Skip to main content

Fishers Face a New Threat: Poisons Used by Marijuana Growers

Posted by Sherri Eng, Office of Communication, Pacific Southwest Research Station, U.S. Forest Service in Forestry
Feb 21, 2017
Rat poison used on illegal marijuana farms pose a threat to fishers (pictured) and other forest animals.
Rat poison used on illegal marijuana farms pose a threat to fishers (pictured) and other forest animals.

Illegal marijuana farms in our nation’s forests are not only threatening the safety of humans in these recreational areas, but are also causing ecological damage to the land. And now, there’s proof that the animals that make the forests their homes are also being harmed.

Rat poison used on illegal marijuana grows in remote areas of California is killing fishers, a cat-sized, weasel-like critter, according to a recent study conducted by a team of scientists from the U.S. Forest Service’s Pacific Southwest Research Station, University of California Davis, University of California Berkeley, Integral Ecology Research Center, Wildlife Conservation Society, Hoopa Tribal Forestry, and California Department of Fish and Game.

Researchers discovered commercial rodenticide in dead fishers in Humboldt County near Redwood National Park, and in the southern Sierra Nevada on the Sierra National Forest. The fishers became ill after eating the rodenticides directly, or by consuming prey that have ingested the poisons.

“The ecological consequences of this are frightening, but not well understood,” says co-author Craig Thompson, a Pacific Southwest Research Station research wildlife ecologist. “These poisons are entering the system not only through prey species, but also through the contamination of the soil and water.”

The researchers deduce that illegal marijuana farms are a likely source, because the fishers in this study were radio-tracked and many were not observed venturing into rural, urban or agricultural areas where rat poison is often found. Illegal marijuana cultivation on public lands is widespread, and some growers apply the poisons to deter a wide range of animals and insects from encroaching on their crops.

“Fishers face plenty of threats to their survival, including predators, disease, and vehicular collisions,” says co-author Kathryn Purcell, a Pacific Southwest Research Station research wildlife biologist. “Adding this new risk factor could mean the difference from a population that is stable or increasing, to a decreasing population. The fact that the problem is so widespread is particularly surprising.”

This new threat could have potential impact on other species already facing declining populations, including wolverines, martens, great gray and spotted owls, and Sierra Nevada red foxes, which may also be exposed to the poison, say the scientists.

Category/Topic: Forestry

Write a Response

CAPTCHA This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.


Brad Forrester
Jul 30, 2012

If we listen to the U.S. government sources long enough, I bet they tell us cannabis is responsible for everything bad from atmospheric erosion to wild life genocide.

We give you $25 billion a year to stop cannabis, what the heck are you doing with the money?

Lenora Tooher
Jul 30, 2012

No need to publish/perhaps just pass this along only...

Lenora Tooher, MS, BS, BS Biology, USEPA Bronze Medal Award-Clinton Administration
Federal Job Applicant for over 400 jobs

Timothy J. Lambert
Jul 30, 2012


“Illegal marijuana farms”, secluded in out-of-the-way places like national parklands, only come about because the law bans marijuana from being grown in LEGAL “farms.” Legal “marijuana farms” would be located in other places besides parklands.

Indeed, the first paragraph in this story spells it out: “Illegal marijuana farms” (Specifically, “illegal” ones – “Illegal,” first word – ) “Illegal marijuana farms” are “threatening the safety” of people, damaging the ecology, and further endangering endangered species.

The Prohibition of marijuana is bad for people, bad for the environment, and even a danger to the animal kingdom.

oh come on
Jul 31, 2012

"The researchers deduce that illegal marijuana farms are a likely source, because the fishers in this study were radio-tracked and many were not observed venturing into rural, urban or agricultural areas where rat poison is often found."

Please get some real evidence before making such a bold headline. The ecologists even said that the fishers consume smaller animals that may have eaten arsenic from people's homes and businesses, not requiring the fishers themselves to venture into human areas. It can take up to several days for poison to kill some rodents.

Ellen Komp
Jul 31, 2012

And yet the US Government squelched the first licensing program for commercial medical marijuana cultivation in California (Mendocino county).

Lenora Tooher
Aug 01, 2012

Just looking at the fisher on that log makes me want to help make sure that we help keep their population and other declining populations stable and increasing as appropriate. What is a possible remedy to this situation?

James Burns
Dec 27, 2012

The illegal grows in National Forests are run by people working for Mexican Cartels according to law enforcement organizations. They are not only using anti-coagulent rat poisons (D Con type), they use deadly poisons to kill marauding bears or any other animal that eats the bait. The Federal Government's EPA knows the dangers of anti-coagulent rodenticides to people, pets and wildlife and tried to ban it but was blocked in court by the manufacturer. The Feds in California (Melinda Haig, Asst.US Atty.) have been too busy busting and harrassing State Law legal and tax paying medical cannabis dispensaries to go out and bust the mega grows on public land. We can stop this damage in one growing season by banning the rodenticide and committing law enforcement resources to these grows instead of wasting them on nickle and dime small grows. Any one with google earth can see the grows on their home computer. I'm sure the Feds can do better since they have access to the satellites.

Rob O
Jul 30, 2013

The rodenticide brodifacoum, the active ingredient in D-Con, has been cited as a hazard to wildlife. Perhaps the marijuana growers could switch from brodifacoum to safer rodenticides. The common rodenticide, diphacinone, is less toxic to mammals and is eliminated more quickly from the rodent's body, according to the California Department of Fish and Game. This rodenticide requires multiple feedings over a few days before killing the target pest. A rat will carry a only a small amount in its body and is therefore much less likely to cause secondary poisoning.

The growers could use EPA approved bait stations to limit exposure to non-target species. The bait stations could be pre-baited for faster acceptance and shorter treatment time with less potential exposure to wildlife. The treatment area could be surveyed daily to remove any dead or dying rodents before a predator takes them. These standard measures might solve the problem.

Legalize marijuana so it can be grown openly on farms and in nurseries where pesticide laws and regulations can be enforced