6 minute read
You Are What You Eat
A new study will determine tarpon feeding habits and prey sources during the seasonal migration.
BY CHRIS SANTELLA
We are what we eat, as the old saw goes. And we travel to places that serve up our favorite foods. Or in the case of tarpon, the forage that’s necessary to survive.
The Tarpon Isotope Study, initiated by scientists at University of Massachusetts Amherst and underwritten by Bonefish & Tarpon Trust, aims to gather data about tarpon feeding behavior at different points in their migration cycles. Determining the connections between prey species and habitats can help inform improved management of forage fish and highlight alterations in water conditions, both factors that impact the long-term health of tarpon populations.
“The conventional wisdom has been that tarpon spawn somewhere in Southwest Florida and then migrate north to feed,” said Dr. Aaron Adams, BTT Director of Science and Conservation. “Acoustic tracking studies have given us a better sense of the fish’s regional movements. The Tarpon Isotope Study will allow us to overlay spatial data points with data points about feeding behavior to give us a better sense of what they are eating as they migrate.”
Isotope analysis can show us what chemical elements are contained in a plant or animal. It’s often used to investigate the flow of energy through a food web; in other words, to determine a person or animal’s diet. “The analysis shows isotopic ratios of carbon, nitrogen and sulfur,” explained Dr. Luke Griffin, a BTT collaborating scientist and Post-Doctoral Fellow at the University of Massachusetts Amherst. “From these data points, we can get a good idea of not only what the fish rely upon for food but also what habitats and ecosystems are important for foraging.”
Stomach pumping could yield similar results, but tarpon are sensitive to the procedure, and isotope analysis provides a more comprehensive approach.
To obtain samples, researchers have relied on fishing guides and anglers to obtain fin clips from fish they’ve caught. “We’ve been able to get people to participate through postings on BTT’s social media channels, and through our newsletters and our longterm relationships with guides and anglers,” Adams continued. “UMass also has a good network within the sport fishing community, and they’ve been able to recruit participants.”
Once they sign up, participants receive a fin clip kit that includes ethanol-filled vials, small scissors, a tape measure, and a note card. Anglers and guides are instructed to clip a centimeter of tissue from the trailing edge of the dorsal fin, place it in a uniquely numbered vial, and record/estimate the tarpon’s size, as well as the date and location where it was caught. A postage paid envelope is provided to facilitate the tissue’s return. “Once we receive a fin clip, we dry it out for 48 hours,” Griffin continued. “Then it’s sent to one of our collaborators, Dr. Michael Powers, at the University of Waterloo, Ontario. There, the fin is pulverized and sent through the machine for analysis. We receive results a few months later.”
During the first year of the Tarpon Isotope Study, approximately 250 fin clip kits were distributed to nearly 100 guides and anglers, resulting in over 300 tarpon fin clip samples from both juvenile and adult fish. From the reports for this year, the project hopes to double the number of fin clips collected during Year 1 across five regions: Northern Gulf of Mexico, West Florida, South Florida, Eastern Florida/Georgia and the MidAtlantic states.
Kellie Ralston, BTT’s Vice President for Conservation and Public Policy, articulated the management implications stemming from the study. “Environmental changes can cause shifts in prey locations, and prey regime. If these shifts can be tied to certain causal effects—say a change in freshwater flows into estuarial habitat—we will advocate for policy to better manage those flows.”
The study may also illuminate the importance of menhaden as tarpon forage, especially for fish migrating north and west. “The management of menhaden on the east coast is fairly structured, though harvest is concentrated, mostly around Chesapeake Bay,” Ralston continued. “The Gulf commercial fisheries are much less regulated.”
Menhaden, also known as bunker or pogy, have been called the most important fish in the sea, as so many larger fish and birds rely on them for food. Gulf menhaden are found from the Yucatán to Tampa Bay, and Atlantic menhaden from central Florida to Nova Scotia. Humans harvest menhaden for use as bait and in fertilizer, paint and cosmetics.
“We’ve seen declines in redfish the last dozen years,” opined Captain Ty Hibbs, who guides out of Delacroix, Louisiana. “Some of that is a result of bad spawns, some a result of bycatch from shrimp boats. But many of us feel that the commercial menhaden boats are the main problem. Not just bycatch, but overharvest. The menhaden industry is currently self-regulated.” Captain Hibbs believes that when the tarpon arrive in Louisiana in July and August, they are looking for menhaden. But this is speculation. The Tarpon Isotope Study may provide the ammunition needed to advocate for more responsible menhaden management in the Gulf.
While results are not definitive as data collection is ongoing, Griffin shared a few observations. “Our data suggest that tarpon are generalists, meaning they rely on prey from both marine and freshwater environments,” he said. “Specifically, these prey items fell into two categories—benthic (prey from the bottom like crabs and pinfish); and pelagic (prey in the water column, like menhaden, mullet, and threadfin). That being said, we found that some individuals were highly dependent on specific environments. For example, several West Florida fish were feeding and residing primarily in freshwater environments. We suspect that additional data will show a similar pattern where some fish are more “specialist” and focus on specific environments and prey. In contrast, others are more “generalist” and feed opportunistically.”
Early results have also shown variations in feeding behavior from tarpon in western Florida and fish in the south and east. Western Florida fish seem to have fed across many different species and environments, while tarpon in eastern and southern Florida seem to rely on a few key prey species. “Results also suggest that juvenile tarpon depend on freshwater ecosystems,” Griffin added, “which are often threatened by development and improper water management.”
Chris Santella is the author of 21 books, including the popular “Fifty Places” series from Abrams. He’s a regular contributor to The New York Times, The Washington Post, and TROUT.
