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Davis, California

Monday, March 4, 2024

Toxicants on marijuana farms threaten weasel communities


Researchers discuss wildlife protection and potential threat to humans

According to a recent UC Davis study, the fisher species has dropped dramatically in the last three years, resulting in their new classification as an endangered species in California. This decline in the fisher community has drawn attention toward possible threats that humans face as well.

Fishers, or mid-sized weasels, can be found in two areas of California: the southern Sierra Nevada and in northern California. Recently, there has been a cultivation of numerous illegal marijuana grow sites close to the natural habitats of the fishers, which are subsequently wiping out the population due to use of rodenticides.  

The head of the study, Mourad Gabriel, a former UC Davis doctoral student and current director of the nonprofit organization Integral Ecology Research Center (IERC), has been studying fishers since 2005. Gabriel began researching the impact of marijuana farms on fishers in 2010. In the first study, his team found four fishers in California that died from rat poison and identified that 79 percent of the fisher population tested had been exposed to “anticoagulant rodenticides.”

“[Anticoagulant rodenticides are] a type of rodenticide which inhibit the clotting mechanisms that are necessary for normal body processes or when an animal has an injury,” Gabriel said. “It targets those specific mechanisms, inhibits the ability to clot, and therefore the animals could potentially bleed out to death.”

Because fishers typically live in isolated forests away from communities and humans, the study required several steps that started with collecting the dead fishers and determining the cause of death. Gabriel’s team included researchers from all different backgrounds, including wildlife biology, veterinary pathology, conservation ecology and toxicology.

Dr. Leslie Woods, a professor in the school of veterinary medicine and a pathologist in the veterinary diagnostic laboratory system, has been responsible for undertaking all the fisher necropsies (the animal equivalent of an autopsy) since 2010.

“I look for changes in the tissues that indicate disease or cause of death and take portions of organs for testing,” Woods said. “The important finding of exposure to anticoagulant rodenticide stemmed from necropsy and finding bleeding into the body cavities, but with no evidence of trauma. When we see this, anticoagulant rodenticide is a suspect cause of death and we send the tissue to our toxicology lab to test it.”

When the tissue is sent to the lab, toxicologist Dr. Robert Poppenga, and his team undertake the procedure of isolating the liquid from the tissue that they want to test, and use mass spectrometers to detect any existing rodenticides.

“It’s very important in this work to make sure if you think you see something, it’s truly what you’re seeing,” Poppenga said. “There’s a lot of quality control that goes into the analysis to make sure what we’re recording is valid.”

Using this procedure, Gabriel and his team published another paper in 2013, which solidified the fact that the more marijuana cultivation there was in the habitats, the likelier it was that the fishers would be exposed. They also discovered a baby fisher that was exposed while it was completely dependent on its mother’s milk. According to Gabriel, this means the fisher was exposed either by its mother’s milk or while it was in utero. This finding shows that fishers can become contaminated from day one of their lives.

In their most recent study published this year, the team lengthened their study to three years to see how the exposure of poisoning would change the results.

“We found that exposure increased from 79 to 85 percent of fishers,” Gabriel said. “We also had eight additional poisoning cases in just three years.”

Over the three-year span, the annual rate of death for fishers from poisonings jumped from 5.6 percent to 18.7 percent, a 233 percent increase.

“This work has exposed a very serious problem in California of contamination of our environment and the danger for our wildlife populations and humans that stray onto these sites,” Woods said.

While the fishers live far enough away from metropolitan areas that the rodenticides does not directly affect humans, Gabriel notes that there are several ways they could still infiltrate human systems. People who drink downstream water in rural committees could easily become affected if the water is contaminated. The toxicants could also enter humans via the species they consume; the research has found that animals such as deer, bears and birds have also been exposed to the rodenticides, which poses a risk to the people that eat them.  

Through their publications, the researchers are determined to bring this rising issue to the forefront of public awareness and provide the best sound science available to help managers make decisions regarding fishers.

“One goal is to continue to monitor for exposure to toxicants in the environment and hopefully, monitor effects of eventual cleanup of these sites on public California land,” Woods said.

According to Woods, the difficult part about cleanup is that there are many grow sites with hundreds of pounds of garbage, toxicants and fertilizer, and limited funding to properly clean them out, along with the risk of exposure. But with such staggering new data, the team knows that the rodenticides could affect other wildlife, and hope their research will jumpstart management decisions to act on conserving the affected species.         

“The fisher has essentially become the canary in a coal mine that is now showing we have an issue,” Gabriel said. “Our end goal is to provide data to make scientifically sound decisions, but also to conserve species that are of conservation concern and stabilize them.”

Written by: Lisa Wong features@theaggie.org


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