A fisher stops to sniff at the ground on a University of New Hampshire farm property in 2022. The university and New Hampshire Fish and Game Department have found that fisher are widely exposed to toxic rodenticides, likely through their preferred prey of small mammals. (Photo courtesy University of New Hampshire)

It’s not easy to spot a fisher. The nocturnal species, with its long, brown-furred body, rounded ears, and short snout, prefers to live a solitary life in dense forest. But sometimes, New Hampshire researchers manage to catch one on a hidden game camera as it inspects a spot on the ground, perks up to scan the forest floor for prey, or runs, with an awkward, loping gait, through the trees.

Scientists are staking out these shy predators to understand more about a decline in their numbers that has worsened in recent years. The cause isn’t fully clear, and is likely multifaceted, they said, but one possible culprit has set off alarm bells: rodenticides, the poisons humans use to target rats, mice, and other rodents, that have caused illness and death in other wild species in the state.

“Unfortunately, there’s a lot of risks out there for wildlife. Rodenticide is growing on that list, now that we’re seeing how broad exposure seems to be,” said Dan Bergeron, chief of the wildlife division at New Hampshire Fish and Game.

Final results from the fisher study are still to come, but the prevalence of rodenticide exposure in the species is becoming clear. Meanwhile, toxic and sometimes lethal effects of the chemicals have been documented in other species of New Hampshire wildlife, including raptors like owls, eagles, hawks, and other birds of prey. Rehabilitators and environmentalists say the situation is getting worse.

“It’s a moment, right now, where we really need to shift,” said Caelin Graeber, an organizer with anti-rodenticide advocacy group Rodenticide Free New Hampshire.

The New Hampshire House is currently considering two bills seeking to ban or limit certain rodenticides, including those that seem to be affecting wildlife the most. But the rodents the chemicals seek to control pose a mounting nuisance and a real threat to public health, some officials and pest control professionals said, and that has complicated the push for a ban.

Focus on ‘SGARs’

Rep. Cathryn Harvey, a Spofford Democrat, is the prime sponsor of House Bill 1018, which proposes a statewide ban on the rodenticides she said are particularly dangerous for wildlife: a class called second-generation anticoagulant rodenticides. The bill contains some carve-outs, allowing use when rodents pose a threat to public health and don’t respond to other means of population control, for example.

Meanwhile, House Bill 1676, from prime sponsor and Republican Rep. Liz Barbour of Hollis, would restrict access to licensed pest control professionals for those chemicals and certain other “high risk” rodenticides.

“Second-generation” anticoagulants were developed in the 1970s, after rodents became resistant to the “first-generation” poisons of the ’40s. They work by disrupting the process of blood clotting, which is required to prevent bleeding. An animal that consumes a lethal dose will typically die over a period of four days to two weeks from widespread internal bleeding, according to the U.S. Environmental Protection Agency.

Affected rodents move sluggishly, making them easy targets for predators that eat small mammals. And if they return to the bait multiple times to feed, the toxic chemicals accumulate in their livers, turning their body into a more potent dose of poison that can be passed on to an opportunistic predator.

Because they persist in animals’ bodies for longer than first-generation rodenticides and other types of common poisons, these newer chemicals are a greater concern for off-target species and have broader ripple effects, according to the EPA.

The four second-generation anticoagulant rodenticides registered in the United States are the active ingredients brodifacoum, bromadiolone, difenacoum, and difethialone. The EPA does not allow the chemicals to be used in products “targeted to consumers,” according to their website, such as pre-loaded bait boxes, but the products are still sold commercially and in large volumes at farm stores and some other retailers.

While their use by pest control professionals is reported to the Department of Agriculture, Markets, and Food, use of the chemicals by unlicensed homeowners is not tracked.

In nature, spillover effects build up

One after another, concerned citizens deliver injured and sick wild animals to Maria Colby. One of a handful of licensed wildlife rehabilitators in New Hampshire, Colby takes smiling photos of the citizens and posts them on the rescue’s Facebook page, thanking them for bringing the patients to her Henniker rescue, Wings of the Dawn.

Many of the animals arriving at Wings are raptors showing signs of rodenticide poisoning: bloody beaks, sluggishness, poor balance or difficulty moving, according to the Merck Veterinary Manual. Colby tests their blood for signs of poisoning and gives the animals food and vitamin infusions. Many die; Colby may then send tissue samples to a laboratory to test for specific rodenticides. The results tend to confirm the poisoning she had suspected.

In 2025, Wings took in 68 animals with signs of rodenticide poisoning, according to Colby’s records, including 14 barred owls, nearly 40 hawks of multiple species, four crows, a turkey vulture, a gull, a fisher, a squirrel, a chipmunk, and four bald eagles. Five lived, while 63 of the animals, about 93%, did not — including the bald eagles, three of which tested positive for more than one type of rodenticide. (Laboratory testing results were not available for the fourth eagle, a juvenile.)

The sample of wildlife seen by rehabilitators like Colby is limited: It consists of animals that fell ill in places visible to humans and were found by someone willing to transport them to get help, said Carol Foss, senior advisor for science and policy at NH Audubon. 

“One suspects that that’s the tip of the iceberg,” she said.

The concentration of a toxin, like rodenticide, as it is passed upward through the food chain is known as bioaccumulation. This phenomenon has historically plagued birds of prey, as in the case of the pesticide DDT, which was widely banned after lethal upstream effects on raptors and other birds were documented in the 1960s and ’70s.

Bioaccumulation of rodenticides can affect other species, too: Their toxicity is indiscriminate, said Bergeron. Pets or any of New Hampshire’s wildlife that feed on rodents, like foxes, bobcats, coyotes, mink, raptors, and more, could wind up inadvertently ingesting some of the poison, which will then accumulate in their system. And the poisons seem to spread: they have also been found in aquatic animals, he noted, though how they wound up there is not yet clear.

