Research reveals why more hooked tarpon are eaten

In research on wave creation recently published in Marine and coastal fisheriesa team of researchers led by biologists at the University of Massachusetts, Amherst, quantified the rate at which great hammerhead sharks eat Atlantic tarpon hooked by fishermen in Bahia Honda, Florida, one of the top tarpon fishing spots in the Florida Keys.

Called “depredation rate,” the team found that 15.3 percent of tarpon hooked by anglers and fought for more than five minutes were eaten while still on the line. But researchers also show that this is not necessarily a sign of an imbalance in the ecosystem. On the contrary, we should expect an increase in reports of predation, especially since great hammerhead sharks, classified as critically endangered by the International Union for Conservation of Nature (IUCN), see their population stabilize in the southeastern United States; the result of decades of conservation and management efforts.

At the same time, angling is an increasingly popular sport, which means there are more chances of encountering humans, fish and sharks. To help manage the health of the tarpon fishery and hammerhead shark population, researchers are advocating solutions that do not impact either species.

Tarpon are one of the most iconic saltwater fish of the Southeast and Gulf states. Many anglers spend their lives dreaming of hooking a tarpon that can easily exceed 100 pounds, known to fight fiercely, often jumping out of the water in their efforts to shake off a hook. Tarpon fishing, which stretches from Texas to the Carolinas in the United States, is by some estimates a multimillion-dollar-a-year industry, and the fish is deeply tied to local culture.

Yet despite the species’ legendary hardiness, the tarpon is classified as “vulnerable” by the IUCN, and its populations appear to have been affected by fishing, degraded water quality and habitat loss. Recently, guides are increasingly reporting that sharks are taking a larger share of tarpon catches in recent years and may actually pose a risk to the survival of the species. But until now, there was no concrete data on depredation rates, making it difficult to make informed conservation decisions, whether for tarpon or hammerhead sharks.

To arrive at the depredation rate and then track the annual movements of tarpon and sharks in a specific area, you need a few things: high-tech acoustic telemetry equipment, sturdy fishing gear, and a comfortable lawn chair.

Acoustic telemetry has recently revolutionized scientists’ ability to track migratory marine species. The technique involves anchoring an acoustic receiver in the water and implanting a small transmitter in whatever you want to track.

In this case, lead author Grace Casselberry, a postdoctoral researcher at UMass Amherst, and her colleagues deployed 16 receivers in a mesh network in the Bahia Honda Channel. They then captured and tagged 51 tarpon and 14 hammerhead sharks. For more than two years, every time one of the tagged tarpon or hammerhead sharks swam within range of the receiver, it recorded that animal’s unique identification, date and time.

Then came the lawn chair. “I sat in that chair for two months,” says Casselberry, “all day watching with binoculars and a long-lens camera as people fished. Every time someone hooked a tarpon, I recorded the time of day, the current, whether the tide was rising or falling, what boats were fishing, how many fishermen were in the area, how long it took them to get the tarpon to their boat, and whether a hammerhead shark had ate the fish or not. I saw a total of 394 tarpon hooked.”.

With all this data, the researchers revealed that the longer the fisherman fought with his tarpon, the more likely it was to be eaten, and that when the fight lasted more than five minutes, there was a 15.3% chance that the tarpon is snatched. by a hammer.

These depredations most often occur on an outgoing current, which was also confirmed by acoustic telemetry data which showed that hammerheads occupied a smaller area in the channel corresponding to where most tarpon are held and fought for. The team also discovered that tarpon tend to congregate in Bahia Honda in the spring, before spawning, and the hammerheads know it. Fishermen too.

“Bahia Honda is most likely a place where sharks and tarpon have been congregating for a very long time,” says Andy Danylchuk, lead author and professor of fish conservation at UMass Amherst. “If there has been less depredation in recent times, this is likely because the population of great hammerhead sharks was dangerously low.” But fishing pressure has also increased in recent decades.

“There are more sharks in the water and also more hooks in the water,” Danylchuk continues, “which is the perfect recipe for more shark-fish-human encounters.” In fact, predation is a growing problem in the United States, as evidenced by the recent SHARKED Act introduced in Congress to help find solutions.

Unfortunately, the more fishermen and guides see their prized fish taken by sharks, the more likely they are to advocate for shark culls.

“It’s taken 30 years to get hammerhead sharks to the point where they’re just starting to recover,” says Casselberry, “and all that work could be undone if we start killing sharks indiscriminately.”

“There is evidence that hammerhead sharks are gravid females,” adds Danylchuk, “and if they are extirpated, it could decimate their numbers.”

None of this means that fishermen should stop fishing for tarpon in Bahia Honda, but it does mean that conservation efforts, both for tarpon and hammerhead sharks, must be guided by solutions that do not have impact on tarpon, hammerhead sharks or fishermen.

Casselberry and his colleagues suggest anglers use fishing gear that will allow them to land tarpon more quickly, reducing fight times and the risk of depredation. They should also avoid fishing during the outgoing tide, when most predation events occur. Anglers using fish finders should monitor for sharks and consider moving when hammerheads are in the area.

“We’re asking fishermen to see themselves as part of the ocean ecosystem, rather than fighting against it,” says Casselberry.