Rat poison used on illegal marijuana grows is killing fishers in the southern Sierra Nevada, according to a recent study conducted by a team of scientists from the University of California, Davis, the U.S. Forest Service’s Pacific Southwest Research Station, UC Berkeley, and the Integral Ecology Research Center in Humboldt County, Calif.

A study published last summer by a team of UC Davis veterinary scientists documented that rodenticides were being found in the tissues of fishers — cat-sized, weasel-like animals that live in rugged portions of the southern Sierra Nevada. Lead author of that study, Mourad Gabriel, a UC Davis Veterinary Genetics Laboratory researcher and president of Integral Ecology Research Center, speculated that the most likely source of the poisons was the illegal marijuana grows found throughout the Sierra Nevada.

The new study, published this week in the journal Conservation Letters, solidifies that link, showing that female fishers who live in areas with a higher number of marijuana sites had more exposure to rodenticides and subsequently had lower survival rates.

"This paper further emphasizes that the number of marijuana cultivation sites, a risk factor just recently discovered, is tied to the survival of female fishers,” said co-author Gabriel.

The findings concern scientists because the fisher is a candidate for listing under federal, Oregon and California endangered species acts, and is considered a sensitive species in the western United States by the U.S. Forest Service.

In this study, scientists reported on the amount of poisons found at over 300 illegal plots and compared the locations of these sites with the home ranges and survival of 46 adult female fishers.

The researchers deduced that illegal marijuana grows are a likely source of the poison in fishers because the animals in the study were radio-tracked, and many were not observed venturing into rural, urban or agricultural areas where rodenticides are often used legally.

Illegal marijuana cultivation on public lands is widespread, and some growers apply large quantities of numerous pesticides to deter a wide range of animals and insects from encroaching on their crops. While the exposure of wildlife to rodenticides and insecticides near agricultural fields is not uncommon, the amount and variety of poisons found at the illegal marijuana plots is a new threat, as regulations regarding pesticide use tend to be ignored by those tending illegal marijuana sites.