Fisher Deaths Skyrocketing From Poisonous Pesticide Used On Illegal Pot Farms, Researchers Say

Illegal marijuana farms in forested areas of California are threatening the survival of a rare weasel-like species known as fishers (Pekania pennant), revealed a recent study by the U.S. Depart of Agriculture (USDA) Forest Service. Forest-dwelling fishers are increasingly vulnerable to toxins associated with marijuana farms; poison-related deaths have increased by 233 percent since 2012.

"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," Dr. Craig Thompson, study co-author from the USDA Forest Service's Pacific Southwest Research Station, explained in a news release. "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."

Fishers are cat-sized mammals characterized by their long slim body and short legs. The native North Americans are efficient hunters and prefer to feed on snowshoe hares, rabbits, rodents and birds. While the animals are typically known to be carnivorous, they also nibble on insects, nuts and berries.

Previous studies have detected rat poisons in the tissues of some fishers living in close proximity of an illegal pot farm in areas of Northern California and southern Sierra Nevada. Male fishers are more susceptible to poisoning than females because males tend to roam more during spring breeding season, when pot growers use rodenticides more frequently.

For their recent study, researchers examined the deaths of 167 fishers that occurred between 2007 and 2014 and found that Northern California fishers are five times more likely to die from poisoning than predation in comparison to their southern Sierra Nevada counterparts. Moreover, 85 percent of fishers had been exposed to multiple types of poison, especially rat poison. In total, poisoning accounted for 10 percent of all documented fisher deaths. In two cases, traces of unidentifiable rodenticides were the cause of death.

"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," Dr. Mourad Gabriel, executive director of the Integral Ecology Research Center and the study's lead author, said in a statement. "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."

When tracking the animals, researchers found that fishers are also threatened by predation from bobcats, mountain lions and coyotes; natural disease, vehicle accidents and other human influences.

Following the study, U.S. Fish and Wildlife Service recently proposed listing fishers in California, Washington and Oregon as threatened under the U.S. Endangered Species Act. On a more local scale, Sierra Nevada fishers have already been listed as threatened under the California Endangered Species Act.

The recent study was published in the journal PLOS ONE.

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