Research reveals why more suspended tarpon are eating

Research reveals why more suspended tarpon are eating

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The hooked tarpon is an easy target for hungry hammerheads. Credit: Captain Bobby Spano

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The hooked tarpon is an easy target for hungry hammerheads. Credit: Captain Bobby Spano

In wave-making research recently published in Marine and coastal fisheriesA team of researchers, led by biologists at the University of Massachusetts Amherst, measured the rate at which great hammerhead sharks eat Atlantic tarpon caught by fishermen in Bahia Honda, Florida — one of the prime tarpon fishing locations in the Florida Keys.

Calling it the “predation rate,” the team found that 15.3% of tarpon that fishermen snatched 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 that the ecosystem is out of balance. Conversely, reports of depredation are expected to increase, especially as great hammerhead sharks, listed as Critically Endangered by the International Union for Conservation of Nature (IUCN), see their numbers stabilize in the southeastern United States; The result of decades of conservation and management efforts.

At the same time, fly fishing has become an increasingly popular sport, which means there is a greater chance of encounters with humans, fish and sharks. To help manage the health of both tarpon fisheries and hammerhead populations, researchers are urging solutions that do not impact either species.

Tarpon are one of the most popular saltwater fish in the Southeast and Gulf states. Many anglers spend their lives dreaming of hooking a tarpon that can easily exceed 100 pounds, and they are known to fight ferociously, often jumping completely out of the water in their efforts to shake the hook. The tarpon fishery, which extends from Texas to the Carolinas in the United States, is, by some estimates, a multi-million-dollar-a-year industry, and the fish is closely linked to local culture.

However, despite the species’ legendary power, the tarpon is listed as “Vulnerable” by the International Union for Conservation of Nature, and its populations appear to have been affected by fishing, deteriorating water quality, and habitat loss. Recently, guides have increasingly reported that sharks are taking a bigger bite out of tarpon catches in recent years, and this may actually pose a threat to the survival of the species. However, to date, there is no consistent data on the rate of depredation, making it difficult to make informed decisions for tarpon or hammerhead conservation.


Awesome hammer takes a rag off the boat at Bahia Honda. Credit: Grace Casselberry

To get to the depredation rate, and then track the annual movements of tarpon and sharks across a given 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. This technique involves installing an audio receiver in the water and planting a small transmitter on whatever you want to track.

In this case, lead author Grace Casselberry, a postdoctoral researcher at the University of Massachusetts at Amherst, and her colleagues deployed 16 receivers in a grid array in the Bahia Honda Channel. They then caught and tagged 51 tarpon and 14 hammerhead sharks. Over the course of more than two years, each time a tagged tarpon or hammerhead swam within range of the receiver, the receiver recorded that individual animal’s unique identity, date, and time.

Then came the lawn chair. “I sat in that chair for two months, watching all day through binoculars and a long-lens camera as people fished,” Casselberry says. “Every time someone tied a tarpon, I would record the time of day, the current, and whether the tide was turning in.” “Or offshore, what boats were fishing, how many anglers were in the area, how long it took them to get the tarpon to their boat, and whether or not the hammerhead had eaten the fish. I saw a total of 394 tarpon hooked.”

With all this data, the researchers revealed that the longer a hunter fought the 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 of the tarpon being snatched. By hammer.

These depredations often occurred on an outgoing stream, which was also supported by acoustic telemetry data that showed that hammerheads occupied a smaller space within the channel opposite where most tarpon hooked and fought. The team also found that tarpon tend to congregate at Bahia Honda during the spring and pre-spawning seasons, and hammerheads know this. So do fishermen.

“Bahia Honda has likely been a gathering place for sharks and tarpon for a very long time,” says Andy Danilchuk, senior author and professor of fish conservation at the University of Massachusetts Amherst. “If there has been less depredation in recent memory, it is likely due to the fact that the population of great hammerheads has been dangerously low.” But hunting pressure has also increased in recent decades.

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

Unfortunately, the more fishermen and guides see their long-awaited sharks being snatched by sharks, the more likely they are to call for a shark cull.

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

“There is some evidence that hammerheads are pregnant females, and if they were culled, it could decimate their numbers,” Danylchuk adds.

None of this means that fishermen need to stop fishing for tarpon in Bahia Honda, but it does mean that conservation efforts for both tarpon and hammerheads must be informed by solutions that do not affect the tarpon, hammerheads, or fishermen.

Casselberry and her colleagues suggest that hunters use fishing gear that allows them to catch tarpon faster, thus reducing fighting times and opportunities for depredation. They should also avoid fishing during the outgoing tide, which is when most depredation events occur. Anglers using fish finders should watch for sharks and consider relocating when hammerheads are in the area.

“We invite fishermen to think of themselves as part of the ocean ecosystem, rather than working against it,” Casselberry says.

more information:
Grace A. Casselberry et al., Depredation rates and spatial overlap between large hammerheads and tarpon in a hot recreational fishing area, Marine and coastal fisheries (2024). doi: 10.1002/mcf2.10277

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