Illegal Pot Farms Are Killing Rare Animals With Bacon-Scented Poison

Marijuana plots hidden in California’s forests are inadvertently poisoning protected mammals called fishers

The weasel-like fisher, an already at-risk animal, faces threats of poison from pot farms. Michael DeYoung/CORBIS

Illegal marijuana growers in California are not only breaking drugs laws, they are also killing animals protected under the California Endangered Species Act. Pacific fishers, small carnivores that look like a cross between a weasel and a teddy bear, are inadvertently being poisoned by contraband farmers who pepper their crop fields with rodenticides.

Fisher populations have been declining precipitously for the last century, and today the animals occupy just 20 percent of their historic range. Researchers estimate that the southern Sierra Nevada fisher population, for example, includes just 300 individuals and fewer than 120 breeding females.

While the biggest threat to the species’ survival is development, marijuana farms exacerbate this already precarious situation. These farms are known to sprinkle poison scented with bacon or peanut butter flavoring to keep pests away from their crops.

A team of California-based researchers first reported the pot-farm problem in 2012, and now they find that the situation is not getting better. According to their latest study, published today in PLOS ONE, 10 percent of dead fishers discovered between 2012 and 2014 were directly killed by poison, and up to 85 percent of all fishers that died contained traces of poison in their bodies.

For the followup study, the team focused on two populations of fishers, one in northwest California in and around the Hoopa Reservation, and another in the more southerly Sierra National Forest. They trapped live fishers and outfitted the animals with tiny radio collars that transmitted GPS coordinates via radio telemetry. The collars also contained mortality sensors, which sent an alert to the researchers if a fisher stopped moving for more than 24 hours.

In total, the researchers recovered 167 bodies. After locating a fallen fisher, they switched into detective mode, examining the area for predator tracks, nearby roads or illegal farms. Fisher bodies, if recovered, were brought back to the lab, where a veterinary pathologist conducted full necropsies.

Predators were technically the most frequent cause of fisher deaths—70 percent—but the team also found that 13 of the animals succumbed to rodenticides. Animals from both populations were poisoned, but poisonings were more prevalent in the northern California group.

Some of the dead bodies contained up to six different commercial toxins, most often ones that inhibit a mammal’s ability to recycle vitamin K. This creates coagulation and clotting problems that eventually lead to massive internal bleeding.

Most of the deaths occurred in the spring, when fishers come out to mate and raise their kits, and all of the poisoned animals were found in remote areas but within the vicinity of illegal marijuana farms.

The team also found that poisons from pot farms are probably weakening the animals and making them more available to predators. Out of 101 deceased fishers that contained enough liver tissue for the researchers to examine, 86 turned up positive for anticoagulant rodenticide exposure.

Fisher predators—including red foxes and spotted owls—that consume poisoned animals might in turn suffer from toxicosis, the team points out, and poisonings likely also extend to omnivorous animals not examined in the study, such as martens.

“We’re showing that [the situation] is not getting better,” lead author Mourad Gabriel, who conducted the research as a doctoral student at UC Davis’ Veterinary Genetics Laboratory, said in a statement. “This is going to get worse unless we do something to rectify this threat.” 

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