Researchers from the University of California, Davis, along with the Wildlife Conservation Society and other partner institutions, said they've found commercial rodenticide in dead fishers in Humboldt County near Redwood National Park and in the southern Sierra Nevada in and around Yosemite National Park.
Illegal marijuana farms in remote areas where fishers live are considered a likely source as some marijuana growers apply the poisons to keep a wide range of animals away from their crops.
Fishers are likely exposed to the poison when eating animals that have already ingested it, the conservation society said in a release Monday.
Fishers in California, Oregon and Washington have been declared a candidate for listing under the federal Endangered Species Act.
Other species, including martens, spotted owls and Sierra Nevada red foxes may also be at risk from poisons, researchers said.
"If fishers are at risk, these other species are most likely at risk because they share the same prey and the same habitat," UC Davis researcher Mourad Gabriel said in a university release.