Invasive blue catfish threaten marine ecosystems in Delaware River Watershed

Editor’s note: This story first appeared on NJ Spotlight News. It is republished here with permission.

“This is ground zero,” Mike Steiger, a fisheries biologist with Delaware’s Department of Natural Resources and Environmental Control, said one morning last month. 

Steiger was standing on a wooden walkway in the Russell Peterson Wildlife Refuge, looking over the steel-gray Christina River, a narrow tributary that twists through Wilmington, Del., and along I-95, eventually spilling into the Delaware River. 

“This is the first place we found them.”

That first find was a decade ago. 

In 2013, a recreational fisherman hooked an unfamiliar, 25-inch brute of a fish while casting into the Christina from a boat ramp in the town of Newport, about two miles from where Steiger now stood. 

The fisherman knew he had a catfish, but it didn’t look like a channel catfish, a common species in the Delaware River watershed. He snapped a photo and sent it off to the natural resources department.

The department forwarded the image to fishery biologists at the Virginia Institute of Marine Science who were quickly able to confirm that the fish in question was a blue catfish. 

The Christina “blue cat” was the first of its kind recorded in the watershed but it would not be the last. 

These days, Steiger and his counterparts in Pennsylvania and New Jersey are bracing for an invasion that has already upended marine ecosystems in Virginia and Maryland.

At first, the Christina blue catfish appeared to be an anomaly. The Delaware natural resources staff had put out an alert to Delaware anglers, asking them to notify the department of any other blue catfish encounters, but no additional reports surfaced for another five years. 

“For a long time, we wondered if someone had stocked this one fish here and that the fisherman just happened to catch it,” Steiger said.

How the blue catfish got in

But then, in 2018, during their annual gill-net surveys in the Delaware River, not far from the mouth of the Christina and adjacent to South Jersey’s Pennsville Township, Steiger and his colleagues hauled up more blue catfish. 

The following year, Steiger finally got what he believes is the answer to the question of how the lone 2013 fish had made it into the Christina: That September, eight big blue catfish were caught in the C&D Canal, the manmade corridor connecting the Delaware Bay with the Chesapeake Bay.

The reports have poured in ever since. 

“I believe the northeast section of the Chesapeake Bay is feeding our infestation,” Steiger said.

Chris Smith, who oversees invasive aquatic species for the New Jersey Department of Environmental Protection, agrees.

“We don’t know for sure if the C&D Canal is directly where they came from,” Smith said. “But we can speculate that it’s the most likely pathway the blue cat took.”

Effect on ecosystem

“Invasive” is a tricky label. 

The channel catfish, which most New Jersey anglers would consider native, was in fact introduced in the state’s waters about a century ago. The largemouth bass, a beloved game species among freshwater fishermen, comes from the Mississippi River basin and the Southeast. 

At this point in history, when humans have so thoroughly and haphazardly scattered plants and animals across the globe, it is less about a species’ origin than it is about the impact it may have on its new ecosystem.

And blue catfish, Steiger said, “are big, angry, slimy, eating machines.”

In the 1970s, blue catfish — named for their silvery-blue hue — were stocked in Virginia to boost recreational fishing in the Rappahannock, York and James Rivers, whose ecosystems had been decimated by decades of pollution. 

Unlike the rivers’ native species, especially the shad and Atlantic sturgeon, blue catfish and other nonnative species, like the northern snakehead, thrive in the kind of low-oxygen environments that result from poor water quality.

Feasting on native species

Though they are native to the Mississippi, Missouri, Ohio, and Rio Grande River basins, blue catfish have also proven to be tolerant of salty water. 

When they reached the mouths of the Rappahannock, York and James, they had the entire Chesapeake Bay before them. Year by year, they crept north, vacuuming up native species, like menhaden, shad, river herring and blue crab.

Steiger and Smith worry that mature blue catfish may also enjoy eating young Atlantic sturgeon, a particularly worrying preference in the Delaware River and Bay, where fewer than 250 spawning adults remain. 

Each fall, Steiger and his colleagues conduct net sampling of juvenile Atlantic sturgeon in the Delaware River, about three miles downriver of the Christina’s mouth, and in recent years they’ve found blue catfish in their nets. 

“We’re only catching little ones, but if a 100-pounder was there, it could definitely eat a juvenile Atlantic sturgeon,” Steiger said. “We haven’t seen it yet, but it absolutely could happen.”

In recent years, biologists from the Delaware Division of Fish and Wildlife have found juvenile blue catfish during their annual Delaware River surveys, raising fears that mature blue catfish could soon be eating the highly endangered, native Atlantic sturgeon.

Read more: Invasive fish in Delaware River pose growing threat, officials warn

Blue catfish are equally voracious when it comes to reproduction — an aquatic version of the rabbit. 

During the spawning season, which runs from late May to June, a female blue catfish will produce up to 8,000 eggs per kilogram (2.2 pounds) of her body weight. The species can grow up to five feet and weigh over 100 pounds — a size that could yield over 360,000 eggs. 

“Snakeheads actually eat each other, so their numbers are kept down that way,” Steiger said. “Blue cats protect their young, so their numbers tend to explode.”

In New Jersey, Smith said, word-of-mouth reports of anglers catching blue catfish have made their way to the DEP for the past decade. But it wasn’t until 2021 that the department received a photo of a blue catfish that was caught in Salem County’s Lower Alloways Creek, a Delaware River tributary that is south of the C&D Canal and close to where the river turns into the Delaware Bay.

That the species is heading toward saltier water is a concern for the DEP, Smith said.

He used the northern snakehead as a cautionary example.

“We’ve confirmed them as far south as the Cohansey River this year, and, most likely, they will continue to expand down to the Maurice River,” he said. Both the Cohansey and Maurice rivers are in Cumberland County.

Anglers not dependable source of data

As of now, though, Smith and his colleagues have not detected any blue catfish below Lower Alloways Creek. But relying on anglers to map the species’ movement through the watershed is problematic. Many don’t bother — or don’t know — to report their catch to the DEP.

To fill that gap, Smith is hoping that, in the coming years, the DEP will invest in eDNA sampling, which is the process of identifying plant and animal species by analyzing the DNA they leave behind in the water column. 

“It’s a great screening tool,” Smith said. “We won’t have to do electrofishing” — a tool that uses an electrical current to temporarily stun fish — “or put a net in the water. We can just take a water sample and get a better indication of the distribution of species.”

Both Smith and Steiger, along with their counterpart in Pennsylvania’s Department of Environmental Protection, Sean Hartzell, are part of the Mid-Atlantic Panel on Aquatic Invasive Species, which brings together state and federal agencies, academic institutions, environmental groups and other stakeholders to develop and coordinate strategies to manage an ever-growing list of destructive invasive species. 

For now, though, the most effective way to stay ahead of the blue catfish’s spread throughout the Delaware River watershed is regular coordination among Smith, Steiger, and Hartzell. 

If an angler reports a blue catfish or a northern snakehead or a freshwater drum — another recent arrival to the watershed — the three biologists will add that information to their growing body of research. 

If one turns up in the Delaware team’s Atlantic sturgeon nets, Steiger will alert the others.

Most importantly, Smith and Steiger said, is that anglers know a blue catfish when they see one, so that they can report it back to their state environmental agency. And then, if they’d like, they can take their catch home for dinner.