Maryland Declares War on Invasive Blue Cats

By Joe Zimmermann, Maryland Department of Natural Resources

The first catch of the afternoon was a white catfish. Saved by its wider head and fewer anal fin rays, it was tossed back into the water.

The blue catfish came next—two in quick succession. “It only takes a few minutes for them to start biting,” said Capt. Marcus Wilson, whose Rock-N-Robin Charter Fishing, operated together with Capt. Robin Payne, runs blue catfish trips on the Potomac River several months of the year.

It ended up being a slower day for the charter boat, but they often catch 50 blue catfish at a go. And with no catch limits, customers end up bringing home a lot of fish.

Anglers across the state are seeing more and more blue catfish, an invasive species that only reached Maryland waters in the 1990s. Eric Packard, a recreational fisherman and a member of Maryland’s Sport Fisheries Advisory Commission, described it as “bizarre” how many blue catfish you can catch.

“I’ll put it this way,” he said. “I’m catching more catfish than I’m catching what I’m targeting.”

The spread of blue catfish is a growing concern for scientists and fisheries managers. The invasive species is quickly becoming abundant in Maryland’s rivers and outcompeting native fish for food and habitat. Blue catfish are preying on blue crab, menhaden, American eel, and other economically and ecologically important species.

There isn’t extensive monitoring data on how many blue catfish are in Maryland, but Branson Williams, the invasive fishes program manager at the Maryland Department of Natural Resources, said looking at harvest data can be a good indication of how much the population has expanded.
In the Potomac River and Maryland waters, commercial landings of blue catfish skyrocketed from 609,525 pounds in 2013 to 4.2 million pounds in 2023, an increase of more than 500 percent.

Looking at Virginia rivers, where blue catfish became established a few decades earlier, also gives a sense of the scale of the issue. In portions of the James and Rappahannock rivers, surveys have found blue catfish making up 75 percent of the total fish biomass—or 3 out of every 4 pounds of fish. Williams noted that survey work in Maryland’s upper Patuxent River has found more than 500 fish per hectare, a density “kind of on par with what people were seeing in the James.”

“We’re not seeing that density everywhere,” he said. “But we could start to.”

With the severity of the problem, blue cats have quickly shot up as a priority among Maryland’s big three invasives, which include northern snakehead and flathead catfish, and are a focus for increasing management efforts and targeted fishing in order to contain the fish.

The Spread of Blue Catfish in Maryland

In some American river systems, the blue catfish is just another fish. Native to the river basins of the Mississippi, Missouri, Ohio, and Rio Grande, blue catfish share these waters with a range of other species. Prized for both their large size and their taste, blue catfish have long been popular targets for sport fishing. Various state agencies stocked the fish in new waterways by the second half of the 20th century, intentionally introducing them for the purpose of building up fisheries, everywhere from lakes in South Carolina and California to rivers in North Carolina and Alabama.

One such effort took place in Virginia. From 1974 to 1985, the Virginia Department of Game and Inland Fisheries released more than 300,000 juvenile blue catfish into waters of the commonwealth, first into the James and Rappahannock rivers, followed by additional stockings in the York River 11 years later.

Blue catfish thrived in the Virginia rivers. Though officials at the time believed the salty Chesapeake Bay would serve as a barrier for the freshwater fish, the blue catfish defied these expectations, tolerating higher salinities and expanding farther north. By the late 1980s, they had arrived in the Potomac. By the 2000s, they became abundant in that river and reached the Nanticoke and Patuxent rivers. DNR scientists believe the wet years of 2018 and 2019 enabled them to spread throughout the Upper Bay.

Now, they’re in every major river in Maryland—and often at high densities.

Mary Fabrizio, a professor at the Virginia Institute of Marine Science who has studied blue catfish, said she’s not totally surprised by their ability to tolerate higher salinity—blue catfish often travel from river to river in coastal waters of the Gulf of Mexico—but she is surprised by how successful they’ve become in a relatively short amount of time.

“You’d think it would take a lot longer, but it didn’t,” she said. “They became very successful very quickly.”

Fisheries biologists say there isn’t a singular reason for the success blue catfish have found in the Chesapeake. Williams said they “check off all these boxes” that allow them to do well as a generalist species in this area. “A lot of times when an invasive species gets into a novel environment, they have the potential to do so much better than in their native range,” he said. “Everything falls in place for them to be pretty successful here.”

One large factor in their favor is their sheer size. In North America, blue cats are the largest catfish and among the largest freshwater fish. They can grow to be 5 feet long and top 100 pounds. An angler in 2011 caught the world record blue catfish at 143 pounds. That fish was reeled in on a lake in their introduced range. The current Maryland record is an 84-pounder caught on the Potomac River in 2012.

“Once they reach a decent size, mortality drops off,” Williams said, even for catfish far smaller than record sizes. “They don’t have predators to worry about.”

Blue catfish also have substantial reproductive capacity—which increases as they get larger. Mature female blue catfish can lay about 3,500 or 3,600 eggs per pound, Williams said. “If you’ve got a 20- or 30-pound fish, that’s a heck of a lot of eggs.”

