The northern snakehead — a sharp-toothed fish that looks like a horrifying hybrid of fish and snake — is spreading throughout the waters of the United States. That’s not good. But if you fish, you can help. It’s easy: Just kill that sucker.

The snakehead is not a fish you especially want to meet when spending a peaceful day on the lake. Aside from everything else, snakeheads — gulp — can breathe air.

And they don’t just look scary. Left unchecked, northern snakeheads can also threaten native species.

“Because they’re out of their natural environment, where they evolved in a food web that has checks and balances between predator and prey, they often grow and reproduce and spread really rapidly,” said Angela Sokolowski, the invasive species coordinator at the Missouri Department of Conservation. “They could outcompete the natives. They essentially are the bigger player, without good competition, and they become resource hogs.”

And it’s not just fish. Snakeheads will happily dine on crustaceans like crayfish as well.

Ms. Sokolowski and officials in other states have been calling on residents to join the fight against the northern snakehead as angler season gets underway.

The northern snakehead is native to Asia. It was first found in the United States in a Maryland pond in 2002, the U.S. Fish and Wildlife Service said. From there to the Potomac River, rivers in Pennsylvania and onward.