When snakeheads were first discovered in a pond in Maryland in 2002, the public panicked. People feared the big, voracious Asian fish would gobble up native species and take over local waterways. Since then, those fears have proven to be only partially correct.

The northern snakehead has established itself firmly in the Potomac River system, with a population estimated at somewhere above 21,000 individuals, ranging through more than 120 river miles (200 kilometers). Growing up to 18 pounds (8 kilograms) and three feet (one meter) long, the "Frankenfish" keep spreading; they have recently been found above Great Falls in the C&O Canal (north of Washington, D.C.), as well as in the upper Chesapeake Bay.

The invasive species were imported legally from Asia for the aquarium and seafood trades until 2004. Now they pose a risk to the embattled wildlife of the continent's largest estuary, but people are fighting back. State officials are working with fishermen to keep the population in check and try to prevent it from spreading even farther. State and federal laws also now prohibit keeping or transport of live snakeheads.

On May 21, Maryland will host a snakehead fishing "derby" to raise awareness about the invader and encourage the public to be vigilant.

We spoke with Joseph Love, a fisheries biologist with Maryland's Department of Natural Resources who has studied snakeheads in the region.

How did invasive snakeheads get into the Potomac River?

Based on genetic evidence, we believe the fish were repeatedly introduced into the Anacostia River [which flows into the Potomac]. Why they were introduced is somewhat of a mystery, although there are two main theories. One is something called prayer release, when people release the fish as part of a cultural tradition. The other is that people were keeping them for food but then released them.

The first snakehead was caught in Doe Creek in Virginia in 2004. Since then it has spread rapidly through the Potomac.

What happened in Crofton, Marlyand in 2002, leading to widespread fear and, eventually, even a cartoon?

In 2002, four adult snakeheads and at least a hundred juveniles were found in a pond in Crofton [west of Annapolis]. The fish was introduced there by a family as a symbolic gesture, because a woman had been suffering from an illness and then became better. So they released the fish as a token of thanks. It wasn't just one fish that was released.

Officials treated the pond with Rotenone, a poison that affects the respiratory system of the fish. The fish were killed but the incident got major coverage in the media and led to laws prohibiting the importing or keeping of live snakeheads. Some people have speculated that all the attention made people who already had the fish nervous, causing them to ditch their pets into rivers, although we don't have any evidence of that.

What impact are the fish having on the environment here?

At first people were concerned that snakeheads would cause extinction of local species, but that hasn't happened. People weren't too concerned about them spreading, but that did happen. Snakeheads reproduce twice a year and both parents closely guard their offspring, which is unusual for our fish fauna, and that has led to high levels of survivorship.

Anglers have feared that snakeheads will outcompete largemouth bass, which were introduced into the Potomac River in the late 1800s. Snakeheads can commonly grow up to 10 or 12 pounds (with 18 pounds being the record), while bass might get to 3 or 4 pounds, and both fish occupy the same niche as opportunistic, top-level predators. For the most part bass seem to be holding their own, although we did find a dominance of snakeheads in one tributary we sampled in Virginia, where we found only a few small bass.

We haven't seen major impacts on prey species, perhaps because snakeheads eat such a diverse diet, from other fish species to invertebrates and even small mammals.

Do any local predators eat them?

Young snakeheads are certainly susceptible to predators, although both parents guard them closely. I've cut open a lot of bass and never found a snakehead in their guts. Snakeheads also grow so fast that they escape a lot of predators. There are birds that consume them, as well as some large catfish. But once the fish grow above around five pounds, it's difficult for herons or eagles to pick them up. We also don't have bears in this area, so the fish get to a size where the only primary predator is going to be a human.

Are snakeheads dangerous to people?

They're not piranhas or sharks and they're not going to swim up to you and nibble on you. But the large ones can be difficult to handle. They have a lot of sharp teeth, on both jaws and going down their throat.

Four years ago, a young child on a boat in a pond in Delaware reached down into the water, toward the flash of gold on the side of a juvenile snakehead. One of the fish's parents lunged out of the water and bit the kid. The child's injury was very superficial but the incident shows that the fish defend their nests.

Some people also occasionally find snakeheads in their yards, where they are deposited by floods. The fish don't pose a human health risk, but people should use common sense when handling them.

Can't snakeheads live for a while outside of the water?

I've put them in the refrigerator and found them alive three days later. They have an organ that allows them to get oxygen out of the air, so they can stay alive for several days out of water, as long as they stay cool and moist. In Asia, they tend to do well in rice paddies and shallow water that occasionally dries out, based on the monsoon cycles. [They can even travel short distances on land and are sometimes called "walking fish."]

How are fishermen responding to the invaders?

We have been encouraging commercial and recreational harvest for a few years, and that seems to be keeping the biomass of snakeheads down and even declining, which has helped minimize their impact. The commercial fishery is small so far, but we are seeing increased interest from anglers using hooks and lines and even bows and arrows. That's great because it is getting people involved in invasive species management. Snakeheads taste good, so that's one incentive. People also enjoy the sense of stewardship and the challenge. The fact is, we don't have enough agency staff to control the snakeheads ourselves.

It's important that people use caution, however, because we definitely don't want people to inadvertently spread the fish even more. We want to fish it to extinction, if possible.

What should people do if they catch one?

Possession of a live snakehead is illegal, so we want people to euthanize the animal after they catch it. We encourage people to serrate it, remove its guts or gill arches, or decapitate it (the fastest way). To subdue it you can knock it on its head but you're unlikely to kill it that way because it has a very thick skull.

Is it possible to get rid of them?

I like to remain optimistic, but so far I've been thwarted in my optimism. It's possible we could still prevent them from spreading and eradicate them from some areas, but in the main river their population is pretty well established. We're not going to be able to poison the whole bay.

Are there other invasive species causing problems in the Chesapeake system?

There's been a lot of recent attention on blue catfish, which have arrived from the Mississippi River system and are spreading around the bay. They can grow up to 85 pounds (38 kilograms) and forage on shad and other fish that people value. In one area we found that 90 percent of the biomass of all fish was comprised of these big catfish. We are trying to encourage a commercial fishery for them.

This interview has been edited.