Stolz, the refuge manager, said it was impossible to tell how many snakeheads are in the refuge, or just what they'll 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 surfaced in Maryland in 2002, generating considerable hubbub because it can survive out of water for a brief time and 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.
Actually, 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 that report didn't reach Stolz until today, 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 doing research in FDR Park. He suspects that 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 it could mean that the fish is showing up in isolated locations - such as the refuge - and maybe breeding there and sending out little fish to other areas.
At some point in the future, they could be all over the region's waterways. The voracious predators 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 they're big and put up a good fight.
So that has kept the numbers of snakeheads somewhat under control. Meanwhile, 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 if he's watching a giant aquatic experiment. "Unfortunately, it's an uncontrolled experiment."
And at the same time, other uncontrolled experiments are taking place. Flathead catfish - native to the Mississippi River basin, but not the East - are spreading. And eating.
Within the Tinicum refuge itself, "invasive species are one of the biggest management issues we have," said Stolz.
Indeed, one of the common fish caught there is another invasive: carp.
Invasive plants are even worse than the fish. The refuge is battling phragmites and purple loosestrife and Japanese knotweed and mile-a-minute vine and garlic mustard and on and on and on.
"They change the habitat for wildlife and make it much less suitable for them," said Stolz. "You change the menu, and the animals don't know how to use it. I tell kids it's like opening up a cereal box and getting sawdust. Or opening a can of nuts and getting nuts and bolts."
Friday, Stolz and other refuge staffers were talking about what to do next. One of the first things on the list: 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 on Twitter @sbauers. Read her blog, GreenSpace, at www.philly.com/greenspace