8 March 2013 Science Briefs

SNAMP PUB #17: Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore

Article Title: Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore.

Authors: Gabriel, M.W., L. W Woods, R. Poppenga, R. A. Sweitzer, C. Thompson, S. M. Matthews, J. M. Higley, S. M. Keller, K. Purcell, R. H. Barrett, G. M. Wengert, B. N. Sacks, and D. L. Clifford.

Research Highlights:

  • The rare pacific fisher typically occurs in remote forest habitats where overlap with humans and homes is minimal

  • On April 15, 2009 SNAMP researchers found a dead fisher that showed massive internal bleeding caused by secondary exposure to anticoagulant rodenticides (ARs)

  • All fisher carcasses collected from throughout California that could be tested were then tested and more than three-fourths had ARs in their systems.

  • Four fisher deaths between 2009 and 2012 could be directly attributed to AR poisoning, a serious number for this rare forest carnivore.

  • The animals exposed to AR were widely scattered across remote forests and park lands, rather than concentrated near residential areas, indicating that the exposure was not due to homeowner use around dwellings

  • The likely source of AR exposure is illegal marijuana “grow sites”, where ARs and other chemicals are used to protect young plants

  • ARs are scattered around thousands of grow sites to control squirrels and field mice, thereby also reducing and contaminating the prey of fishers and other forest carnivores

  • Regulatory changes are needed to reduce AR-linked mortality of fishers and other wildlife in our forests and park lands

There is growing concern that anticoagulant rodenticides (AR) (these are substances that act on rodents by inhibiting normal blood clotting) are harming wildlife. These poisons are commonly used to control rodent pests in and around farms and homes. Anticoagulants work by inhibiting the body’s ability to process vitamin K-1, causing the blood to lose its ability to clot. Animals ingest ARs either by eating AR-laced bait or by eating animals that have eaten such bait. Numerous species of wildlife that are not the target of pest control efforts, including endangered species, have been found to have consumed ARs in these ways. It was previously thought that the greatest danger of exposure was in agricultural, urban or peri-urban settings where large quantities of AR compounds are often used. Very little is currently known about the danger to wildlife away from these areas.

Fishers (Martes pennanti) were once found throughout west coast of North America, but now have declined to the point that are rare and absent from large portions of their historic range. In Washington, Oregon and California, the fisher is now a candidate for listing under the Endangered Species Act. This study assessed the magnitude of AR exposure and poisoning using fisher carcasses collected from 2006 to 2011 as part of a collaborative effort studying threats to fishers in California. Additionally, analysis of where deaths occurred was used to look for sources or patterns of AR in the environment.

Fisher records and carcasses from three different studies were used in this research. In northern California, the Hoopa Valley Reservation Fisher Project participated. In the southern Sierra Nevada, the Sierra Nevada Adaptive Management Project and the USDA Forest Service Kings River Fisher Project provided information and carcasses. In each study area, fishers were captured in box traps and radio-collared. The collars emit a signal when an animal dies and carcasses are collected as quickly as possible. Carcasses were frozen until a complete necropsy to determine causes of death could be performed by a certified pathologist at either the California Animal Health and Food Safety Laboratory System (CAHFS) or at the University of California Davis Veterinary Medical Teaching Hospital, Davis, CA. Liver samples were screened for seven different ARs and the amount of AR in the system was measured. These anti-coagulants are commonly sold in hardware and garden stores for pest control, and include “2nd generation” types that have been recently developed to overcome the resistance some pest animals have developed to older types of rodenticides.

For each fisher, the area used by the animal during three different time periods was determined from records based on collar signals during the animal’s life. The first time period was the area used during the entire time the animal was collared until its death. The second time period was the area used for the six months prior to death, and the third was the area used during three months prior to death. The shorter, recent time periods were important to study because some ARs do not last long in the body, so if they were found, it would be possible to determine a recent source of AR exposure. Researchers looked for patterns in where the exposed animals were found, to see if exposure was found everywhere or mostly in particular areas.

Forty-six of the 58 fisher carcasses tested (79%) were exposed to one or more AR chemical. The number of AR compounds detected per animal ranged from 1 to 4. Both first and second generation ARs were detected, with brodifacoum being most common and detected in 44 of the 46 (96%) exposed fishers, followed by bromodiolone (16 of 46; 35%), diphacinone (8 of 46; 17%), chlorophacinone (four of 46; 9%), difethialone (one of 46; 2%), and wafarin (one of 46; 2%). There were many causes of death, including predation, diseases, and cars. Four deaths were determined to be directly due to internal bleeding caused by AR consumption. Two of the four fishers killed by ARs were from the southern Sierra Nevada, and two were from northern California.

It was possible to put together complete information about areas used by the fishers for 42 monitored fishers, 12 from the northwest and 30 from the southern Sierra Nevada (19 from SNAMP, 11 from KRFP). There were no clusters of fisher exposed to AR during any time period, or for specific AR compounds, type of AR, or numbers of ARs per fisher in any of the study areas. The lack of clustering indicates that fishers are encountering these poisons in remote, natural forest regions within their home ranges.


  1. Anticoagulant rodenticides are a new and previously unknown cause of death for fishers, and may be a serious threat to California’s fishers.

  2. The high rate of exposure to 2nd generation, or more recently developed, AR compounds (96% of exposed fishers) was both surprising and troubling; 2nd generation ARs are more likely to kill immedi¬ately after consumption, but also stay in the animal’s system for a longer time than the older types of ARs.

  3. Sub-lethal AR exposure may harm fishers by reducing normal blood clotting by depleting the squirrels, field mice, and other small animals that fishers prey on.

  4. Lack of spatial clusters of exposed individuals shows poisoning is widespread and irresponsible use of ARs is continuing despite recent regulatory changes. Further study is needed to identify sources of ARs in the forest, including illegal grow sites.

  5. The published article includes two tables and eleven figures providing more detailed information about ARs in California fishers.

Full Reference:
Gabriel, M.W., L. W Woods, R. Poppenga, R. A. Sweitzer, C. Thompson, S. M. Matthews, J. M. Higley, S. M. Keller, K. Purcell, R. H. Barrett, G. M. Wengert, B. N. Sacks, and D. L. Clifford. 2012. Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore. PLoS One 7:7. Article Number: e40163. (http://www.plosone.org)

Article Web Link.

For more information about the SNAMP project and the Fisher Team, please see the Fisher Team website here.


1 Ara Marderosian says...

Efforts to get the California Legislature to limit or ban the sale of anticoagulant rodenticides are on-going.

In the true spirit of adaptive management, perhaps the most effective way to stop the anticoagulant rodenticides killing of Pacific fisher (and other carnivores, which are also being poisoned), to prevent illegal marijuana “grow sites,” and to stop the illegal and unregulated use of public and private lands may be to legalize and regulate the sale and use of marijuana to generate taxes from a legal marijuana commodity.

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