Marijuana Farm Pesticides Are Killing Fishers, a Rare Forest Mammal

Poisons from illegal marijuana grow sites are killing increasing numbers of fishers in California. Here, Mourad Gabriel of the Integral Ecology Research Center stands among all the trash at a illegal marijuana grow site in Northern California. Mark Higley/Hoopa Valley Tribal Forestry

Fishers, a rare forest mammal in the weasel family, are being poisoned by illegal marijuana farms in California. And the situation appears to be getting worse.

According to a study published today in the journal PLOS ONE, 10 percent of dead fishers examined were killed by poisons, says study co-author Mourad Gabriel, a scientist with the Integral Ecology Research Center. The rate of annual poisoning deaths has more than doubled since 2012, when Gabriel and co-authors first showed in a study that the animals were being poisoned by pesticides left at marijuana grow sites.

Illegal marijuana farms and plots on public lands are a huge problem in California. In 2012 alone, authorities found and destroyed nearly 870,000 illegal marijuana plants on public and tribal lands, and these grow sites are far from benign; 315,000 feet of plastic hose, 19,000 pounds of fertilizer and 180,000 pounds of trash were taken out of these sites in 2012 by the U.S. Fish and Wildlife Service.

The more pressing concerns, though, are the rodenticides and other poisons. Growers scatter these about the land to keep rodents from eating the growing marijuana plants, which contain enough sugar and other nutrients to make them attractive to animals, as well to prevent chewing on plastic tubing or getting into stashed human food, Gabriel says. These poisons are often mixed with meat or fish, and attract fishers, as well as other large mammals like bears and skunks.

In the study, Gabriel's team collared 156 fishers throughout the state, using devices that could relay their location and also if they weren't moving, and likely dead. The team recovered the bodies of nine fishers that had been poisoned to death. The paper also found that 85 percent of fishers had the poisons in their system, an increase from 79 percent in 2012.

Bill Zielinski, a researcher with the Forest Service's Pacific Southwest Research Station who wasn't involved in the study, says the true impact of the poisons may be higher than the mortality statistics suggest. For example, the study found that the average fisher had 1.7 types of poisons in their bodies, and some as many as six. That's more than previous tests indicate, and Zielinski says it's likely that by being exposed to these toxicants, fishers may act more sluggish or otherwise behave in an erratic way that's more likely to get them killed, possibly by larger wildlife.

But it's not just fishers that scientists are concerned about. Fishers "may be our 'canaries in the coal mine.' I worry that with the density of grow sites on public lands in California, that entire biological communities may be affected," including land and aquatic wildlife, he says.

Other research has suggested that the poisons may be reducing populations of spotted owls and salmons as well. "As a conservationist I see fishers and all wildlife species, as having an integral part in the ecosystems health. If you remove one," or several, "you're going to have a system failure at some points," Gabriel says.

This fisher, brought to UC-Davis for a necropsy, was poisoned by anticoagulant rodenticide (rat poison) found on an illegal marijuana grow site in Northern California. UC-Davis