtesy of Grace Casselberry

When depredation occurs, Atlantic tarpon are eaten by hammerhead sharks instead of being released relatively unharmed. Since anglers try to target the largest trophy fish, those tarpon are the ones most at risk of being eaten. If the number of large Atlantic tarpon starts to dwindle because of depredation, the tarpon population may decrease, since large fish typically lay more eggs. If Atlantic tarpon numbers decline, anglers may no longer be interested in them and the Florida Keys economy could suffer.

Hammerhead sharks are also negatively affected by their depredation behavior. Taking advantage of an easy meal could make them less fit for survival. If anglers are frustrated with sharks eating their tarpon, the sharks themselves may be targeted. Because depredation is reportedly occurring at higher levels inside the sanctuary, Casselberry’s research will give her insight into how anglers are shaping animal interactions in a protected environment.

“Florida Keys [National Marine Sanctuary] is accessible for recreation, which is great, but we need to take into account how our activities are influencing natural predator-prey interactions,” says Casselberry. While hammerhead sharks probably do eat Atlantic tarpon in other contexts, Casselberry believes they don’t consume tarpon as “often or as easily as they’re able to when the tarpon is busy trying to get off the fishing line.”

two people tagging a hammerhead shark — Casselberry works with her advisor Dr. Andrew Danylchuk to tag a hammerhead shark. Without knowing where hammerhead sharks are within the sanctuary, it is difficult for sanctuary managers to take the best course of action to reduce depredation. Photo courtesy of Grace Casselberry

Studying shark movement isn’t new to Casselberry. While pursuing her masters degree at the University of Massachusetts Amherst, she examined the movement of sharks in Buck Island Reef National Monument in the U.S. Virgin Islands. Now, she’s not only tracking the predators, but also the prey. Atlantic tarpon movement patterns are not well-studied. Casselberry’s fellow doctoral student, Lucas Griffin, tagged 150 tarpon of different ages and sizes to start tracking their movement, sharing the data with Casselberry to use in her study.

“Knowing where animals go is one of the biggest pieces that you need to effectively conserve a species,” Casselberry says. “You cannot make any kind of management decision if you don’t know where [fish] are. From a sanctuary standpoint, you have this boundary trying to protect an ecosystem. You need to know what is using that [protected] ecosystem.”

From March to June 2019, Casselberry embarked on her first field season in Florida Keys National Marine Sanctuary. She tagged great hammerhead sharks with electronic tracking mechanisms and visually surveyed the sanctuary for interactions between Atlantic tarpon and hammerheads during the peak tarpon fishing season. To help her understand fish movement the remainder of the year, acoustic receivers throughout the sanctuary “listen” for tagged fish. Without knowing where the fish go, it is difficult to recommend effective solutions for depredation.

grace cleaning an acoustic receiver — Casselberry cleans acoustic receivers, which are used to detect fish. As tagged tarpon or shark swm by the receiver, it takes note of their movement. “I can have those receivers listening for me all year long, so I only have to be down [in Florida] for a field season to track the animals,” Casselberry says. Photo © Dana M. Bethea

Once Casselberry has a better idea of where hammerhead sharks and Atlantic tarpon are within the sanctuary and what factors contribute to depredation, she can work with resource managers and the sanctuary to test potential solutions.

“Understanding the interactions between anglers and fish is important for managers to make informed decisions to help protect these recreationally important species,” says Sarah Fangman, superintendent of Florida Keys National Marine Sanctuary. “This study will provide critical information about the potential impacts of shark depredation on targeted tarpon.”

Casselberry also hopes to discuss with fishing guides various solutions, like shark deterrent devices. Angler support will be key to successfully implementing solutions.

"[Atlantic tarpon] is my favorite fish to catch. We have progressed over the years and learned how to catch more and more tarpon. It's good for business, but we are hurting ourselves by losing more than what the sharks would take anyway," says Captain Grubb. "The importance of this research is to find a way to still catch tarpon, yet not lose too many to the sharks. I hope we can learn as much as possible so that we have many more years of catching such a magnificent creature."

A few decades from now, you and your grandchildren are enjoying another bright, sunny day in the Florida Keys. You recount to the kids the tale of a hammerhead shark taking your prize Atlantic tarpon. They listen intently as you tell them of a scientist named Grace Casselberry, who started investigating depredation in Florida Keys National Marine Sanctuary, the managers who implemented strategies to reduce it, and fellow anglers who provided their support. As you cast your line into the blue waters, you’re sure a hammerhead won’t interfere.

Yaamini Venkataraman is a volunteer social media intern for NOAA’s Office of National Marine Sanctuaries and a graduate student at the University of Washington.