Increasing numbers of fishers are being exposed to and dying from rodenticides used on illegal marijuana grow sites in California, a new study warns.
Fishers are mid-sized weasels that live in isolated forests in California, Oregon, and Washington. The West Coast fisher population was proposed for federal listing as a threatened species in late 2014. In 2015, the southern Sierra Nevada population was listed as threatened under the California Endangered Species Act.
Fishers most affected live in the Sierra Nevada range near Fresno and in forests stretching from Humboldt County to Redding in Shasta County, says Mourad Gabriel, who began his research as a doctoral student with the Veterinary Genetics Laboratory at the University of California, Davis.
Gabriel’s studies in 2012 and 2013 were the first to link rat poison and illegal marijuana farms to fisher deaths.
In the 2012 study, Gabriel recorded four fisher deaths in five years due to rat poisoning from illegal marijuana grows.
The new study, published in PLOS ONE, examined the deaths of 167 fishers, with 129 of them suitable for necropsies. The study found that, between 2012 and 2014:
- Eight more poisoning deaths occurred, a 233 percent increase in the annual rate of poisoning deaths over the 2012 study.
- 69 percent of all poisoning cases were in the spring, when fishers mate and raise their kits.
- Exposure rates to rodenticides rose from 79 percent to 85 percent.
- There were as many as six different rodenticides in one animal.
The full study period, from 2007 to 2014, showed that predation caused the majority (70 percent) of fisher deaths, but rat poisoning connected to marijuana grow sites accounted for 10 percent of fisher deaths.
An additional fisher from 2008 was reclassified as a poisoning death, bringing the total number of poisoning deaths between 2007 and 2014 to 13.
“We’re showing that it’s not getting better,” Gabriel says. “Fishers are the flagship species. We have to think of so many species, like Sierra Nevada red foxes, spotted owls, martens—they all are potentially at risk. This is essentially going to get worse unless we do something to rectify this threat.”
“We’re sort of a one-stop shop with a tremendous resource of people and broad array of tests that can be brought to bear about why an animal died,” says coauthor Robert Poppenga, a professor and veterinary toxicologist with the California Animal Health and Food Safety Laboratory System, where the necropsies were conducted.
“The thing that intrigued us early on was the detection of anticoagulant rodenticides in these fishers. They’re out in the middle of nowhere. Yet, based on post-mortem testing, more than 85 percent have ARs in their system.”
Anticoagulant rodenticides inhibit the ability of fishers and other mammals to recycle vitamin K. This creates a series of clotting and coagulation problems, which can lead to uncontrollable internal bleeding.
The poison, which is often enhanced with bacon, fish, and peanut butter “flavorizers,” is scattered around illegal marijuana grows in remote sites to deter pests from the crop.
Other researchers from UC Davis and from UC Berkeley and Humboldt State University are coauthors of the study.
Source: UC Davis