Tribal Researchers Help Write Book on Fishers

Hoopa Tribal Forestry crews use traps to capture fishers, and then they do measurements and take blood for testing before releasing them. Some of the fishers are given radio collars that can be tracked. / Photo courtesy of Hoopa Tribal Forestry

By KRISTAN KORNS, Two Rivers Tribune

Hoopa Tribal Forestry researchers spent years studying fishers, and even helped write the book on fisher conservation as part of an international effort to understand the small creatures and their cousins, martens and sables.

Wildlife Biologist Mark Higley said, “What we do is track fishers, and we’re using radio telemetry to follow them.”

Higley pointed to a map of the forests around Hoopa marked with pins. “This board shows the last known locations of fishers with radio collars.”

Fishers are carnivores that weigh between four and 13 pounds. They feed mostly on small animals like squirrels and wood rats.

Higley wrote a chapter in the latest book on fishers: Ecology and Conservation of Martens, Sables, and Fishers: A New Synthesis.

Higley and several others at Hoopa Tribal Forestry have spent years tracking and studying the animals.

Supervising Technician Aaron Pole said his crew is using tomahawk line traps to capture the animals, and then they do measurements and take blood for testing before releasing them.

“We’ll catch all the usual carnivores. If they’re not fishers, we just let them go,” Pole said. “It’s a full time job just trapping the fishers, and we’ll select some to put collars on.”

The collars send out a radio signal that the researchers can track to see where the fishers go.

“Each fisher has their own range, and you figure it out after a while,” Pole said.

The crews will recapture every animal once per year, check their bodyweight and health, and replace the radio collars.

The thick closed-canopy forests around Hoopa are a good environment for fishers, with plenty of tan oaks to feed the squirrels and wood rats that they feed on, but the researchers saw the population drop sharply.

Higley said, “The population crashed between 1998 and 2005. It went from an estimate of 52 fishers per 100 kilometers down to 12 fishers per 100 kilometers. It seems to be recovering slowly.”

Their main threats are bobcats, mountain lions, and humans.

Deforestation and logging can put fishers in danger from bobcats in the open, but lately the biggest human threat to the animals is marijuana farmers using poisons to protect their forest plantations from rats.

The poisons, known as rodenticides, cause rats and other animals to bleed internally.

Higley said, “We’ve concluded from autopsy work that 24 percent of male fisher mortality is from the rodenticides, but it could be as high as 35 percent.”

The researchers also found that even those who died of other causes still had traces of the rodenticides in their bodies.

“Every animal that we’ve autopsied has been exposed to rodenticides,” Higley said. “It has some concentration in its liver tissue that is detectable.”

Higley described how the crews found one of the poisoned animals.

“Within the radio collars they have a mortality sensor, and it stopped moving around the landscape,” Higley said. “So we went and picked him up and he was dying.”

He’d lost half of his bodyweight, and had to be euthanized.

“The animals infected lose one-third to half their bodyweight over the course of two to three weeks,” Higley said, “and parasites infect them. Fleas, ticks, and internal parasites like worms.”

As deadly as the rodenticides are, the researchers found that predators killed 65 percent of female fishers, which are smaller than the male fishers.

“We have one PhD student at UC Davis who does DNA analysis to confirm which predator killed each animal we found,” Higley said. “Males are killed by mountain lions or coyotes, and bobcats are the number one predator of female fishers.”

But in one case, it was a pack of dogs.

“We had a fisher who was in a live trap, and dogs tore the trap apart and killed the fisher,” Higley said. “Normally, even bears can’t get the box trap apart, but those dogs spent so much time and effort at it.”

Despite humans, bobcats, coyotes, mountain lions, and even dogs, some fishers are lucky enough to die of old age. Their average lifespan is between three and five years, but some can live much longer.

“We have one that is a little over nine, which is very unusual,” Higley said.

The work done by researchers in Hoopa not only helped write the book on fishers, but has spilled over into other fields.

UC Davis Graduate Student Researcher Nicole Stephenson came to Hoopa to study ticks, which affect fishers, but can also jump to farm animals, pets, and even humans.

Pole said, “It all kind of connects. We all live in the same place and we’re all tied together. So the more you know, the better it is. That’s what I like about research.”