Impacts of Rodenticide and Insecticide Toxicants from Marijuana Cultivation Sites on Fisher Survival Rates in the Sierra National Forest, California

Abstract

Secondary exposure of wildlife to pesticides has been well documented, yet exposure is typically associated with agricultural or wildland-urban interface areas. Wildlife in undeveloped areas is generally presumed free from risk. In 2009, a male fisher was found dead in the Sierra National Forest and subsequent necropsy revealed that the animal died of acute rodenticide poisoning. Follow-up testing revealed that 85% of fisher carcasses recovered by two research projects in the previous three years tested positive for rodenticides. Concern arose that exposure could predispose an animal to mortality from other causes, and that the underlying role of toxicants would escape notice. Further investigation indicated that the most likely source was the numerous illegal marijuana cultivation sites currently found on public lands throughout the western United States. To determine whether the presence of cultivation sites predisposed fishers to mortality from other sources, we related survival rates to the presence and number of cultivation sites found within that animal's home range over the past 10 years. Likelihood of exposure was related to the presence of cultivation sites, and female fisher survival was influenced by the number of cultivation sites within its home range. We discuss the conservation implications of this unexpected relationship.

Exposure to rodenticides in Northern Spotted and Barred Owls on remote forest lands in northwestern California: evidence of food web contamination

“…Our data confirm previously reported exposures in remote forest wildlife where both Northern California and Southern Sierra Nevada populations of fishers have been exposed to ARs at high prevalence (85% of all fishers tested; Gabriel et al 2012, Thompson et al 2014). For fishers, AR exposure occurs through the thousands of illegal clandestine marijuana cultivation sites throughout their range on public and tribal lands (Thompson et al 2014, Gabriel et al 2015.…”

“…For fishers, AR exposure occurs through the thousands of illegal clandestine marijuana cultivation sites throughout their range on public and tribal lands (Thompson et al 2014, Gabriel et al 2015. The use of not only the ARs brodifacoum or bromadiolone, but other first and second generation ARs, in addition to neurotoxicant rodenticides like bromethalin, have been documented in large quantities (10-90 lbs.…”

“…However, emerging use of AR in remote forest settings has recently been documented in the western United States (Gabriel et al 2012(Gabriel et al , 2015. The widespread use of AR at illegal clandestine marijuana cultivation sites on public and tribal lands has been a significant source of wildlife exposures in these California habitats (Gabriel et al 2012, Thompson et al 2014. Concurrently, accumulating data have demonstrated deleterious impacts from ARs for a rare forestdwelling carnivore in California, the fisher (Pekania pennanti), as indicated by exposure rates, population survival trends, and direct cause of mortality (Gabriel et al 2012, Thompson et al 2014.…”

“…The widespread use of AR at illegal clandestine marijuana cultivation sites on public and tribal lands has been a significant source of wildlife exposures in these California habitats (Gabriel et al 2012, Thompson et al 2014. Concurrently, accumulating data have demonstrated deleterious impacts from ARs for a rare forestdwelling carnivore in California, the fisher (Pekania pennanti), as indicated by exposure rates, population survival trends, and direct cause of mortality (Gabriel et al 2012, Thompson et al 2014. In California and other western states, fishers inhabit remote forests and require deteriorating mature and old-growth forested habitat structures for offspring rearing.…”

Mortality risks and limits to population growth of fishers

“…Known causes of death, which may be limiting numeric and spatial recovery of fisher populations, include exposure to disease and debilitating injury (Aubry and Raley , Larkin et al , Keller et al , Gabriel , Lewis ), predator attacks (Truex et al , Aubry and Raley , Wengert et al ), incidental or targeted fur trapping (Lewis and Zielinski , Koen et al , Lewis ), vehicle strikes (Krohn et al , York , Chow , Lewis , Spencer et al ), entrapment in water tanks or other human structures (Folliard , Truex et al , Davis ), and direct consumption and secondary exposure to rodenticides and insecticides (i.e., toxicants) at trespass marijuana grow sites in public wildlands (Gabriel et al , Thompson et al ). Several reports suggest that fisher survival may vary between sexes or according to season associated with period‐specific life‐history events (e.g., reproduction, breeding movements, dispersal; Powell and Leonard , Lewis , Sweitzer et al ), higher energetic costs during winter (Powell ), or timing of exposure to toxicants (Thompson et al , Gabriel et al ). Moreover, important denning and resting habitats used by fishers in western North America (i.e., mature and late‐successional mixed conifer forests; Raley et al , Weir et al , Zhao et al , Aubry et al , Schwartz et al ) appear at elevated risk from large forest fires (Thompson et al , Kane et al ).…”