The fisher, a threatened mammal species related to weasels, has lived in the Sierra National Forest for far longer than there has been human habitation in North America. Recent research, however, shows that a very human vice may be threatening the fisher population in Northern California.
In 2009, researchers report, a male fisher was necropsied after being found dead in the national forest. Toxological screening showed that the cause of death for the fisher was acute poisoning from a commercial rodenticide.
When scientists realized that the fisher had been poisoned, they started looking back at other fishers that had been recovered in the park in previous years. Rodenticide was present in 85 percent of the total fishers.
However, the scientists were initially unsure of what caused the rodenticide to be present in the first place. Typically, the only places that rodenticides are used are agricultural and urban environments. Because the Sierra National Forest is generally considered to be undeveloped land, risks to fishers from rodenticides were thought to be low.
According to scientists writing in the journal Conservation Letters, the culprit was illegal marijuana farming. While California has broadened its medical marijuana laws to allow some dispensary activity, federal raids on marijuana dispensaries and farms have resulted in illegal cultivation continuing, sometimes on federal lands like the Sierra National Park.
In order to keep rats and other animals from eating the marijuana crop, growers use large quantities of anticoagulant rodent poisons. Many of the toxins developed to create these rodenticides were originally designed as nerve poisons during World War II. Today, however, these toxins find their way into the soil and water around the cultivation sites, poisoning other animals. While the fisher is a carnivore, its prey is contaminated with the poisons.
When the fisher eats prey that contains the anticoagulant toxins, scientists say they will not necessarily die right away. Often, they will be weakened or made ill by small amounts of the poison, leaving them more vulnerable to their natural predators. Most of the animals studied that tested positive for toxins had been killed by predators, rather than dying directly of the toxin's effects.
U.S. Forest Service wildlife ecologists said that illicit growing operations may be a factor in animal deaths that is often overlooked, because previously weakened animals are likely to be considered dead from predation if they display wounds from predators. The study shows that it may also be necessary to do toxicology screenings on these animals to make sure that they were not harmed by the effects of nearby poisons.