Stolz, the refuge manager, said it was impossible to tell how many snakeheads were in the refuge or what they would do.
But he did know one thing: "They are very aggressive fish," he said. "And they will eat a lot of stuff."
The Asian species first showed up in Maryland in 2002, generating considerable hubbub because it can survive out of water briefly and can even sort of slither across land. They turned up in one of the lakes in South Philadelphia's Franklin Delano Roosevelt Park in 2004.
Because the lake is connected by tidal sloshings to the Schuylkill and then the Delaware, fisheries officials knew it was probably only a matter of time before the fish made a break for it and showed up in one of those rivers. Sure enough, in 2005, they had their first confirmed catch - off a pier north of the Navy Yard.
After that, there were reports of other catches in other places, including some New Jersey rivers.
It turns out that a New Jersey biologist caught a snakehead in the refuge last year and a report was made to the U.S. Geological Service. But the report didn't reach Stolz until Friday, when he sent out notifications of Thursday's catch.
Rich Horwitz, senior biologist with the Academy of Natural Sciences of Drexel University, has been monitoring the snakehead issue for years and conducting research in FDR Park. He suspects the species is still in the early stages of colonization.
"Sometimes with invasives that you see, they are introduced and then you don't see much of them, and all of a sudden they appear everywhere," he said.
So the fish could be showing up in isolated locations - such as the refuge - and maybe breeding there and sending out little fish to other areas.
At some point, the voracious predators could be all over the region, and they could put a serious dent in native fish populations.
But Horwitz has seen some interesting developments at FDR Park. Now that anglers know the fish are there, they've been targeting them - not so much for altruistic reasons, but because snakeheads are big and put up a good fight.
That has kept the number of snakeheads in the FDR lakes somewhat under control. They have been eating native fish, but it turns out that native fish are also eating them.
Small snakeheads are "wonderful prey" for bass, Horwitz has found.
The local sunfish population has remained abundant. But killifish and eels may be suffering, although assessing the population is a logistical problem.
"Exactly what will come out of it is hard to say," Horwitz said.
It's as though he's watching a giant aquatic experiment. "Unfortunately, it's an uncontrolled experiment."
Greg Murphy, a fisheries biologist with the southeast regional office of the state Fish and Boat Commission who tracks the reports, said anglers who think they've caught a snakehead should take a photo.
About half the snakehead reports he received last year turned out to involve other species.
The snakehead can be confused with the bowfin, a native fish being considered for inclusion on the state's endangered or threatened species list.
Anyone who catches one of those is encouraged to release it immediately back into the water.
Not so the snakehead.
One of the first things on the to-do list Stolz and other refuge staffers were coming up with on Friday was to put up signs in fishing areas to help people identify snakeheads. And instructing them not to throw the fish back.
Contact Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or follow @sbauers on Twitter. Read her blog, "GreenSpace," at www.philly.com/greenspace.