Rat poison used on illegal marijuana farms may be sickening and killing the fisher, a rare forest carnivore that makes its home in some of the most remote areas of California, according to a team of researchers led by University of California, Davis, veterinary scientists.

Researchers discovered commercial rodenticide in dead fishers in Humboldt County near Redwood National Park and in the southern Sierra Nevada in and around Yosemite National Park. The study, published July 13 in the journal PLoS ONE, says illegal marijuana farms are a likely source. Some marijuana growers apply the poisons to deter a wide range of animals from encroaching on their crops.

Fishers in California, Oregon and Washington have been declared a...