UNIVERSITY PARK, Pa. — Flathead catfish — native to the Mississippi River basin — were first detected in the Susquehanna River in Pennsylvania in 2002, according to the U.S. Geological Survey. In the two decades since then, the invasive species has spread throughout the river basin. The impact of the large predator on the waterway’s food webs and ecology was unknown, but now a research team is beginning to understand what Susquehanna flatheads are eating and how their presence is affecting native aquatic species in the river.

The findings, which the team said state fisheries managers can use to better manage the waterway, were recently published in Transactions of the American Fisheries Society. The researchers from Penn State, the Pennsylvania Fish and Boat Commission, the U.S. Fish and Wildlife Service, and the U.S. Geological Survey, reported that flatheads are opportunistic feeders preying on dozens of aquatic species. The invaders are voracious and have become the apex predators in the river — some specimens are known to grow more than 4 feet long and nearly 70 pounds in Pennsylvania waters. They have great potential to affect the abundance of riverine fish species, according to the researchers.

 “We conducted the first flathead catfish population diet study in a mid-Atlantic region waterway because it was unknown how this invader is impacting food webs and which species may be at greatest predation risk,” said study coauthor, Megan Schall, associate professor of biology at Penn State Hazleton. “Flathead catfish in waters where they are not native pose a threat to aquatic communities.”

To determine what the flathead catfish were eating, researchers collected 576 of them over two years using electrofishing and hoop net capture methods, as well as collection from the Conowingo Dam fish lift, with 241 individuals having recoverable stomach contents. The researchers extracted DNA from those contents and identified prey tissues using a molecular genetics technique known as DNA bar coding. Researchers conducted genetic sequencing of selected stomach contents at the Genomics Core Facility at Penn State’s Huck Institutes of the Life Sciences.

In all, researchers identified 47 different prey species. Thirty-four of the species detected were at a low frequency, occurring in less than 2% of fish sampled. The most common were rusty crayfish, margined madtoms — a variety of catfish typically 2 to 3 inches long, considered to be a species of conservation concern — and shiners, which are silvery minnows. While frequency of occurrence of prey species differed depending on the length of flathead catfish, rusty crayfish were found in the stomachs of almost half the fish sampled in the study.

Among other prey species identified were blueback herring, gizzard shad, northern hog sucker, shorthead redhorse, goldfish, river chub, bluntnose minnow, longnose dace, white catfish, brown bullhead, channel catfish, flathead catfish, banded killifish, rock bass, green sunfish, bluegill, smallmouth bass, white crappie, black crappie, white perch, greenside darter, yellow shield darter and walleye.

Not surprisingly, the researchers said, with increasing flathead catfish size comes a proclivity to feed on larger prey. In general, flatheads over 24 inches long primarily consumed fish species, and fish below 24 inches generally fed on a mix of invertebrates and fish species.

Learning that flathead catfish in the Susquehanna River basin are consuming a diverse set of prey species, including species of recreational importance — smallmouth bass, rock bass, channel catfish and walleye — and species of conservation importance — blueback herring and margined madtom — will help fisheries managers who oversee the river, according to Schall.

“Identifying prey species consumed by this invasive predator is one of the first steps in elucidating potential community-level impacts from this actively expanding aquatic invasive species,” she said. “That knowledge is vital for determining potential management solutions to a novel invader.”

The discovery that flathead catfish in the Susquehanna River are consuming so many rusty crayfish — themselves an invasive species that are largely supplanting native crayfish species in the U.S. Northeast — was especially interesting, said research team leader Julian Avery, assistant research professor of wildlife ecology and conservation in the College of Agricultural Sciences.

“These invasive species likely ended up in the Susquehanna River because of human tampering — anglers moving them around — and the scale of what can go wrong with ecosystems as a result is eye-opening,” he said. “We are only now coming to grips with the effect these aquatic invaders are having on river ecosystems.”

Also contributing to the research were Geoffrey Smith, Pennsylvania Fish and Boat Commission, Division of Fisheries Management; Aaron Maloy and Jason Coombs, U.S. Fish and Wildlife Service, Northeast Fishery Center, Lamar, Pennsylvania; and Tyler Wagner, U.S. Geological Survey, Pennsylvania Cooperative Fish and Wildlife Research Unit at Penn State.

The Pennsylvania Sea Grant, Penn State Commonwealth Research and Development Program and Mid-Atlantic Panel on Aquatic Invasive Species provided funding for this research.