Charlie Wandrei first flipped through photographs of beaming anglers holding monster flathead catfish about five years ago at the Pennsylvania Sportsman’s Show in Harrisburg. He was hooked.

“I was absolutely excited,” said Wandrei, who’s from Adams, MA. “I live near the Connecticut River, which has a lot of catfish, but they don’t get nearly as big as those.”

Wandrei got a photo of his own in June, smiling widely while holding a 35-pound lunker pulled from the Susquehanna River.

The characteristics that make the flathead catfish, Pylodictis olivaris, attractive to anglers are the same ones that worry scientists. Weighing up to 100 pounds, they are voracious feeders that vacuum up any smaller fish they can fit into their gaping mouths. But they are not native to the Susquehanna, and they are growing larger and faster there than they do in their native range, the Mississippi River basin.

Flatheads are the “other” invasive catfish in the Chesapeake Bay watershed. Most of the excitement and concern to date has been focused on blue catfish, another native of the Mississippi and Ohio river basins. It was introduced as a new recreational fishery in the James, Rappahannock, and York rivers during the 1970s and 1980s, and has since spread to virtually every major Bay tributary. Like flatheads, blue catfish can grow to massive sizes — the largest recorded catch was 102 pounds from the James in 2009, according to the National Oceanic and Atmospheric Administration Chesapeake Bay Office.

Flatheads haven’t spread as rapidly, apparently because they’re less tolerant than blue catfish of the saltier water in the Bay. They’ve been introduced in 17 states, including several places in Virginia and the Potomac in Maryland, according to the U.S. Geological Survey.

As for how the flathead got to Pennsylvania, it could have been moved by an angler, or maybe it snuck in with a stocking of young channel catfish, introduced in the 1800s. However it happened, the first reported sighting in the state of a flathead was in 1991 in Lancaster County, according to NOAA’s Bay office.

Pennsylvania is just beginning to study the impact that flatheads may be having on the Susquehanna’s other fish; it takes a lot of live fish, on which they feed exclusively, said Geoff Smith, Susquehanna River fisheries biologist, who is leading the study for the Pennsylvania Fish and Boat Commission. So far, the largest Susquehanna flathead captured by his team weighed 40 pounds and was nearly 4 feet long.

“We’re not really sure what we’re up against,” Smith said. “Anglers really like it but they don’t understand the ramifications — and some don’t care — and they move fish.”

Scientists have only recently been studying the impact, said Bruce Vogt, chair of the Bay Program’s Invasive Catfish Task Force. Of the two, blue catfish have attracted more attention and research dollars, he said. They are expanding their numbers and range rapidly, and their ability to occupy tidal rivers and tolerate salinity up to 14 parts per thousand opens up a lot of real estate in the Bay. Blue cats’ diet includes blue crab, striped bass and shad — species that the Bay program has been trying to restore for years.

But combined, the two species spell trouble, biologists said. In some parts of Virginia’s James and Rappahannock rivers, invasive catfish dominate all fish species — making up almost 75 percent of the biomass, said Vogt, who is also ecosystems manager for NOAA’s Chesapeake Bay office.

“Flatheads are limited in where they can spread compared to blues,” Vogt said. “Unless someone picks them up and moves them, their spread will be slower and more contained.”

That may be true, but flatheads may have more of an impact than their blue cousins on some anadromous species that are in trouble, such as American shad, blueback herring and alewife, according to a recent study conducted on a section of the James River in Virginia.

In other watersheds where they’ve been introduced, flatheads have been known to deplete the red-breasted sunfish population and make a dent in native catfish as well.

Even so, Susquehanna fisheries managers say they haven’t seen any evidence that the flatheads are depleting any fish yet, said John Arway, executive director of the Pennsylvania Fish and Boat Commission. That could be because the river is so productive, he said — or it could be that they do coexist with other species in the river.

“We will continue to watch, but they don’t meet my definition of invasive yet,” Arway said. “Personally, I don’t think they are going to have any impact on the Susquehanna.”

The commission started getting angler reports of catches around 2002 in the lower part of the river, below the York Haven Dam, about 12 miles south of Harrisburg, Smith said. Then, around 2013, reports came in from anglers above the dam as far as 14 miles upriver of Harrisburg, near where the Juniata meets the Susquehanna.

“That happened around the same time we were focusing on channel catfish, so we decided to take a closer look,” Smith said. “It was obvious they were moving north.”

Smith’s crew collected nearly a ton of flatheads from the Susquehanna the last two summers. The fish are being collected to determine age and growth rate for the state’s first-ever catfish management plan.

“It was pretty impressive, that first set of nets,” Smith said. “That net was so full we weren’t sure we could get it in the boat.”

Data collection isn’t complete — but the oldest fish found so far was 18 years old in the lower Susquehanna, dating introduction of the species to earlier than originally thought. Younger fish have been picked up farther upriver.

It’s not clear what might be done to curtail the population of flatheads or limit their spread, should managers decide they’re enough of a threat to other species. Unlike blue catfish, there’s no commercial fishery for them, and recreational anglers want more of both, not less.

When they first began showing up in the Susquehanna in the early 2000s, the fish commission told anglers to kill as many as possible. The “kill on sight” policy was removed from the commission’s website a few years ago and limits on flatheads and channel cats are a generous 50 a day.

Wandrei, the Massachusetts angler, takes his annual vacation in Lancaster County with fishing guide Rod Bates. Bates’ guide service, Koinonia, has taken anglers from all over — even China — out in search of big flatheads.

Bates would like to see the commission create a trophy fishery of flatheads and institute a catch-and-release policy to preserve the larger older fish.

“I started fishing for flatheads to make up for [the] closed season for bass a few years ago,” Bates said. “Flatheads and channel cats now make up about 35 percent of my business.”

Smith suggested the jury’s still out on flatheads’ impact on the Susquehanna.

“Anglers are lobbying — talking to commissioners, saying, ’Look at this great resource,’” the state biologist said. “But we need to know what they (flatheads) are doing and what things they are harming. Anything that moves in and blows up in abundance like this is taking away from something.”