“Don't touch any bottles or cans you find,” Craig Thompson told us as we wound our way through oak and manzanita. “And if anything looks fresh, tell me.” Thompson is a research ecologist with the U.S. Forest Service and has been studying fisher populations in California for years, but it's in the last six that his job has gotten a lot more complicated — and dangerous.

We were walking up a trail toward the site of an illegal marijuana grow, discovered and cleaned up by law enforcement officers about 18 months ago. I'd parked our car less than a quarter of a mile away, in front of a residential house and next to an old fire road that soon petered down to a walking trail and then into almost nothing. We could still easily hear the road traffic, but we couldn't see it. That proximity to the road yet being virtually invisible is exactly why this site was chosen by illegal growers to raise thousands of pot plants on public land.

The issue of plots of public land hijacked by illegal pot growers is not new. Anyone who has wandered around the Humboldt area of California knows not to stray off trail when hiking. But the extent of California's pot problem — the areas where it has spread and the depth to which it's draining the land of water and wildlife alike — is coming to light through years of careful research. The credit for blowing the whistle on the issue goes to one species in particular; in fact, one individual animal in particular. The mysterious death of a fisher, a carnivore in the weasel family, in April 2009 connected the dots between the threatened species' struggle to reestablish populations in what seems like perfectly suitable habitat, and the ongoing problem of trespass grows on public land.

A Pacific fisher, sometimes referred to as a tree otter, awakens from slumber after getting outfitted with a radio-collar. These close relatives to the wolverine were once almost hunted to extinction for their fur. Today impacts from marijuana grows mightA Pacific fisher, sometimes referred to as a tree otter, awakens from slumber after getting outfitted with a radio collar. These close relatives to the wolverine were once almost hunted to extinction for their fur. Today impacts from marijuana grows might sound the death knell for the species. Studies have shown that fisher populations could only sustain about a 10 percent death increase before sending the species on a precipitous decline. (Photo: Morgan Heim)

The particular fisher was a collared male and part of a study. He had been tracked for most of his life. When his collar sent up a mortality signal, a technician found the fisher's body. He looked perfectly healthy, save that he was dead. Confused about what might have caused the death of what otherwise seemed to be a healthy, uninjured animal, the team sent the fisher in for a necropsy. The animal had died of acute rodenticide poisoning.

Rodenticide poisoning is an awful way to go, and sadly, a way that more and more predators are dying after eating prey killed by or dying from rodenticide. The poison is an anticoagulant. A large dose will kill an animal outright. “It opens the holes in veins and arteries, and the blood literally just leaks out,” explains Thompson. “It's a mess inside then.”

If an animal takes in a sublethal dose — say a coyote eats a couple of rats that have ingested rodenticide — the animal can die from tiny injuries that simply won't stop bleeding. “There's a lot of records of animals coming into wildlife rehab that end up dying from rodenticide poisoning, but it's small injuries,” says Thompson. “They'll bleed out, essentially. I read that one great horned owl bled out from a mouse that bit it on the toe.”

While there are plenty of records of animals ingesting rodenticide, it's usually animals that are near human habitation. But this fisher created a worrying mystery. First, fishers live deep in forests and don't tend to hang out anywhere near humans. Second, this was a collared male tracked most of his life, so the researchers knew for a fact he wasn't going near any places where rodenticide might be. How could it have died from acute poisoning?

Stumbling upon a grow is a constant hazard for researchers studying the Pacific fisher. Even though part of the study area, some places are off limits for safety purposes.Stumbling upon a grow is a constant hazard for researchers studying the Pacific fisher. Even though part of the study area, some places are off-limits for safety purposes. (Photo: Morgan Heim)

Mourad Gabriel, a wildlife disease ecologist with the University of California Davis, performed the necropsy. He had questions not only about how this male fisher got into rodenticide in the middle of a forest, but if it might have happened to fishers elsewhere. He analyzed archived tissue samples from all the animals sent in by the researchers on the fisher study and discovered that 85 percent of the animals tested positive for some amount of rodenticide.

