Flatheads grow fast in the Susquehanna River system, attain large body
sizes and can eat a variety of prey. Because adult flatheads have few natural
predators, they can exert strong control over the ecosystem.
Credit: Sam Stukel, USFWS
When Flatheads Become Invaders
By Keith “Catfish” Sutton
A Cautionary Tale From the Susquehanna
Flathead catfish are legendary among anglers for their power, size and willingness to eat just about anything that swims. In their native Mississippi River basin, they’re an important part of the ecosystem and one of America’s most respected sportfish.
But in the wrong river, a flathead can become something very different—an apex predator capable of reshaping entire fish communities. A new study from Penn State, the U.S. Geological Survey (USGS) and the Pennsylvania Fish and Boat Commission shows just how dramatically that can happen.
The research, published on September 4 in the journal Ecology, confirms what many Pennsylvania anglers have suspected for years: flatheads in the Susquehanna River are no longer just newcomers. They’ve risen to the top of the food web, displacing long-established native predators and forcing other fish to scramble for ecological footing.
How One Fish Became the River’s Top Predator
Flatheads first showed up in the lower Susquehanna in 1991. Since then, their population has grown rapidly. According to the study’s lead author, Olivia Hodgson, the Susquehanna offers near-perfect conditions for them to thrive.
“Flatheads grow fast in this river system, attain large body sizes and can eat a variety of prey,” Hodgson said. “With few natural predators, they can exert strong control over the ecosystem.”
That combination—fast growth, large size and the ability to feed on nearly anything—has allowed flatheads to climb to the very top of the river’s food web. Using stable-isotope analysis, researchers measured what various species were eating and how energy moved through the ecosystem. The results were striking.
Flatheads now occupy the highest trophic position in the system—higher even than longtime apex predators like smallmouth bass and channel catfish.
A River Food Web Under Pressure
In areas where flatheads were present, channel catfish showed a measurable drop in trophic position. In simple terms, they’ve been pushed lower on the food chain. Researchers believe this may be because channel cats avoid flatheads or because flatheads outcompete them for preferred prey.
Even more telling, every species studied, from bass to minnows, showed broader, more overlapping diets in sections of river with flatheads. That means fish are changing what and where they eat, likely because they’re competing with or trying to avoid being eaten by the invader.
“These findings support the trophic disruption hypothesis,” Hodgson explained. “When a new predator enters an ecosystem, it forces existing species to alter their diets, behaviors and ecological roles. Over time, this can destabilize the food web.”
For anglers, that may translate into fewer native fish, changed feeding patterns and long-term declines in some recreational fisheries.
Why Flatheads Hit Hard Outside Their Home Range
Flatheads are often introduced unintentionally and sometimes by anglers who want a trophy-sized sportfish in their local water. But in rivers where they don’t belong, the story is almost always the same:
- They grow faster than native predators.
- They reach larger sizes.
- They eat a broader range of prey.
- Nothing eats them.
In systems like the Susquehanna, that combination allows them to take over quickly. This study is one of the clearest scientific demonstrations of how far-reaching those impacts can be.
A Tool for Understanding — and Managing — Invasions
The research team relied on stable-isotope analysis, a method that tracks the carbon and nitrogen signatures in fish tissue. Because animals incorporate the chemical “fingerprint” of what they eat, these signatures act like a long-term record of dietary habits and habitat use.
By comparing isotope patterns among species, and between invaded and uninvaded areas, the scientists could detect shifts in diet, habitat, competition and predator-prey relationships.
The project analyzed 279 fish and 64 crayfish, offering one of the most detailed snapshots yet of how an invasive predator restructures a riverine food web.
What This Means for Anglers
Flatheads are exceptional sportfish, and many anglers love catching them. But this study highlights a hard truth: in places where they don’t belong, they can put tremendous pressure on native fisheries.
Anglers can help by:
- Never transporting live fish between waters.
- Reporting unusual catches in waters where flatheads are not native.
- Harvesting flatheads where state agencies encourage removal.
- Staying informed about regulations and management plans.
Flatheads will always be a valued part of their native rivers. But as this research shows, when they become invaders, they can reshape an entire river system, often in ways that are invisible until it’s too late.
Understanding these impacts is the first step toward protecting the waters we all rely on for fishing, recreation and healthy fish populations.
(Keith “Catfish” Sutton has written four books about fishing for whiskerfish, including “Hardcore Catfishing: Beyond the Basics.” He has twice received the Conservation Communicator of the Year Award from the Arkansas Wildlife Federation.)
