09/14/2026
Illegal ma*****na plantations hidden inside national forests are poisoning some of the rarest carnivores in North America with rat bait that can smell like peanut butter. Later research found anticoagulant rodenticides in 85 percent of the California fishers tested.
More than 90 percent of tested mountain lions have also carried these poisons, although their exposure comes from many settings. For fishers living in remote forests, researchers found a direct connection to illegal grow sites on public land.
Wildlife disease ecologist Mourad Gabriel first reported the connection in 2012 while studying fishers from the Sierra Nevada and the mountains of Northern California. His team tested 58 carcasses collected far from residential neighborhoods and legal agriculture. Forty-six had been exposed to at least one anticoagulant rodenticide, and four had died from the poisoning.
UC Davis pathologist Leslie Woods conducted the necropsies. Many of the fishers carried more than one rodenticide, suggesting they had encountered poison repeatedly or at different locations. This was especially troubling because a fisher may cover several square miles of forest while hunting.
The poisons interfere with blood clotting. A mouse that eats the bait may remain alive for days while bleeding internally, giving a fisher plenty of time to catch it. The rodenticide stored in the mouse then enters the fisher. Poison can also reach fishers directly.
Manufacturers add flavors such as bacon, cheese, or peanut butter to make bait attractive to rodents, and those smells may attract other animals investigating a grow site.
A study in the Sierra National Forest found an average of 5.3 illegal cultivation sites within each fisher’s home range. Growers used rodenticides to protect cannabis plants from rodents that chewed the stems for moisture. Investigators also found insecticides, fertilizers, diverted streams, camps, and long irrigation lines running through the surrounding forest.
These sites could contain thousands of plants spread among several clearings. The people tending them often remained in the forest for months, applying chemicals that were banned or restricted in the United States. When a plantation was raided and abandoned, removing the plants did not automatically remove the poison scattered around it.
Follow-up research published in 2015 found that exposure among fishers had increased. Nine additional fisher deaths were attributed to pesticide poisoning, bringing the documented total to 13. Predation still caused most recorded fisher deaths, but researchers found that rodenticide exposure could leave an animal bleeding, weakened, or less able to survive an attack that might otherwise have been escaped.
The contamination was not confined to fishers. Anticoagulant rodenticides have also been detected in northern spotted owls, martens, foxes, bobcats, and mountain lions. Gabriel called the fisher a flagship species because its carcasses revealed a problem moving through much of the forest food web. Among the fishers that could be followed, one animal lived in a territory containing 16 separate illegal grow sites.
California legalized recreational cannabis in 2016, but that did not legalize unlicensed plantations on federal land. Trespass grows continued operating alongside the legal market.
Image is for illustration purposes only
Source: Gabriel et al. (2012), PLOS ONE / Thompson et al. (2014), Conservation Letters / Gabriel et al. (2015), PLOS ONE / Wengert et al. (2021), PLOS ONE / UC Davis / Integral Ecology Research Center / U.S. Forest Service.
