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Pot Growers' Rat Poisons Hurting Wildlife More Than Expected

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Pacfic fishers in California are taking a big hit from illegal rat poison use | Photo: USFWS

A mid-sized weasel proposed for federal protection is in more danger than previously thought from illegal use of rat poisons at "trespass" pot grows in California's forests. According to a new study, a large majority of Pacific fishers subjected to post-mortem testing had been exposed to a range of anticoagulant rat poisons, including supposedly safer alternative poisons.

The study, published Wednesday in the journal PLOS-One, examined 167 Pacific fishers that died in California forests between 2012 and 2014. Scientists at UC Davis' California Animal Health and Food Safety Laboratory System determined that ten percent of the cat-sized weasels' deaths were directly caused by rat poisons. Predation by other animals accounted for 70 percent of the total fisher deaths, but even those animals were highly exposed to rat poisons. Overall, 85 percent of the animals necropsied showed they'd been exposed to as many as six different anticoagulant rat poisons.

Pacific fishers are rare enough that the U.S. Fish and Wildlife Service is considering listing the West Coast population as an endangered species. Fishers in the southern Sierra Nevada are listed as Threatened under California's Endangered Species Act. The study found that those southern Sierra fishers, as well as fishers in the state's northwestern "Emerald Triangle," are especially hard hit by poisoning.

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Pacific fishers are highly reclusive, shying away from human contact. That reduces the likelihood that fishers are getting their poison at residences. But illegal pot growers prefer the same remote, untraveled forests as fishers, and they often use rat poisons as a component of bait designed to kill wildlife that might otherwise damage their lucrative crops.

Anticoagulant rat poisons kill by interfering with blood clotting. Animals that ingest such poisons usually die from massive internal hemorrhaging, and as those poisons aren't broken down in the process they're passed on to scavengers that then consume the initial victims.

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Rat poison found at a single pot grow site | Photo: Mourad Gabriel/Integral Ecology Research Cente
 

Given the potential financial loss from losing even a single bud to hungry wildlife, pot growers are known to scatter large amounts of rat poison in an attempt to depress the local wildlife population. How much is a "large amount"? According to a synopsis of a cleanup effort covering seven illegal grows in the Shasta-Trinity National Forest in the last week of October, the smallest amount of rodenticide found at any of the sites cleaned up was more than 51 pounds. A few ounces of most such poisons are more than enough to kill a large mammal.

Pot growers commonly mix rat poisons with food to increase the likelihood that wildlife will eat it, including peanut butter, fish, and bacon -- a confluence of weed and bacon that even the most ardent hipster will likely have trouble endorsing. That enhanced bait is then scattered around the grow site and its immediate surroundings.

And that poses a huge threat to a range of wildlife on beyond the Pacific fisher. "Fishers are the flagship species," said Mourad Gabriel, lead author of the study, who has been studying the effects of illegally placed rodenticides on the fisher for several years.

A 2012 study by Gabriel, who works with the Humboldt County-based Integral Ecology Research Center, was the first to link mortality in fishers to rodenticides used at pot grows. In the years since, the peril to the small predator has only increased: known fatal poisonings have more than doubled.

"We have to think of so many species, like Sierra Nevada red foxes, spotted owls, martens -- they all are potentially at risk," said Gabriel. "This is essentially going to get worse unless we do something to rectify this threat."

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Mourad Gabriel of Integral Ecology Research Center stands amid an illegal marijuana grow site in Northern California. | Photo: Mark Higley/Hoopa Valley Tribal Forestry
 

In addition to Gabriel, the study was conducted by scientists from UC Davis, UC Berkeley, Humboldt State, the Forestry department of the Hoopa Tribe, the U.S. Forest Service, Wildlife Conservation Society, and the California Department of Fish and Wildlife.

For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.

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