Evidence suggests that some degree of broad exposure is occurring in New Hampshire wildlife. Preliminary data from a collaborative effort between the University of New Hampshire and the Fish and Game Department shows that fisher and foxes across the Northeast are being exposed to the chemicals.

Over an area stretching from Pennsylvania to Maine, more than half of about 600 fisher tested were positive, Bergeron said at a public hearing on HB 1018 Feb. 11. And in New Hampshire and Vermont, that proportion was significantly higher: about 93 to 94%, he said.

“We found it essentially everywhere we looked, in fisher,” he said.

But what exactly that means isn’t yet clear. Exposure is one thing, Bergeron said; a concurrent decline in fisher and fox population totals, as seen in trapping harvest reports over the past 25 or so years, is another. Some studies are beginning to show a correlation between population decline and rodenticide exposure, but a causal link hasn’t yet been drawn between those factors.

While a high enough dose of rodenticide can kill a fisher, Bergeron said he was also interested in the possibility that lower levels — like those almost ubiquitous in the New Hampshire fishers sampled — were also causing negative health impacts, like reduced productivity or a depressed immune system. If rodenticide is making animals it doesn’t outright kill more susceptible to infection, that would also contribute to a population decline.

“One of the things we’ve noticed in some of our furbearing species, fox and fisher, was there seem to be higher levels of canine distemper virus,” Bergeron said. “There could be a correlation. … We’re hoping to learn that with this study.”

Fisher fulfill an important ecological role in New Hampshire’s forests, said Remington Moll, a professor at UNH who has researched the species, including in collaboration with the Fish and Game Department. In addition to controlling small rodents, fisher are one of the only endemic species able to kill porcupines, keeping their populations in check. They also contribute to soil health: Fisher can travel miles looking for a meal, helping disperse fungi over that distance via the spores trapped in the guts of their prey. 

“They are really an indicator species of general forest structure and health,” Moll said.

There are lots of pressures facing populations of fisher, foxes, and other furbearers. Factors beside rodenticide, such as interspecies competition, habitat fragmentation, and vehicle strikes are also likely to be exerting pressure on the populations, he said. 

But even though scientists don’t yet have the whole picture, the high rates of rodenticide exposure documented in fisher are concerning, Moll said. 

“That really speaks to the widespread presence of rodenticides on the landscape,” he said.

Renewed attention

Conversations around rodenticides have gained steam in New Hampshire in recent years. A previous attempt to ban the poisons came before the Legislature in 2023; Rodenticide Free New Hampshire formed in 2025, and has encountered a lack of public awareness on the topic, said Graeber.

Now, “it’s become more of an issue. It’s more widely used,” Foss said.

At the hearing for HB 1018, legislators wondered whether a growing mouse and rat population was contributing to the problem.

Department of Agriculture, Markets, and Food Commissioner Shawn Jasper said rat populations were growing in rural areas, and Adam Carace, CEO of Pest-End pest control services and chair of the policy committee for the New England Pest Management Association, said his experiences in the field over the past few years supported that idea. 

Since the COVID-19 pandemic, “we’re seeing rat activity in areas that we’ve never seen them before,” Carace said.

New Hampshire-specific research on mouse and rat population growth is lacking. Studies in urban areas have found, though, that climate change and increased development can bolster rat populations.

Meanwhile, rodenticide use has increased since the pandemic, with use of second-generation poisons rising about 60% from 2020 to 2022, according to a preliminary report compiled by a state task force in 2023.

Can poison be phased out?

The Feb. 11 hearings featured debate between those in favor of an outright ban on second-generation rodenticides, as is proposed in HB 1018, and those who want them used only by licensed professionals, as is proposed in HB 1676.

Rodenticide Free New Hampshire has advocated for the former.

“It doesn’t matter if you’re a professional because of the way the [poison] works,” said Graeber. “When we put it all in [pest control companies’] hands, the problem is just going to get worse.”

Graeber called for integrated pest management, an adaptive method of pest control that involves using multiple approaches and saving poison for a “last resort.” She didn’t believe, she said, that pest control companies were really following those steps.

Carace said they were, and advocated for licensed professionals to continue being able to use the products. Untrained consumers sometimes misuse the poisons, placing them outside and away from structures or overdoing dosages, he said. But he added that rodenticides have sometimes been the only way his team has been able to gain control of a rodent infestation after other means failed.

In a survey of 10 pest management companies included in the Rodenticide Review Task Force’s preliminary report, however, second generation rodenticides were the only pest control method identified by all companies as part of their base-rate package.

Alternative means for rodent control are developing, according to Foss, who plans to order a set of electric fence-style ground plates designed to be laid across doorways to keep rodents out of buildings. Also on the market are contraceptives, or rodent birth control, showing promising results — but the tried and true method of exclusion, such as by plugging foundation holes with steel wool, remains a great option, she said.

To Graeber, the boom in mice and rats is evidence that poisons are ineffective and should be abandoned. 

“The pest problems get worse and worse. Clearly, these poisons are not a long-term solution… Increasing our biodiversity is,” she said, noting that the poisons are also killing the natural predators of mice and rats who would otherwise help keep them in check. Rodenticide Free New Hampshire has paid for a roadside billboard in Hooksett to protest poisons and raise awareness of their effects.

As Foss noted, rodenticide is sold to consumers in 10-pound buckets; it will take a long while to see its use wind down, even if a ban is passed. For now, scientists said they will continue to monitor wildlife for exposure and effects — and Colby will continue sending dead raptors’ tissue samples off to the lab.