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Predators and Prey: Understanding Shark Depredation in Our Fisheries
Grace A. Casselberry and Dr. Andy J. Danylchuk, University of Massachusetts Amherst
If you’ve been tarpon fishing in the Keys, Boca Grande, or Tampa Bay, you’ve likely had at least one run-in with a shark in the last few years. Maybe you found your fish unexpectedly bitten off at depth by an unseen shark, or a great hammerhead has gotten up close and personal with both your fish and your boat. Hooked fish can be relatively easy targets for foraging sharks, which are likely attracted by the sounds and electromagnetic signals of a fight. In a way, it’s all about a cost-benefit analysis; it uses less energy to catch a struggling fish that seems hurt than to chase down one that is healthy and free swimming. Why wouldn’t a shark choose to go after an easy meal?
At first, it might have been exciting to see a rare predation event, but after the fifth, tenth, twentieth time, you’re fed up. As one guide put it in a recent survey on shark-angler interactions run by our lab, “I often fish the Tortugas and we’re forced to fish around the sharks. They make us move from areas within minutes where in the past we’ve fished that same spot for hours. It’s like they know boats equal food.” Shark depredation, the full or partial removal of a hooked fish by a shark before it is landed, is reportedly increasing in U.S. recreational fisheries, including the tarpon fishery. However, accurately quantifying depredation is quite difficult, especially in recreational fisheries where anglers are decentralized, plus there is no official system in place to report when a depredation event occurs.
But in Bahia Honda in the Keys, where tarpon aggregate in large numbers, it doesn’t take long to hear stories from guides and anglers about how their catch is chased down and chomped by a great hammerhead before it is landed. Given that this is likely a tarpon prespawning aggregation site, high depredation rates could pose a significant threat to tarpon stocks. Similarly, with some shark populations still on the decline, simply removing sharks from the equation is also not an option, especially given the important role they play in marine ecosystems.
To better understand the factors that contribute to depredation, our collaborative team from UMass Amherst and Carleton University began using a multifaceted approach in 2019 to study the depredation of tarpon by sharks in Bahia Honda. The first part of the project uses three different but complementary tagging techniques to quantify the site fidelity of great hammerhead sharks to Bahia Honda, and whether they synchronize their movements to match those of their potential tarpon prey. Great hammerheads caught under or near the Bahia Honda Bridge are tagged with color-coded numerical cattle tags attached to their dorsal fins to allow for visual identification, either by researchers observing the angling action, or by guides and anglers. This helps determine how frequently depredation is actually occurring in Bahia Honda and whether the same sharks are repeat offenders.
From an observation station on the old railroad bridge in Bahia Honda State Park, armed with high-powered binoculars and an umbrella to beat the heat, a scientist observed nearly 400 tarpon hook-ups from 55 boats over the course of the 2019 tarpon season. Just over 25 percent of hooked tarpon were successfully leadered and released, while at least 15 percent were eaten by sharks, either while on the line or immediately after release. The rest of the tarpon spit the hook or broke the leader.
Two dorsal-tagged great hammerheads were spotted eating hooked tarpon in Bahia Honda, while another was observed herding a school of free-swimming tarpon on the flats near the Seven Mile Bridge. However, the vast majority of feeding sharks were untagged, indicating that while this behavior may be learned, many great hammerheads likely come to Bahia Honda to feed on free-swimming tarpon, taking advantage of hooked tarpon while in the area as part of their normal prey pursuit. Preliminary results show that longer fights and the more tarpon jumps increased the likelihood of tarpon mortality. A tarpon that jumped two or more times within the first 10 minutes of the fight had a much higher probability of mortality than a tarpon that jumped fewer than two times in the first ten minutes of the fight. The likelihood of tarpon mortality also increased significantly for fights over 20 minutes long.
Great hammerheads in our study are also being tagged with acoustic transmitters to determine whether the same sharks frequent Bahia Honda year after year, whether their movement patterns align with that of tarpon also tagged in this area, and how far these sharks travel between tarpon seasons. In addition to a cluster of acoustic receivers deployed in Bahia Honda pass that allows us to look at the finer scale movement patterns of both the great hammerheads and the tarpon, our project relies on a vast network of receiver stations in the Florida Keys, the Gulf of Mexico, and along the east coast of the United States. To date, preliminary results show a mix of movement patterns for greater hammerheads that are caught in Bahia Honda Channel. While some remain in the Florida Keys year-round, others make long-distant migrations north in the summer, only to return to Bahia Honda the next season, very similar to tarpon. In 2020, the team began satellite tagging great hammerheads to learn more about where they go when they are not present within the acoustic array. The vast majority of sharks tagged in Bahia Honda during this study are large, mature, female great hammerheads. It is very likely that Bahia Honda is a feeding ground critical to reproductive biology, just like the tarpon, with the added benefit of a free lunch served by anglers.
Collectively, the dynamics of what’s happening among sharks, tarpon, and recreational anglers in the Bahia Honda Channel is complex and is of great conservation concern. However, depredation is not unique to Bahia Honda, or even the broader Florida Keys, and it is important to put local issues in a broader context. As such, we conducted a nation-wide survey of coastal anglers and guides, including those that frequent the Keys, about their interactions with sharks. Results from this aspect of our study show that depredation is happening regularly across the east coast of the US but is particularly frequent in Florida. Nearly 150 guides and 400 anglers responded to the survey, with 77 percent of respondents having experienced depredation at least once while fishing recreationally. Among guides, 75 percent reported that depredation had increased dramatically in the last five years and 87 percent had experienced depredation with clients. Half of guides felt that depredation had a negative effect on the fishing experience for their clients.
With more people out on the water than ever before, and no historical data to quantify depredation rates in the past, it is difficult to know if depredation really is increasing or if there are just more opportunities for these encounters to happen. Regardless, it is important to work to minimize depredation to keep our fish stocks healthy and fisheries sustainable. In addition to modifying fishing practices, this could include testing shark deterrent devices for recreational fishing, which are currently in development, in areas where depredation is frequent. As responsible tarpon anglers, we need to find ways to sustainably compete with the natural predators in the ecosystem.