“That was kind of the sky-falling moment. We had no idea what was going on,” says Thompson. But Thompson didn't have to wait long for answers to arrive. In conversations with law enforcement officers, researchers realized that the chemicals they were finding in fishers were the same as the ones commonly found at grow sites. “We started digging into it deeper, started realizing the scope of the problem and what these sites are baited like. Since then, [we've] gotten more and more focused on this as a real problem.”

The poisoning isn't accidental. Bacon and cheese-flavored baits, or poisoned hot dogs and other baits, are intentionally left out to kill any animals that come close to the grow site.

“In the process of trying to defend their plants, they'll actually ring the area with tuna cans with pure carbofuran mixed into the tuna, which is extraordinarily lethal, and they'll just put them around the perimeter. These guys [are] living out here, so they don't want bears or foxes out here messing up their camp.”

In early 2010, the researchers discovered that the rodenticide was coming from these trespass grows. In that time, despite publishing a few papers on the effects of the grows on wildlife, things aren't changing. “We're still at a 70 to 85 percent exposure rate. And the bottom line is it's getting worse.”

A mysterious pink substance inside a Gatorade bottle sets off alarms with Greta Wengert, the Associate Director of Integral Ecology Research Center. The research team is cleaning up a grow outside Quincy, Cali. The substance, suspected to be carbofuran, iA mysterious pink substance inside a Gatorade bottle sets off alarms with Greta Wengert, the Associate Director of Integral Ecology Research Center. The research team is cleaning up a grow outside Quincy, Cali. The substance, suspected to be carbofuran, is a powerful and commonly used neurotoxin used by growers to rid the area of wildlife. An exposure of even a tiny amount, from touch, ingestion or breathing, can lead to severe illness and death. (Photo: Morgan Heim)

In 2013, Gabriel and a team of researchers published a paper showing that a fisher was purposefully poisoned by growers with rodenticide. It was the first confirmed intentional poisoning of a fisher by growers, using carbamate insecticide. A study published in November in the journal PLOS ONE, shows that increasing numbers of fishers are dying from a growing variety of rodenticides found at trespass pot farms. The study reports that 69 percent of the poisoning cases occur in the spring, which is particularly bad since this is the time of year when fishers mate and raise their kits, and that as many as six different rodenticides can be found in a single animal.

But it isn't just fishers and other predator species that are at serious risk from the chemicals used at the grow sites. So too is the general public. Thompson warned me not to pick up any bottles, such as water or soda bottles. The reason is because growers will mix chemicals in them and thus they may contain concentrated amounts of chemicals. I had a warning, but a child hiking with parents may not realize that such an innocuous item like an old bottle could be deadly. The growers also dig shallow reservoirs to store water, into which they usually pour chemicals before watering the plants. They cover the reservoirs up with branches and tarps to disguise it. Falling into one can be lethal – not from drowning but from the chemicals soaked into a victim's system on contact.

"I was looking at a map the the law enforcement guys had of a lot of the grows and I saw one that had this big list of chemicals they pulled out of it. It was about a hundred yards up-slope of a place called Lewis Creek, which is just above Oakhurst here, where I've taken my kids to play. That was a watershed moment for me, that this is not an academic thing. These are my kids playing in the water."

In the lab, samples from water sources are tested for a broad range of chemicals. Long lists of things have been detected from single sources, including drugs, anticoagulants, pesticides and insecticides. “There are chemicals they pull out of [the sites] In the lab, samples from water sources are tested for a broad range of chemicals. Long lists of things have been detected from single sources, including drugs, anticoagulants, pesticides and insecticides. “There are chemicals they pull out of [the sites] that are things you could never buy here," says Mike Filigenzi of the California Animal Health and Food Safety Laboratory, pictured here. "Stuff that's so concentrated it's just not available to the general public.” (Photo: Morgan Heim)

In the process of producing a crop, growers will drain the streams dry of water, poison wildlife, and leave behind lethal amounts of chemicals. All of this happens on public land, where any person, along with their kids and pets, might easily stumble across a grow site.

