In California, a little weasel called the “fisher” may be exposing a big environmental problem: Research shows that the species is being heavily exposed to, and even killed by, rat poison used on illicit marijuana cultivation sites. And scientists are concerned they’re not the only animals being affected.

A new study, released Wednesday in the journal PLOS ONE, investigated the causes behind fisher mortality in California and identified a common form of rat poison — which you wouldn’t normally expect to find in the middle of their remote forest habitat — as an “emerging threat” to the species.

After collecting and performing necropsies on more than 150 dead fishers in three California locations between 2007 and 2014, the researchers concluded that predation by other animals, such as bobcats or coyotes, was the top cause of death, accounting for about 70 percent of all the mortalities. Natural diseases were the next most common cause, accounting for about 16 percent of the deaths. But they also concluded that 10 percent were killed by rodenticide poisoning.

The researchers also found that, even if it wasn’t the ultimate cause of death, 85 percent of the animals are being exposed to rat poison, which shows up in their bodies even if they were eventually killed by something else. This is worrisome, as fishers are still recovering from severe declines in California and aren’t completely out of the woods yet.

Fishers are found throughout parts of Canada, bits of the Great Lakes region, and the Pacific states. They’re mid-sized, omnivorous weasels that dwell exclusively in forest habitats. Once prized for their fur, fishers experienced widespread declines in the 1800s and early 1900s thanks to trapping. Today, although they’ve recovered to a certain extent, they still occupy a fraction of their historic range, and fishers in the western states have been a particular concern. The U.S. Fish and Wildlife Service considers West Coast fishers a distinct population segment, which is currently proposed for a “threatened” listing under the Endangered Species Act.

In the state of California, fishers are further divided into two separate populations: the Northern California segment and the Southern Sierra Nevada segment. The latter was listed as threatened earlier this year under the California Endangered Species Act. Altogether, this population segment is believed to number only about 300 individuals, said Richard Callas, a senior environmental scientist with the California Department of Fish and Wildlife. Statewide, there could be anywhere from 1,000 to 4,500 individuals, according to the Department.

These concerns over fisher conservation were what led the scientists to initiate their research. The new study builds on past research that was also conducted by lead author Mourad Gabriel, executive director of the Integral Ecology Research Center. Gabriel was involved with two other papers, one published in 2012 and the other in 2013, which included earlier efforts to document rodenticide poisoning in fishers. The new study combines data from the previous two papers with more recently collected data.

While examining the data for the new paper, Gabriel said he and his colleagues were hoping to answer the question of whether rodenticide poisoning cases were “dissipating” in the state. “Unfortunately, what this paper signified was it wasn’t,” Gabriel said. “The problem is still there. It hasn’t curtailed, it has actually increased.”

Between 2007 and 2011, Gabriel and his team found the rate of toxicosis in fishers was about 5.6 percent. But between 2011 and 2014, the toxicosis rate rose to 18.7 percent. Altogether, the toxicosis rate for the whole study period was about 10 percent.

And exposure rates have increased as well, according to the research. In the earlier study period, 79 percent of collected fishers had been exposed to rodenticides. In the latter study period, this number rose to 85 percent.

Why researchers think illegal marijuana is the cause.

But why are threatened forest weasels consuming a lot of rat poisoning, a substance usually used in homes, not forests?

The conclusion that marijuana cultivation sites could be the culprit behind the poisoning was part of a long process, according to Gabriel. In the early stages of the research, he and his colleagues were puzzled to find rat poison in fishers and started asking around to try and figure out where it might have come from. The Forest Service didn’t use it. Neither did camping grounds or privately owned energy companies that ran cables through the California forests.

It wasn’t until Gabriel gave a presentation, during which he explained some of his early findings and expressed his confusion over the rat poison, that the answer became clear. “After the presentation, several law enforcement officials that were present there [came up] and said, ‘We know your answer, and your answer is marijuana cultivation,’” Gabriel said.

Gabriel began accompanying law enforcement officers on raids of illicit marijuana grow sites, which are frequently located in remote places in the California forests — prime fisher habitat. He found that they were right: growers were placing large amounts of rat poison at the grow sites to keep rodents in check. In his earlier research, Gabriel found that fishers living in territories that included cultivation sites were more likely to have been exposed to rodenticides, further support for the idea that marijuana cultivation was the culprit.

The cultivation of marijuana is permitted in California only for medical use, and growers must register with the state government. The forest cultivation sites being targeted in the government’s raids are unregistered, illicit grow sites, likely intended for the illegal selling of the drug.

And while predation remains the biggest cause of fisher deaths, Gabriel suggests that poisoning could be contributing to those high numbers as well, by weakening otherwise healthy individuals and making them easier prey.

Fishers are omnivores, so the poisoning can come about in two ways, according to Gabriel: Either the fishers eat the poison directly, or they consume a poisoned animal, like a rat or a squirrel.

Because of the poison’s tendency to move up the food chain, Gabriel is also worried that other wildlife could be affected as well — even wildlife that humans hunt and eat, such as deer or quail. Some of his current research is focused on testing other animal species for rodenticide exposure (he’s already found a handful of northern spotted owls that tested positive for exposure). He’s also testing water systems near marijuana cultivation sites to see if the poison could be leaching into streams.

As it stands, law enforcement raids are only controlling the poison problem so much, according to Gabriel. He estimates that law enforcement officials are only aware of about 15 to 25 percent — perhaps as much as 40 percent, in a high estimate — of marijuana cultivation sites in the state, meaning a significant number of them continue to fly under the radar. And while raids may shut down a site temporarily, Gabriel said he observed that there aren’t sufficient resources to clean out all the sites in a way that removes all traces of toxins and makes them inhospitable to future growers.

“We need to have some level of continuous support to remediate and document these grow sites,” Gabriel said. “Each and every grow site that is detected and eradicated needs to have funding allocated to remediate it.”

To the best of his knowledge, Gabriel said, he’s the first person to conduct research on the effects of toxins from illicit marijuana grow sites. But his research has quickly captured the attention of other scientists, including officials from the California Department of Fish and Wildlife. In fact, his research was considered in the fisher species status review that led to the threatened listing for the Southern Sierra Nevada population.

“It’s clearly a threat to fishers and some other species that may be exposed to toxins that are used at marijuana grow sites,” said Callas, the California Fish and Wildlife scientist. But there’s still more research to be done for a better understanding of the problem, he added. Gabriel’s research sampled fishers in three specific locations in California — one in Northern California and two in the Southern Sierra Nevada — and these areas constitute a “relatively small portion of their range,” Callas said.

“So the question is…how representative is that of fishers elsewhere in California,” he said. A larger-scale study, which samples in a wider swath of the state, could help to answer that question.

Still, he said, “the exposure rate where they have studied fishers has been high.” And as the new research shows that it’s been growing over the last few years, the threat is nothing to take lightly, Gabriel said, adding: “That is a big red flag.”