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. 

“Unfortunately, these early investigations of rodenticide exposure in fishers that are associated with illegal marijuana cultivation on public lands might only be the tip of the iceberg with regard to overall impact of pesticides on wildlife at these sites,” said co-author Robert Poppenga, a professor with the California Animal Health and Food Safety Laboratory System at UC Davis. Poppenga’s laboratory provided rodenticide analysis in support of the investigation.

While some fishers have died from either directly consuming flavored rodenticides or by consuming prey that had recently ingested the poisons, exposure may also predispose animals to dying from other causes. Exposure to lower doses or to combinations of the poisons results in slower reflexes, reduced ability to heal from injuries, and neurological impairment. This leads to death from other causes, such as predation or road traffic.

Fishers in the southern Sierra Nevada are highly susceptible to pesticide exposure because their diet consists of small mammals, birds, carrion, insects, fungi and other plant material. Numerous dead or dying insects and small mammals are often found in the vicinity of illegal marijuana sites.

The conservation implications of this study are far-reaching.

“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 wildlife ecologist with the U.S. Forest Service’s Pacific Southwest Research Station and the study’s lead author.

This new threat may also impact other species already facing declining populations, including the wolverine, marten, great gray owl, California spotted owl and Sierra Nevada red fox, which may also be exposed to the poisons, the researchers said.