Use of rat poison on illegal pot farms threatens rare carnivore

Jonathan Marker | Science Recorder | June 30, 2013

Use of rat poison on illegal pot farms threatens rare carnivore

A fisher is a rare carnivore in the southern Sierra Nevada.

According to a recent study conducted by a team of scientists from the U.S. Forest Service’s Pacific Southwest Research Station, UC Davis, UC Berkeley, and the Integral Ecology Research Center, the use of rat poison on illegal marijuana grow sites is killing fishers, a rare carnivore in the southern Sierra Nevada.

A total of 58 fisher carcasses were analyzed in the study, 79 percent of which yielded traces of rat poison in the tissues.  The full results of the study, “Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore,” can be found at the PLOS ONE web site.

In a 2012 study published by the same team of scientists, they found that a variety of deadly rat poisons presented in the tissues of fishers. At the time, they speculated that the use of rat poisons as pesticides for illegal pot farms throughout the Sierra Nevada was the most likely cause.  The newest study confirms this hypothesis, as the scientists found that female fishers who lived near numerous pot farms had lower survival rates because of their exposure to rat poison.  The fisher is a candidate for inclusion on Federal, California, and Oregon endangered species acts, and is described as a “sensitive species” by the U.S. Forest Service.

The scientists concluded that the illegal pot farms are to blame for the fishers’ problem with rat poison. The scientists used radio tracking tags and equipment to observe their movements over the course of the study.  The fishers were not observed to have traversed agricultural, rural, or urban areas; if they had, legal use of rat poisons could have sufficed as an explanation for the problem.  Though it is not uncommon for wildlife to be exposed to various pest control poisons in agricultural areas, the extent of the use of rat poison and other pest control poisons at the illegal pot farms is most concerning to the scientists.

“In marijuana cultivation sites, regulations regarding proper use of pesticides are completely ignored and multiple compounds are used to target any and all threats to the crop, including compounds illegal in the U.S.,” said Dr. Kathryn Purcell, a co-author of the study and PSW wildlife biologist.

Because the fishers’ diet consists of small, birds and mammals, fungi, carrion (rotting animal carcasses), insects, and some plants, rat poison ingestion is not always direct.  During the study, scientists reported this force multiplication effect of the rat poison on the natural life cycles of the fishers.


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