Pot Growers’ Use of Rat Poison Killing Rare Carnivores
DAVIS, California, July 16, 2012 (ENS) – Rat poison used on illegal marijuana farms is sickening and killing the fisher, a rare forest carnivore that inhabits some of the most remote areas of California, finds a team of researchers led by veterinary scientists at the University of California, Davis.
Researchers discovered commercial rodenticide in dead fishers in two widely separated locations – Humboldt County near Redwood National Park on the California coast and also in fishers more than hundreds of miles away in central California’s Sierra Nevada mountains in and around Yosemite National Park.
Distribution of the poisoned fishers indicated widespread contamination of fisher range in California, the researchers concluded.
The study, published July 13 in the journal “PLoS ONE,” says illegal marijuana farms are a likely source of the rodenticides. Some marijuana growers apply the poisons to deter a wide range of animals from encroaching on their crops.
The study also involved researchers from UC Berkeley, the U.S. Forest Service, the California Department of Fish and Game, the nonprofits Integral Ecology Research Center and Wildlife Conservation Society, and Hoopa Tribal Forestry.
Fisher captured for examination (Photo by Michael Schwartz, U.S. Forest Service)
“Our findings were very surprising since non-target poisoning from these chemicals is typically seen in wildlife in urban or agricultural settings,” said lead author Mourad Gabriel, a UC Davis Veterinary Genetics Laboratory researcher and president of the Integral Ecology Research Center.
“In California, fishers inhabit mature forests within the national forest, national parks, private industrial and tribal community lands nowhere near urban or agricultural areas,” explained Gabriel.
The anticoagulant rodenticides harm fishers by compromising their blood clotting and recovery abilities and decreasing their resilience to environmental stressors.
Fishers likely become exposed to the rat poison when eating animals that have ingested it, the scientists say. The fishers also may consume rodenticides directly, drawn by the bacon, cheese and peanut butter “flavorizers” that manufacturers add to the poisons.
A member of the weasel family, the fisher, Martes pennanti, has been declared a candidate species for listing under the federal Endangered Species Act in California, Oregon and Washington.
Other carnivorous species, including martens, spotted owls, and Sierra Nevada red foxes, also may be at risk from the poison.
“If fishers are at risk, these other species are most likely at risk because they share the same prey and the same habitat,” said Gabriel. “Our next steps are to examine whether toxicants used at illegal marijuana grow sites on public lands are also indirectly impacting fisher populations and other forest carnivores through prey depletion.”
Fisher in a California forest (Photo by Teresa Benson, U.S. Forest Service)
The researchers analyzed 58 fisher carcasses and discovered that 79 percent of them had been exposed to anticoagulant rodenticides. Brodifacoum, a second-generation rodenticide, was found in 96 percent of the exposed fishers.
Second-generation rodenticides are more toxic because they can be lethal after a single ingestion. It can take up to seven days before clinical signs appear, so the poisoned animal can be a risk to predators for days before it dies.
“I am really shocked by the number of fishers that have been exposed to significant levels of multiple second-generation anticoagulant rodenticides,” said pathologist Leslie Woods of the UC Davis California Animal Health and Food Safety Laboratory System, which conducted the necropsies.
Anticoagulant rodenticides inhibit the ability of fishers and other mammals to recycle vitamin K. This creates a series of clotting and coagulation problems, which may lead to uncontrollable bleeding.
Exposure to the poison was high throughout the fisher populations studied, complicating efforts to pinpoint direct sources. The fishers, many of which had been radio-tracked throughout their lives, did not wander into urban or agricultural environments. But their habitat did overlap with illegal marijuana farms.
The study cites multiple examples of confiscation of marijuana plants and discovery of associated rodenticide use in the region. In 2008 alone, the study notes, more than 3.6 million marijuana plants were removed from federal and state public lands in California, including state and national parks.
The researchers describe a recent example in which more than 2,000 marijuana plants were removed by law enforcement officials less than 7.5 miles from one of the study areas. Large amounts of rodenticide were observed around the marijuana plants and along plastic irrigation lines.
California National Guard soldiers are lowered into a marijuana farm on public land, 2010 (Photo by Staff Sgt. Emily Suhr, California National Guard)
The fisher deaths occurred between mid-April to mid-May, the optimal time for planting young marijuana plants outdoors and the time when seedlings are especially vulnerable to pests.
This is also when fishers are breeding and raising their young.
Co-author Wildlife Conservation Society scientist Sean Matthews points out that fisher kits are dependent on mother’s milk until 10 weeks of age, and dependent young fishers are abandoned when their parents are killed by rodenticides.
Kits also may die from drinking poisoned mothers’ milk. The study is the first to document a neonatal milk transfer of rodenticides in fishers as a dead six-week old kit tested positive for rodenticide.
Matthews said, “Fishers play a vital role in the forests of the Sierra Nevada Mountains and the Pacific Northwest. With a body the size of a house cat and the disposition of their larger cousin, the wolverine, fishers keep forest rodent populations in check and are one of the only predators with the tenacity to regularly prey on porcupines.”
“The findings in this paper could signal a looming conservation threat for other species as well as fishers,” Matthews said. “Depletion of rodent prey populations upon which fishers and other animals feed, along with the anticoagulant poisoning threat might affect the Sierra Nevada red fox, wolverine, California spotted owls and other rare carnivores that inhabit the region.”
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