Populations of the fisher, a rare forest-dwelling mammal, are likely being poisoned as a result of illegal marijuana-growing operations in the southern Sierra Nevada range, according to the results of a recent study. (Photo : Flickr / digizen )

Populations of the fisher, a rare forest-dwelling mammal, are likely being poisoned as a result of illegal marijuana-growing operations in the southern Sierra Nevada range, according to the results of a recent study.

The marijuana plant itself is not a threat to the fisher, but the liberal amounts of pesticides and rat poisons placed on and around the crop are ingested by the small forest creatures that the fisher preys upon, which in turn sickens the fishers and threatens their survival.

About the size of a housecat, the weasel-like animal dwells in the rugged portions of the Sierra Nevada and is considered a sensitive species in the western United Stated by the U.S. Forest Service.

Researchers from the U.S. Forest Service's Pacific Southwest Research Station (PSW), University of California, Davis, University of California, Berkeley and the Integral Ecology Research Center conducted the study, which found evidence that female fishers that live in areas with a higher number of marijuana farms had more exposure to rodenticides and subsequently lower survival rates.

Exposure of wildlife to pesticides is widespread and well documented. But the use of rodenticides and insecticides around illegal marijuana cultivation sites is "a fundamentally different scenario than has been previously addressed," the researcher wrote in their study.

Illegal marijuana grows were pinpointed as the likely source of the fishers' sickening after radio-collared fishers were not observed straying into urban, rural or agricultural areas where pesticides were used legally.

"In [illegal] marijuana cultivation sites, regulations regarding proper use of pesticides are completely ignored and multiple compounds are used to target any and all threats to the crop, including compounds illegal in the U.S.," wildlife biologist and study author Kathryn Purcell said in a statement.

In the vicinity of illegal marijuana grows, numerous dead or dying insects and small animals are often found, the researchers report.

However, the association between fisher mortality and marijuana growing operations remains "strong yet speculative," the study authors wrote, noting that "Determining a cause and effect relationship would require novel testing procedures and either an experimental framework or an extremely challenging, logistically difﬁcult collaboration between the scientiﬁc and law enforcement communities, given the inherent dangers of visiting and monitoring these sites."

Still, the link between forest animals dying from pesticide poisoning is cause for concern.

"By increasing the number of animals that die from supposedly natural causes, these pesticides may be tipping the balance of recovery for fishers" said Craig Thompson, a PSW wildlife ecologist and the study's lead author.

The complete study can be found on the U.S Forest Service website.