When these eggs are released, blue catfish demonstrate behaviors of parental care, with males building nests and protecting eggs, which helps spawning success.

Add to that the fact that blue catfish are highly migratory and will disperse over large areas, much more mobile than the other catfish in Maryland. DNR surveys on the Patuxent River found one blue catfish that moved 35 miles in a few weeks, said Mary Groves, the department’s Southern Region Manager for the Freshwater Fisheries Program, who has studied blue catfish. A 2017 study, with co-authors including Groves and Fabrizio, found that tagged blue catfish in the Potomac traveled an average of 15 miles, with one covering 70 miles in 66 days.

On top of all these factors, VIMS researchers found that they even seem to be able to survive long periods with little food.

“The more we learn, the more we realize this organism is built for success,” Fabrizio said.

And wherever food is available, blue catfish eat well. Their voracity is both a driving factor for their spread and a pressing concern to fisheries managers and biologists.

“They’re very well adapted to eat whatever is available,” Groves said. “To be honest with you, I don’t recall ever seeing a skinny blue catfish, I just don’t.”

The Ecological Effects of Blue Catfish

Though catfish have a reputation as bottom feeders, blue cats are fast, big predatory fish that will pursue prey throughout the water column, with the varied diet of an opportunist.

A 2018 study on the stomach contents of blue catfish found that specimens in the James River consumed 80 different species, from fish and mollusks to birds and turtles. DNR monitoring on the Patuxent River found blue catfish to eat about 40 species—as well as rocks, chicken bones, a doll arm, an entire 16-ounce Coke bottle and electronic parts. “They really do eat an awful lot of different things,” Williams said.

In part, blue catfish disrupt the balance of the ecosystems they enter. But they also directly target species of economic value and conservation concern.

Using data on blue catfish diet and abundance, researchers in 2023 estimated that blue catfish prey on 441 tons of blue crab in the James River alone each year, as well as 108 tons of menhaden and 68 tons of American eel in the same river. American eels are regionally in decline and considered endangered by the International Union for the Conservation of Nature.

Their bigger sizes simply mean more food items are available to blue catfish, Williams said. While younger catfish focus on invertebrates, larger ones mainly target other fish. Migratory fish are known to be on the menu, including shad and herring.

“The run of anadromous fishes on the east coast is probably one of the factors that lead to [blue catfish] success,” Fabrizio said. “There are a lot of juveniles and returning adults that could serve as prey to the blue catfish.”

Aside from direct predation, blue catfish outcompete other fish for food and for habitat space, including the white catfish, Maryland’s one native catfish that is harvested commercially. (Aside from the invasive flathead catfish, the widely established channel catfish were also introduced, though much earlier, in the late 1800s and early 1900s.)

“We saw that ourselves in our survey work,” Groves said. “In places where we expected to find white catfish, we were now finding blue catfish, and anglers were telling us the same thing.”

There’s also been speculation that blue catfish could serve as a vector for the spread of invasive Asian clams and zebra mussels, as blue catfish consume bivalves and these species are able to survive the passage through the digestive system of the fish.

They are known to eat native clams too, their stomachs sometimes filled with so many shells that Groves said “when you pick them up, they rattle.”

Managing Blue Catfish

For Rocky Rice, a lifelong waterman, blue catfish have already resulted in a changing fishery.

He started catching blue catfish as something extra between striped bass and blue crab seasons—now he catches blue crabs and rockfish to fill in when he’s not catching blue catfish, which is his main operation. He said it’s a different fishery than his father and grandfather knew, but it’s also resulted in profitable work.

“It’s actually become a valuable asset for watermen when the season is closed for other fish,” he said. “They’re here to stay. All we can do is to do our best to catch them and minimize any effects they may have on native species.”

Increasing the blue catfish harvest is a primary goal for Williams, who became DNR’s first invasive fishes program manager in November. He hopes to help build up fisheries for the invasive fish in addition to promoting blue catfish to recreational anglers.

Efforts are underway in the department and across the state. The department is awarding grants for proposals on the removal of invasive fish, and a number of recreational fishing groups are holding blue catfish tournaments this year. The Maryland Department of Agriculture is increasing its marketing of wild-caught Chesapeake blue catfish to chefs, consumers, restaurants, grocery stores and distributors to encourage more people to buy, eat and sell the fish.

Though state officials say that federal inspection rules slow the processing of blue catfish, new federal funding from the 2024 agriculture appropriations bill grants funding for overtime inspections. The bill also designates blue catfish for school lunch and domestic food assistance programs, and it provides Maryland DNR with $500,000 in funding for invasive catfish monitoring and study.

Maryland does not currently have a blue catfish monitoring program, and one of Williams’ priorities is establishing one. He is also working to compile incidental information about blue catfish from other surveys across the department. A fuller understanding of blue catfish, their life cycles, and behaviors in Maryland waters can help inform where to focus management decisions.

With how established blue catfish have already become, scientists tend to think it is a question of management, rather than ever fully getting rid of the species.

“Eradication is just not possible,” Fabrizio said. “The best option we have is control.”