Toxic rodenticide at illegal grow sites continue to poison wildlife like Pacific fisher

In a previous newsletter, CSERC staff described the extremely toxic poisons that are repeatedly found at marijuana plantations on forest lands throughout the Sierra Nevada region. Illegal marijuana grow sites divert precious water from forest streams, produce huge amounts of discarded trash, and routinely result in poisonous rodenticides placed widely around the pot plantations to reduce damage to the crop.

Earlier rodenticide products often killed rodents only if enough of the bait was consumed in multiple feedings. The latest “second generation” rodenticides are so highly poisonous that not only are rodents, deer, and other animals killed immediately when they consume the baits. Vultures, foxes, coyotes, fishers, bears, and other carrion-eating species can also die after feeding on the animal that first consumed the poison. The secondary poisoning shows how incredibly toxic the second-generation rodenticides can be.

Federal and state wildlife scientists who study the rare Pacific fisher have tested fishers killed this past year by vehicles or predators, as well as those found without any obvious cause of death. According to research results discussed at a recent Southern Sierra fisher workshop session that CSERC staff attended, all 22 fishers tested this year showed some level of contamination by rodenticides. That means that even for animals that don’t ingest enough to die immediately, the poison can still cause them to stagger, become disoriented, or become so weak that they may blunder out into traffic on forest roads or easily fall victim to lurking predators.

Tragically, with literally hundreds of illegal grow sites in the Sierra Nevada that have been identified from the air or through reports, there is insufficient capacity for law enforcement to visit each discovered site. At many sites, the illicit growers leave behind garbage, chemicals, and rodenticides. Limited agency staff and resources means that many low priority sites never get cleaned up, so poisons continue to pose risk long after a site is abandoned. For the Pacific fisher, which is already rare across the region, the threat posed by rodenticides adds another hurdle to recovery. It is unclear whether the new state legalization of marijuana use may gradually lead to a phasing out of illegal grow sites – something that would greatly benefit wildlife.

Dr. Mourad Gabriel (shown at right at a grow site in 2015) of the Integral Ecology Research Center is one of the leading experts about rodenticide risk to wildlife and the threats posed by grow sites. His efforts to publicize the problem have been instrumental in making agencies and politicians aware of the need for more effective enforcement and clean up of toxic-laden sites, especially on public forest lands.

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