by Joe Eaton
(Full article from RATS Tales December 2022)
For years, there have been only two ways to confirm that a mammal or bird has been exposed to an anticoagulant rodenticide (AR): examining a blood sample from a living animal or liver tissue from a dead one. A team led by University of Washington graduate student Kayla Shively has just field-tested a third method in Washington state. Wildlife biologists have used stable carbon, nitrogen, and hydrogen isotopes in hair to reveal the movements and diet of wolverines, mountain lions, bats, even elephants—and hair samples now offer a new tool for detecting rodenticides.
In a pilot project funded in part by RATS through a grant from The Summerlee Foundation, Shively and her colleagues analyzed fur from fishers that had been released in the Cascade Range in a reintroduction program. Their results, presented at a recent Wildlife Society conference, suggest that fur may provide a reliable, sensitive, and noninvasive way of detecting ARs. The scientists also employed mind-boggling genomic tools to determine what the fishers had been eating, with results that appear to have yielded more questions than answers about exposure sources and pathways.
The fisher is a midsized (males weigh up to thirteen pounds, females up to six) member of the weasel family, closely related to the eight species of martens. Found only in Canada and the northern US, it’s reputed to be the only predator that can kill an American porcupine. Fishers were eliminated from much of their former range by trapping, predator control programs, and habitat loss. In California, biologist Mourad Gabriel and other researchers have shown that fishers are susceptible to ARs, with illegal cannabis grows a significant source of exposure.
The National Park Service, the Washington Department of Fish and Wildlife, and the regional nonprofit Conservation Northwest have partnered to bring fishers back to Washington, where they had been effectively extirpated by the 1950s. Young animals from Canada were translocated to the Olympic Peninsula in 2008, the North Cascades near North Cascades National Park in 2015, and the South Cascades near Mount Rainier National Park in 2018.
Despite the reintroductions, the species is still listed as endangered in the state. Because of disappointing survival rates for relocated animals, researchers decided to look at AR poisoning as a potential risk.
The project focused on the North and South Cascades. Each released fisher was radiotagged; if a signal stopped moving, whatever was left of the animal was retrieved for analysis. Sometimes only the transmitter remained. However, 24 fur samples and 18 liver samples were collected and screened for ARs. For the diet study, fisher scat was obtained with the help of specially trained dogs in the North Cascades and at known fisher denning sites in the South Cascades. Genetic analysis confirmed that all the droppings were produced by fishers. Previous diet studies had relied on stomach contents of dead fishers or visual identification of prey from bones and other hard parts in their scat. For this project, a technique called metabarcoding allowed the researchers to identify the exact prey species the fishers had consumed, information that might clarify exposure pathways.
Shively says a combination of fur and liver samples were screened for both first-generation ARs (FGARs) and second generation ARs (SGARs). The fur test paid off. “In some specimens, ARs were detected in fur but not liver, because of time differences in when the AR was incorporated into fur,” she explains. While ARs have a short life in blood and persist longer in liver, they remain in hair until the hair is shed. The test was able to detect concentrations as low as a tenth of a part per billion. Overall, concentrations were low. However, one individual, found near a ski resort, was positive for five different ARs and had suffered internal hemorrhaging.
ARs were found in six of ten fishers in the North Cascades and six of fourteen in the South Cascades, a lower exposure rate than the 85 percent Gabriel reported from California. The SGARs brodifacoum and bromadiolone were the most prevalent, with lower incidences of the SGAR difethialone and the FGARs diphacinone, warfarin, and chlorophacinone, and an average exposure to 1.77 different compounds. The scientists had expected to see the FGAR chlorophacinone, historically used by the timber industry in Washington State to kill mountain beavers, primitive rodents that posed a threat to young trees. Although fishers are known to prey on mountain beavers, chlorophacinone was present in only two samples. (Shively says chlorophacinone use has declined in the state.) Exposure wasn’t associated with proximity to developed areas.
What in the fishers’ diet might account for these exposure patterns? The menus in the two regions were strikingly different.
North Cascade fishers had been eating a lot of small birds, present in 80 percent of the samples. Large bird components had been reported in earlier diet studies, including one in the southern Sierra Nevada where snowshoe hares and porcupines were absent. Snowshoe and other hares and rabbits were important in Washington, more so in the South Cascades than the North Cascades. Mountain beavers were the second most common prey in the South Cascades, a minor element in the North. Porcupines composed a small portion of the diet in both regions, trailing squirrels, shrews, voles, skunks, and beavers; some larger species may have been scavenged. Exotic rats and mice, the usual rodenticide target species, weren’t identified in any of the samples.
The different species compositions in diet might have something to do with the fishers’ birthplaces: Alberta for the North Cascades, British Columbia for the South Cascades. The Alberta source was closer to agricultural areas; the Alberta fishers, although translocated as juveniles, might have already developed a different set of prey preferences.
According to Shively, the areas where the samples were collected represent a mosaic of land uses: national park, national forest, land owned by timber companies and a public utility, ski resorts, private residences in the wildland/urban interface. “It’s a big patchwork,” she says, “tricky to interpret.” One potential exposure pathway would be “home use of ARs, at cabins and households.” Cannabis grows in Washington State aren’t as numerous as in California, and the researchers discount them as an exposure source.
Fishers don’t necessarily shun human dwellings; a female on the Olympic Peninsula reportedly subsisted on cat food and suet from bird feeders. And they’re highly mobile. “These were recently released fishers,” Shively notes. “We don’t know how long it takes them to establish territories. They could be making big movements, covering large parts of the landscape.” For most of the year male fishers typically have a 15-square-kilometer home range, versus 10 for females; in the denning season, female ranges shrink while male ranges expand.
While fishers could be visiting human habitations, exposed prey could also be bringing ARs into wilderness areas.
Shively says there’s not much data on ARs in birds in the Cascades ecosystem. Recent studies on Eurasian sparrowhawks in Great Britain and songbirds in Germany suggest the possibility that songbirds in the Cascades could have acquired ARs from bait boxes (or secondarily from bait-eating invertebrates) before being predated by fishers.
Shively, whose interest in fishers was sparked by sightings during field work with wolverines and foxes, says she and her team hope to analyze the scat samples themselves for AR residues: “This could help us better characterize the landscape and landowner factors contributing to risk.” In the meantime, hair analysis opens new possibilities for noninvasive monitoring of large elusive carnivores like mountain lions and bobcats, both impacted by ARs in Southern California and likely elsewhere.
And if it works for fur—why not feathers?
