Stories from the Blue: Grace Casselberry

April 2024

Meet Dr. Nancy Foster Scholar, Grace Casselberry. Grace protects the ocean's top predators while studying the interactions between great hammerhead sharks (Sphyrna mokarran) and Atlantic tarpon (Megalops atlanticus) in Florida Keys National Marine Sanctuary. Tarpon are one of the most iconic saltwater fish in the Southeastern and Gulf states, and the great hammerhead shark is listed as a Critically Endangered Species by the IUCN. When a shark snatches a fish that an angler is fighting on their fishing line before it can be landed, this interaction is called a "shark depredation" event. Depredation has a negative impact on the livelihoods of fishing guides. At the same time, it has taken 30 years for great hammerhead shark populations to begin to recover. These sharks are wild animals that belong in these habitats, and they need to eat. The information Casselbury has collected indicates where hammerheads take advantage of tarpon fishing in the national marine sanctuary, and could help resource managers and fishing guides reduce this interaction.

A recent study in Marine and Coastal Fisheries that Casselbury is the lead author on, quantifies the rate at which great hammerhead sharks are eating Atlantic tarpon hooked by anglers at Bahia Honda in Florida Keys National Marine Sanctuary. The data in this study revealed that depredation events at Bahia Honda most often take place during an outgoing tide, and that the longer the angler fought a tarpon on the line, the more likely it was to be eaten by a shark. Furthermore, when the fight lasted for more than five minutes, there was a 15.3% chance that the tarpon would be snatched by a hammerhead.

In order to protect the recovering population of hammerhead sharks while also supporting anglers, Casselberry and her colleagues suggest that anglers use fishing gear that will allow them to land tarpon faster, thus reducing fight times and the opportunity for depredation.