Science

July 30, 2025 — The following was released by the Science Center for Marine Fisheries:

What do some of the Gulf of America’s most iconic predator fish really eat? Scientists at the University of Southern Mississippi are moving closer to answering that question with a groundbreaking new study that delivers some of the most detailed insights to date on the Gulf food web. To do this they’ve combined two powerful approaches: cutting-edge stable isotope analysis and an extensive meta-analysis of hundreds of published stomach content studies dating back to the 1950s.

Despite the ecological and commercial importance of species like red drum, summer flounder, and spotted sea trout, our understanding of their interactions with prey species has been limited. This new research challenges long-standing assumptions, most notably the idea that Gulf menhaden is a primary food source for these key predators.

Funded by the Science Center for Marine Fisheries (SCEMFIS), a member of the National Science Foundation’s Industry–University Cooperative Research Center program, the study provides a comprehensive understanding of the Gulf food web and charts the trophic interactions that structure it. The findings have fishery management implications for several of the species evaluated in the study. Most notably, Gulf menhaden was not found to be a primary food source for any of the predator species studied.

“We looked at some 30-plus predator species, many of them exceptionally well-studied. We did not find any single species where we would say Gulf menhaden was the most important fish in their diet,” said Dr. Robert Leaf, one of the authors of the study and Director of the School of Ocean Science and Engineering at the University of Southern Mississippi.

“When you look at the information that we have, what we find is that Gulf menhaden are a prey item—certainly they play a role in the trophic dynamics of predators—but not to the extent of other prey items, which are also very important—in fact, more important,” Dr. Leaf continued.

Traditionally, determining what predators eat has been limited to analyzing samples of their stomach contents. This provides information about what predators have eaten recently, but it offers only a limited snapshot and does not reveal long-term dietary patterns or prey availability. To bridge this gap, the study incorporates new data, developed from analyzing stable isotope levels taken from predator tissue samples.

Stable isotopes are heavier forms of elements, like carbon and nitrogen, that are present in all species and at all points in the food web. Because these isotopes do not decay, they accumulate in predator species in different proportions, depending on the diet of the predator. By analyzing the levels of stable carbon and nitrogen isotopes in predator species, the study authors are able to determine what types of diet sources the predators generally rely on, as well as what trophic level they predominantly feed on. This technique offers a much broader view of predator diets than stomach content analysis alone.

“When an animal eats a prey item, there is a differential uptake in the carbon and the nitrogen,” said Dr. Kevin Dillon, another author of the study and an Associate Professor at the University of Southern Mississippi’s Gulf Coast Research Laboratory. “So we can measure those small differences to try to piece this together and look at each organism’s trophic position within that food web. So we’re able to tell from a fish’s isotopic signature whether the fish was eating phytoplankton or if it was eating another fish that had eaten phytoplankton.”

The study pairs the data from the stable isotope analysis with a meta-analysis of hundreds of previously published stomach content databases dating back to the 1950s. Integrating the two data sources into a single modeling framework provides clearer insight into the role of low-trophic-level species in the Gulf. The scientists found that species like red drum, summer flounder, and spotted sea trout are general, opportunistic feeders that do not rely solely on a single prey species. Instead, their diets vary depending on factors such as seasons, prey availability, and other climatic conditions.

Species like Gulf menhaden are important parts of the diet, but there is no single prey species that these predators overwhelmingly rely on. There is no “most important” prey species in the Gulf.

While this study focused on the Gulf ecosystem and food web, its insights may have broader relevance for similar species and predator–prey dynamics in other regions. On the East Coast, for instance, Atlantic menhaden play a role comparable to their Gulf counterparts, serving as forage for many similar predators—including striped bass, summer flounder, weakfish, and bluefish—which are also common in the northern Gulf of America. While additional research is needed, the Mid-Atlantic ecosystem likely exhibits similarly complex trophic dynamics and variability in predator diets.

8-Minute Video Brings the Science to Life
An eight-minute YouTube video released alongside the study features Dr. Robert Leaf and Dr. Kevin Dillon explaining how their team used stable isotope analysis and decades of stomach content data to map the Gulf’s complex food web. Viewers will see the inside of the university’s isotope lab, close-up footage of the specialized equipment used to analyze predator tissue samples, and field scenes from the Mississippi coast where the research team collected specimens.

Dr. Leaf and Dr. Dillon walk through how their findings overturn long-held assumptions about the role of Gulf menhaden in predator diets—revealing instead a diverse, seasonally shifting prey base that includes crabs, shrimp, anchovies, and Atlantic croaker. They demonstrate how isotopic tracers such as carbon-13 and nitrogen-15 help identify what predators eat over time and how high they sit in the food web.

Also featured is graduate student Calvin Chee, who offers a field-level perspective and personal observations about the Gulf’s biodiversity. Together, the scientists describe how combining isotope data with traditional stomach content analysis provides a more complete, long-term view of predator–prey interactions.

About the Methodology
Around the world, scientists have increasingly turned to this emerging method of combining stable isotope analysis (SIA) and stomach content analysis (SCA) to better understand what animals eat. Stomach content analysis provides a direct but short-term snapshot of recently ingested prey, while stable isotope ratios in muscle tissue reflect assimilated diet over longer time frames. When used together, the two methods offer complementary perspectives on trophic relationships across seasons, habitats, and life stages.

This dual approach has now been used successfully in ecosystems around the globe from tropical rivers to polar seas, including nine species in the Eastern Adriatic Sea, coral reef groupers in Australia’s Great Barrier Reef, and Nile perch in East Africa’s Lake Victoria.

No such combined study has been conducted involving menhaden species in U.S. waters—until now.

Note on Naming: The body of water traditionally known as the Gulf of Mexico has been renamed “Gulf of America” within U.S. federal usage, following Executive Order 14172 issued earlier this year. As of early 2025, U.S. agencies, including the Coast Guard, and major platforms like Google Maps, Apple Maps, etc. (for U.S. users) have adopted the new name in compliance.