Biologists in the U.S. are questioning whether rat poison used on illegal marijuana farms is inadvertently killing a vulnerable forest predator, one that is also a species of concern in B.C.

The fisher, a small weasel-like forest animal, is currently being considered for the endangered species list in the states of Washington, Oregon and California — and biologists are considering the role that outlaw marijuana plantations may have in its decline.

Mourad Gabriel, a researcher based at UC Davis, says the animal is normally a fierce predator but appears defenceless in the face of rat poison.

The toxin was found in 80 per cent of a sample of 58 dead fishers in Northern California.

"We're finding lots of chemicals out there. We're finding also banned chemicals that have been banned by Canada, United States and other nations because of the high toxicity to humans and wildlife," Gabriel said.

Gabriel suspects most of the fishers consumed the rat poison through the bodies of its prey, but some ate it directly, he said.

As other researchers have also found, Gabriel says the likely culprit is marijuana farmers using rat poison to protect their valuable crops.

"They're using it to deter rodents from chewing on plastic irrigation pipes ... to deter rodents from chewing on their seedlings," he said.

In B.C., where grow-ops are also found on forest land, the fisher is blue-listed as a "species of special concern."

The U.S. Fish and Wildlife Service is just starting to study the connection, and says it does not know if B.C. marijuana farms are also using rat poison.

With files from the CBC's Jason Proctor and the Associated Press