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Pot–Grower’s Poison Taking Toll on Rare Fishers

A new study by the Wildlife Conservation Society (WCS), the Integral Ecology Research Center, the University of California Davis and other partners shows that imperiled fisher populations are being poisoned by the use of anticoagulant rodenticides (AR) on public and community forest lands in California–likely those used illegally by marijuana growers.

The researchers conducted necropsies among 58 fishers and found that 46 of the animals (79 percent) were exposed to one or more of the toxic ARs, and four had died directly from AR toxicity.

The fishers were from two different populations: one occurring on tribal, private, and public lands in northwestern California and another within the Sierra National Forest in central California. Distribution of the poisoned fishers indicated widespread contamination of fisher range in California.

Fishers are likely exposed to AR when eating animals that have already ingested the rodentcide. They may also be drawn to the poison directly by bacon, cheese and peanut butter “flavorizers” that manufacturers add to attract rodents. According to the authors, it is unlikely the fishers were exposed to AR used legally at or near agricultural or residential areas as these settings are not suitable habitat. Nor did animals tracked by telemetry collars during the study venture into those environments.

Instead, the exposure points were likely encountered where AR is used illicitly as part of illegal marijuana cultivation in remote areas that overlap with fisher habitat. The study cites multiple examples of confiscation of marijuana plants and discovery of associated AR use in the region and notes that in 2008 alone, more than 3.6 million marijuana plants were removed from federal and state public lands in California, including state and national parks.

Members of the weasel family, fishers were once widely distributed throughout North America’s west coast but have incurred significant population decline and extirpation from portions of their former range. Populations inhabiting Washington, Oregon, and California have been designated a Distinct Population Segment (DPS) and declared a candidate for listing under the federal Endangered Species Act (ESA).

Study co-author and WCS Scientist Sean 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. As fisher populations declined, they took refuge in the last remaining portions of mature forest in the Sierra Nevada and coastal mountains. Now a new threat has emerged in these remaining refuges.”

AR may also harm fishers by compromising the animal’s blood clotting and recovery abilities, decreasing its resilience to environmental stressors, and abandonment of dependent young due to direct mortality of adults killed by AR. During the study, the first documentation of a neonatal milk transfer of AR in fishers was recorded as a deceased six-week old kit was tested and found to have AR in its system. (Kits are dependent on mother’s milk until ten weeks of age.)

Matthews said, “The findings in this paper could signal a looming conservation threat for other species as well as fishers. As we discuss in the study, 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.”

In their conclusion, the authors consider heightened awareness in removing AR when marijuana grow sites are dismantled and further regulation restricting the use of AR to only pest management professionals.

The study, Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore, appears in the July 13, 2012 edition of the online journal PLoS ONE.

Co-authors of the study include: Mourad W. Gabriel of the Integral Ecology Research Center and the Veterinary Genetics Laboratory at the University of California; Sean M. Matthews of the Wildlife Conservation Society; Davis (UC-Davis); Greta M. Wengert of Integral Ecology Research Center; Benjamin N. Sacks of Veterinary Genetics Laboratory at UC-Davis; Leslie W. Woods and Robert Poppenga of the California Animal Health and Food Safety Laboratory System at UC-Davis; Rick A. Sweitzer and Reginald H. Barrett of the Sierra Nevada Adaptive Management Project at the University of California at Berkeley (UC-Berkeley); Craig Thompson and Kathryn Purcell of the Pacific Southwest Research Station—Sierra Nevada Research Center, United States Forest Service; J. Mark Higley of the Wildlife Department, Hoopa Tribal Forestry; Stefan M. Keller of the Department of Pathology, Microbiology and Immunology at UC-Davis; and Deana L. Clifford of the Wildlife Investigations Laboratory of the California Department of Fish and Game.

Scott Smith | Newswise Science News
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