Flathead catfish, large opportunistic predators native to the Mississippi River basin, are rapidly reshaping ecosystems where they invade. A new study by Penn State, the U.S. Geological Survey (USGS), and the Pennsylvania Fish and Boat Commission reveals just how much impact these invaders are having on the Susquehanna River in Pennsylvania, where flatheads were first detected in 1991.
Over the last three decades, their numbers have exploded — and so have concerns about their ability to decimate native and recreational fisheries.
“Flatheads grow fast in this river system, attain large body sizes and can eat a variety of prey,” said Olivia Hodgson, lead author and Penn State master’s student in ecology. “Because adult flatheads have few natural predators, they can exert strong control over the ecosystem.”
Apex Predators Outcompeting Native Fish
The study, published September 4 in Ecology, confirmed that flathead catfish now occupy the highest trophic position in the Susquehanna River. Their dominance places them above long-standing resident predators such as smallmouth bass and channel catfish.
In fact, channel catfish were observed feeding lower on the food chain in areas invaded by flatheads. Researchers suggest this shift is either because they are being outcompeted for food or because they are actively avoiding flatheads.
“This supports the trophic disruption hypothesis, which states that when a new predator enters an ecosystem, existing species are forced to change their diets, behaviors, and roles in the food web,” Hodgson explained. “Over time, this can destabilize entire ecosystems.”
Stable Isotope Analysis Reveals Food Web Shifts
To measure these changes, researchers used stable isotope analysis — a powerful ecological tool that traces what animals eat and where they feed by analyzing chemical markers in their tissues.
By examining 279 fish and 64 crayfish, including flatheads, smallmouth bass, channel catfish, minnows, and crayfish, the team revealed clear evidence of dietary overlap and resource competition.
The results showed:
- Broader diets among all fish species in invaded areas.
- Greater trophic overlap, suggesting direct competition with flatheads.
- Shifts in carbon and nitrogen isotopes, indicating changes in both diet and habitat use.
“Stable isotope analysis allowed us to see how invasive flatheads reorganize the river’s food web,” Hodgson said. “We could infer resource use, competition, and even displacement from habitats once dominated by native species.”
Why It Matters
Flathead catfish are considered invasive apex predators with the ability to reshape entire freshwater ecosystems. Their spread into the Susquehanna River underscores the ecological risks they pose:
- Disruption of native predator-prey dynamics
- Declines in recreational fisheries like bass
- Potential long-term destabilization of aquatic ecosystems
As Hodgson emphasized, this research shows that invasive predators do more than reduce native populations — they alter the flow of energy and interactions across the food web.
Research Team and Support
In addition to Hodgson, contributors included Tyler Wagner (USGS and Penn State), Sydney Stark (Penn State), Megan Schall (Penn State Hazleton), Geoffrey Smith (Pennsylvania Fish and Boat Commission), and Kelly Smalling (USGS New Jersey Water Science Center).
Funding was provided by Pennsylvania Sea Grant and the U.S. Geological Survey.