A study by Mississippi-based researchers found that menhaden are not a primary food source for Gulf of Mexico predator species like red drum, summer flounder, and spotted sea trout.
“The study provides a comprehensive understanding of the Gulf food web and charts the trophic interactions that structure it,” according to a summary from the University of Southern Mississippi. “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.”
The question of menhaden’s role in the Gulf ecosystem is a perennial, hot-button issue in fisheries management. The impact of the region’s commercial menhaden fleet is routinely challenged by recreational and other user groups.
The project, funded by the Science Center for Marine Fisheries (SCEMFIS) combined the latest in stable isotope analysis “and an extensive meta-analysis of hundreds of published stomach content studies dating back to the 1950s” from Gulf fisheries research.
“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 professor Robert Leaf, one of the study authors 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,” said Leaf.
Marine biologists have long tried to determine what predator fishes eat by analyzing samples of their stomach contents. Those samples can tell what fish have eaten recently, “but it offers only a limited snapshot and does not reveal long-term dietary patterns or prey availability,” according to the researchers. “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 carbon, nitrogen and other elements 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,” according to the USM summary. “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 Kevin Dillon, a study author 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,” said Dillon. “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.
The Gulf study may be relevant to other regions, including the U.S. East Coast, where “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,” the researchers note.
