The target is most often beaver.
But Wildlife Services body-grip traps which consist of two square bars of steel that snap together like the jaws of a crocodile have killed more than 7,800 animals by mistake over the past six years, more than any other agency tool.
The list of non-target victims killed since 2000 includes more than 85 species, most native and many aquatic, from wood ducks to snapping turtles, green-winged teal to alligators.
No casualty draws more concern than the chocolate-brown river otter, which is the focus of widespread conservation efforts.
"People love to see them," said Megan Isadore, co-founder of the River Otter Ecology Project in the Bay Area. "They are charismatic carnivores. They are playful, beautiful and a little bit shy so when you see one, it's kind of exciting."
On occasion, otters are killed on purpose when they threaten aquaculture facilities. But 84 percent of otters caught in Wildlife Services body-grip traps since 2006 2,350 of the 2,800 animals were killed by mistake, records show.
"The image of the federal government accidentally killing hundreds of rivers otters each and every year is especially unfortunate in light of the money and effort expended in at least 21 states to reintroduce this member of their native mammal fauna," wrote Michael Mares, president of the American Society of Mammalogists in a March letter to Wildlife Services.
William Clay, deputy administrator of Wildlife Services, acknowledged the devices kill broadly. "It's not as specific a trap as some of the other ones," he said.
Agency records show that for every 45 beaver killed on purpose in body-grip traps since 2006, one otter has died by accident. Turtles are common collateral damage, too. Ninety-nine percent of all turtles killed in the traps are unintentional. "There is going to be some non-targets taken," Clay said. "We just do what we can to minimize that."
Body-grip traps were invented in Canada in 1957 and are considered to be more humane than other traps because they often kill quickly by crushing an animal's neck or when placed underwater drowning the animal.
But often does not mean always. The Wildlife Services body-grip trap that caught Maggie a border collie-Irish setter mix owned by Denise and Doug McCurtain and their four children in Oregon last year took at least 20 minutes to end her life.
"She was still breathing. Her eyes were open," said Denise McCurtain who rushed to Maggie's side to help free the animal from the trap near a neighborhood pond.
"The look in her eyes when I went down there, I will be haunted by that forever. There is no way that was humane at all."