“Odds are good that people bring their kids up here and they're having a picnic for the day, and should one of those kids find the wrong bottle, that's a disaster waiting to happen. I think if people understood the risk that is posed by what appears to be really harmless things in the woods, that could make an impact. Whether it's hikers, bikers, fishermen, target shooters...we're a quarter-mile from the houses right there.”

Fishers are just one victim of illegal and thus unregulated pot grows. Luckily, they're a victim getting desperately needed help. This fall brought positive news for fishers in both Washington and California.

In Washington, where fishers are listed as an endangered species, approximately 80 individuals will be released over the next 14 months in the Cascade Mountains, a habitat where the species hasn't been seen in over half a century. Meanwhile in California, four fisher kits were introduced into Yosemite National Park in an effort to reestablish a population in an area where the species hasn't been seen since perhaps the early 1900s.

The reintroduction in Yosemite underscores how fragile the species' survival really is. Only two populations of fishers survive in California. We know that historically the fishers lived in the Yosemite area, and the area has been protected for over 70 years from logging and other threats to fishers. However, the species has yet to repopulate the area. The reason, we now know, is because of the high mortality rates from other human-caused dangers.

An orphaned Pacific fisher kit lives inside an incubator at the Fresno Chaffee Zoo. Its mother appears to have died of predation, but rodenticides from pot grows are found in over 80 percent of all dead fishers. This means doing everything possible to givAn orphaned Pacific fisher kit lives inside an incubator at the Fresno Chaffee Zoo. Its mother appears to have died of predation, but rodenticides from pot grows are found in over 80 percent of all dead fishers. This means doing everything possible to give fishers a second chance. This kit will be reintroduced into Yosemite National Park a few months later. (Photo: Morgan Heim)

Researchers have learned that a 10 percent swing in mortality survival is enough to keep a population of fishers from expanding into historical habitat and recovering the population. Acute rodenticide poisoning alone accounts for 13 percent of the known moralities of fishers. Even more of the animals are dying from ingesting sublethal amounts that cause them to die from smaller injuries, illnesses and other issues that wouldn't normally kill them. Thus, fishers are well above that 10 percent mortality rate and it is due primarily to rodenticide. In fact, the chemicals from pot grows even affect the survival rate of fisher kits. “We are pretty sure it can be passed in milk. We did have a nursing kit that died that tested positive for it. So maybe it got it before birth, but most likely it was in the milk.”

The rates of death from rodenticide alone appears to be enough to keep the fishers from being able to recover their numbers, despite years of protection. Fishers are being considered for endangered species status in California, which would provide them with even more protections. But whether it is enough to save the species locally, only time and continued effort can tell.

Fishers aren't just facing individual growers leaving out poisons. They are also up against the economic and political pressures across the country that create a high demand for marijuana and sluggish movement on the regulation and control of growing practices. Though Thompson has long been part of an uphill battle for saving this important forest predator from extinction, he still has hope.

"I think for the most part, the public is unaware of the industrial scale of all this. The pot that's grown here is sold nation-wide. It's not just a California thing. For that reason, I think that if the public understood the damage that is being done and the risks that are being created, it would motivate everyone to search for solutions."

All that glitters is not gold. Electric tea lights illuminate divots from the cultivation of marijuana in High Sierra National Forest. Each divot would hold three plants at a time. This plot -- one of several on the hill slope -- contained about 1200 planAll that glitters is not gold. Electric tea lights illuminate divots from the cultivation of marijuana in High Sierra National Forest. Each divot would hold three plants at a time. This plot -- one of several on the hill slope -- contained about 1200 plants, considered a small grow. (Photo: Morgan Heim)

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.