Fishers, relatively rare cat-sized relatives of the weasel, are found in limited regions of North America and are increasingly falling victim to poisons used by illegal marijuana production operations in California.
Illegal marijuana farms in the forests of California are endangering fishers, relatively rare carnivorous mammals that are increasingly being poisoned by the toxicants known to be used in illegal grow operations, according to the United States Department of Agriculture’s Forest Service.
According to a recent study published in the journal PLOS One, researchers found that the annual rate of poisoning deaths of fishers (Pekania pennant) rose 233 percent compared to a study in 2012. The toxicants, including rat poison and other rodenticides, were discovered to be associated with illegal marijuana farms on public and tribal lands specifically in Northern and Southern California.
Previous studies showed that rat poisons were found in the tissues of this cat-sized relative of the weasel family when they lived in proximity to illegal marijuana cultivation sites in rugged portions of Northern California and the southern Sierra Nevada. The new study examined the deaths of 167 fishers and revealed the degree to which the creatures are being harmed by the illegal crops.
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Researchers found that Northern California fishers were five times more likely to die from poisoning than predation compared to fishers in the southern Sierra Nevada.
85 percent of fishers showed exposure to rat poison, and the animals were, on average, exposed to multiple types of poison accounting for 10 percent of all documented mortalities.
"We know that a 10 percent change in mortality rate is enough to determine whether fishers in California are able to expand their population size or not," said Dr. Craig Thompson, a Pacific Southwest research wildlife ecologist and one of the study's authors. "Now we know that rodenticide poisoning alone is enough to keep fisher populations suppressed in the state, even without accounting for the fact that low doses of these poisons also cause the animals to be lethargic and susceptible to disease, which in turn increases the potential for other sources of mortality."
A team of researchers from the U.S. Forest Service's Pacific Southwest Research Station, Integral Ecology Research Center, University of California - Davis, University of California - Berkeley, and partner organizations conducted the study from 2007 to 2014 at three research sites — one in Northern California and two in the southern Sierra Nevada.
"This study further solidifies the need for continuing to remediate and remove these threats to fishers and other species of conservation concern within our public lands," said Dr. Mourad Gabriel, executive director of the Integral Ecology Research Center and the study's lead author. "I hope the next steps can focus on rectifying the harmful effects of this clandestine activity so that they do not stem the years of conservation efforts of stabilizing the California fisher populations."
The U.S. Fish and Wildlife Service recently proposed listing fishers in California, Washington, and Oregon as threatened under the U.S. Endangered Species Act, and the California Department of Fish and Wildlife has already listed fishers in southern Sierra Nevada as threatened under the California Endangered Species Act.
Based on materials provided by the USDA Forest Service - Pacific Southwest Research Station
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