Rodenticide at Trespass Grows Is Killing Fishers

Photo by Airam Dato-on via Pexels.

A Pacific fisher weighs about as much as a house cat and hunts like something much larger. It is a forest carnivore in the weasel family, and researchers have recovered fishers dead with rat poison in their tissue, hundreds of yards from the nearest road and miles from any farm.

The source is usually not a farm at all. Trespass grows on public and tribal land have no licensed applicator, no product label and no record of what was spread where. Whatever goes onto the ground stays there, and the rodents, then the predators, carry it onward.

This article lays out the full picture: what the carcass research found, what the newest IERC study changes, and what a responsible licensed grower does differently. Some details below are described in general terms because I could not verify them against primary sources, and I flag those where they come up.

Why the Pacific Fisher Is the Animal to Watch

Fishers are forest-dependent. They need mature, structurally complex forest, with big trees, dense canopy and plenty of downed wood for dens and cover. California's populations are small and fragmented, which makes each one fragile.

The conflict zones are well known. The Emerald Triangle (Humboldt, Mendocino and Trinity counties) is a hotspot for trespass cultivation, and the southern Sierra Nevada has its own cluster of sites. Both overlap with fisher range.

Mourad Gabriel, executive director of the nonprofit Integral Ecology Research Center (IERC), first suspected the cannabis link in 2011. He went on to document it in carcass after carcass.

Population math is the reason this matters. Fishers are low-density predators with slow reproduction, so losing even a handful of breeding females can change what a local population looks like within a few years. A species that is rare to begin with has little buffer.

One caution before going further. The federal and state Endangered Species Act status of the Pacific fisher differs by population, and I have not confirmed the current listings. Check USFWS and the California state wildlife agency for the latest status instead of relying on any summary, including this one.

How Rat Poison Travels From a Grow Site to a Fisher

The mechanism is simple. Trespass growers often scatter anticoagulant rodenticide around plants and along irrigation lines, because rodents chew drip tubing and seedlings. Bait goes down in the open, in quantity, and nobody comes back for it.

Fishers eat small mammals, so they can be poisoned secondarily by eating rodents that fed on the bait. Bait left exposed can also be a direct hazard. Either way, the poison moves up the food chain from a single patch of ground.

Anticoagulants interfere with blood clotting. An animal can die of internal bleeding, and a sublethal dose can leave it weaker against everything else the forest throws at it: cold, hunger, disease, a bad winter. A fisher that survives exposure is not necessarily unharmed.

What makes trespass sites different is the absence of any system. They sit on public and tribal land, so there is no label to follow, no licensed applicator, no recordkeeping, and no cleanup when the grow is abandoned. The bait often outlasts the crop.

Licensed cultivation sits in a different world. Legal operations fall under pesticide rules and inspection, with products that have to be used according to their labels. Trespass grows fall outside any tracking system, so the harm is real while the data on it is thin.

What the Fisher Carcass Data Actually Shows

Across three studies, roughly 79 to 85 percent of tested fisher carcasses were positive for rodenticide, showing exposure was consistently high and slightly higher in the 2013 Sierra National Forest study.

The numbers are striking, but they come from different samples and methods, so treat them as a pattern rather than one rate. Here is what each source reported:

• 2012 IERC study (PLoS ONE): 46 of 58 fisher carcasses tested had been exposed to an anticoagulant rodenticide, roughly four in five. It was the first documentation of widespread exposure in the species linked to trespass grows.
• 2013 Conservation Letters paper: Craig Thompson and Kathryn Purcell of the USDA Forest Service found that 85 percent of fisher carcasses recovered in the Sierra National Forest tested positive.
• Same Thompson and Purcell work: female fisher survival was influenced by the number of grow sites within her home range.
• Scientific American: of 129 radio-collared fishers whose cause of death could be determined over eight years, poison at illegal grow sites killed 13.
• High Country News, citing Mother Jones: at least 48 of 60 fishers collected by Gabriel tested positive.

Notice the gap between exposure and death. Most carcasses carried poison, but the radio-collar data attributes a smaller share of deaths directly to it. Exposure is widespread. Confirmed poisoning as the cause of death is rarer, though it can still weaken animals that die of something else.

A habitat-overlap model built on nearly 1,500 trespass grow sites in California and southern Oregon also reported heavy overlap with fisher habitat in the northern Sierra Nevada. I could not confirm the exact percentage from the excerpt I had, so I am not quoting a figure here.

The Newest IERC Study: Rules Are Working, Gaps Remain

IERC has released what it calls the largest-ever study of its kind, with a September 2026 release. I have not confirmed the journal, so check the full paper. Its headline finding is encouraging: California's rodenticide restrictions are working, and anticoagulant exposure in these forests declined within a few years of the rules taking effect.

The study also flags newer rodenticides rising in their place. I could not verify which compounds it names, so I will not guess. The pattern matters regardless: close one door and the pressure finds another.

It reports that up to 46 percent of animals carried at least one rodenticide. The excerpt I saw did not name the species, so do not read that as a fisher figure until you have checked the full article.

The gap in the data is structural. State pesticide-use records capture only licensed commercial applicators. They miss household purchases and illegal use, and IERC says trespass cultivation remains untracked by any regulatory database. I am describing California's restrictions in general terms, since I have not verified specific bill numbers or the dates of Department of Pesticide Regulation rules.

Balance is warranted. Hanson, a researcher quoted in High Country News, considers the lack of high-severity fire a greater overall threat to fishers than rodenticide. And the Northern Sierra Fisher Reintroduction Project is relocating fishers to the southern Cascades and northern Sierra Nevada, so the animals are not simply fading out. Poison is one pressure among several, but it is one that growers can actually remove.

What Licensed Growers Should Do Differently

Licensed growers have no excuse and plenty of options. Start with exclusion, because a rodent that cannot get in does not need killing:

1. Seal gaps in walls, foundations and around cable and duct penetrations.
2. Keep door sweeps tight.
3. Trim vegetation back from structures.
4. Store nutrients, soil amendments and bagged media in sealed bins.

When rodents do get in, use snap traps or enclosed multi-catch traps and check them daily. Skip anticoagulant baits, especially anywhere outdoors or near drainage. A poisoned rodent is slow to die and easy to catch, and raptors, foxes and fishers eat it.

Outdoor growers near wild land can protect drip lines with buried or shielded tubing. Reduce cover and food sources around the garden instead of baiting. Clean ground, short vegetation and sealed feed make a site unattractive without poisoning anything.

Buy from licensed, tested sources and avoid anything of unknown origin. Trespass-grown product carries the pesticide risk into the supply chain. On the front end, Seedtiva offers quality genetics suited to clean, well-managed grows, though results vary with climate, setup and genetics.

If you come across a suspected trespass grow while hiking or hunting, leave and report it to local law enforcement or land managers. Do not enter. Sites can be hazardous and contaminated, and the poison is only one of the dangers.

The fisher is a warning for other forest predators. The poison pathway that reaches a forest carnivore also reaches owls, and IERC's 2018 work found rodenticides in 70 percent of sampled northern spotted owls in the same region. The newest study suggests regulation can bend the curve even when the source stays hidden.

The unresolved piece is the visibility problem. Trespass grows are invisible to the data systems that measure progress, so nobody can say how much of the remaining exposure traces back to them. What growers do control is their own ground: every licensed grow run without anticoagulants is a small, practical contribution, and it costs a few traps and some diligence.

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