A new study conducted by the University of Southern Mississippi has revealed more details of the diets of common predators in the Gulf of Mexico – currently referred to as the Gulf of America by the U.S. government – and the findings and methodology could impact fisheries management of key commercial species.
The study, funded by the Science Center for Marine Fisheries (SCEMFIS), combined both extensive existing data and new stable isotope analysis to determine what common predators in the Gulf eat. The study – which was summarized in a video on the SCEMFIS YouTube channel – looked at more than 30 different predator species and used hundreds of published stomach content studies dating back to the 1950s.
SCEMFIS is a member of the National Science Foundation’s Industry-University Cooperative Research Center program and is, in part, funded by a range of fishing industry partners including Lund’s Fisheries, Atlantic Capes Fisheries, and Bumble Bee Seafoods.
The study revealed that across all species – including commercially important species like red drum, summer flounder, and spotted sea trout – no predator had a single “most important” prey species as part of their diet and instead relied on a web of other species as food sources. That finding includes Gulf menhaden, which was not found to be a primary food source for any of the species studied.
That finding could have direct implications for fisheries managers as they begin to consider more ecosystem-based methodologies for setting fishing quotas. NOAA released a draft of an ecosystem-based fisheries management roadmap in 2024. The goal behind the new management methodology is to better consider a given species’ role in the wider ecosystem when making decisions on quotas.
Menhaden has been a species in the crosshairs of multiple stakeholders, as the Atlantic States Marine Fisheries Commission faces criticism over quotas for the species and wrangles with how to determine fishing pressure while also considering menhaden’s role as a prey species under new ecosystem management considerations. NGOs and critics of the fishery have claimed the science on menhaden is lacking and attributed declines in predator species in the Chesapeake Bay like striped bass or osprey to the existence of a large commercial fishery for the species.
Study author and Director of the School of Ocean Science and Engineering at the University of Southern Mississippi Robert Leaf told SeafoodSource he would not directly tie the Gulf study’s results to the East Coast – but said the study provides a roadmap for similar studies elsewhere.
“We could use the results of this study as a testable hypothesis for the Mid-Atlantic,” he said.
The study did reveal a few themes, with an overarching one being the ecosystem is always more complex than a direct relationship between one predator species and one prey species.
“I think we all understand that that is probably not accurate, but that hasn’t really reached the stakeholders very well – the inaccuracy of that,” Leaf said.
For both menhaden and Gulf predator species, population dynamics are complex and interwoven across multiple trophic levels – and the new study revealed some of that complexity by giving a long-term look at the dietary patterns of major predators.
One way this study differed from those in the past is the use of stable isotopes to determine what diets individual species were consuming. Stable isotopes are slightly heavier versions of common elements like carbon and nitrogen that can be detectable in all species in the food web in certain amounts. Because they are stable, those isotopes stick around in animals at certain concentrations – and that can be analyzed to determine what each species was eating in the food chain.
“When an animal eats a prey item, there is a differential uptake in the carbon and the nitrogen,” University of Southern Mississppi Associate Professor Kevin Dillon said in a release. “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. 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.”
Using that information coupled with longstanding data on stomach contents for key species, the two data sources were put into a new modeling framework to grant greater insights into how the different predator and prey species interact.
Leaf said the study’s use of existing data became enhanced through increased computational capability, which allows decades-old information to be leveraged in new ways.
“We have a lot of information sitting around, ready to be used. That’s one of the things we like to do in my lab is to search around and say, ‘How can we leverage all of those pieces of information and put them together?’” he said.
Other regions of the U.S. could, then, theoretically, use similar longstanding data accumulated over years of federally funded studies to perform dietary analyses of fish species, which would in turn provide another way of forming a better understanding of fisheries dynamics. Leaf called the methods of the study a “complementary approach” to the Ecopath with Ecosim model – a way of evaluating the ecosystem effects of fishing.
“That is the modeling framework that is used in a lot of ecosystem-based assessment models,” Leaf said. “This is a complementary approach that I think is informative because it has different pieces inside of it, handles the data in a different way, and has different assumptions built in. Generally, it’s useful to look at a problem in a number of different frameworks because they all have different assumptions and data sources.”
Leaf said that an additional look could be helpful for fisheries management councils who are considering how to evaluate prey species and determine fishing quotas with metrics outside of just saying, “There’s this many, so we can fish this much, and that’s it.”
“It’s these complementary approaches that will be most useful because they’re going to be most accurate,” he said. “They’re going to actually describe this very, very complex system. The ecosystem dynamics are so complex that you really need to leverage every piece of information that you possibly can.”
Leaf also said one of the benefits of the study and others like it is the low cost of entry. It doesn’t need new expeditions or expensive outputs of labor; it just needs existing monitoring data and the right analysis.
“Many state resource agencies, and of course the federal government, are spending a lot of money under the umbrella of environmental monitoring. Here’s a way at least one part of that monitoring data can be brought into the same umbrella. If we can use modeling frameworks and maximize the efficient use of existing data – yes, we can still admit all that complexity – but we can wrangle that complexity into something that’s more understandable.”
