Carnivore Mystery: Why Fishers Thrive in East, Not West

For weasel-like fishers it’s a good time to live in the East. The fierce little carnivores are reclaiming historic habitats, including the Bronx, New York, where police have photographed one fisher.

But it’s a different story for fishers in the West, which haven’t been as successful in repopulating areas they once roamed in the Pacific and Northwest. New research published online in Animal Conservation investigates reasons for the regional differences, finding fewer predators and better food for fishers in the East, says co-author Roland Kays, a zoologist with North Carolina State University and the N.C. Museum of Natural Sciences.

“The eastern fishers are expanding because they face fewer large predators like wolves, wolverines and cougars,” says Kays, a faculty member in the College of Natural Resources. “Because they have less competition for food, eastern fishers can also eat more large prey, which has helped them evolve a larger body size.” Comparisons show that the eastern fishers have measurably larger skulls than their western counterparts.

Weighing 4 to 12 pounds, fishers are classified as medium-sized or mesopredators who are known as “some of the most ferocious hunters around, pound for pound,” Kays says. With lightning speed and climbing skill, fishers prey on squirrels, rabbits, mice and even porcupines, using their agility to deliver a skull-crushing bite while dodging quills.

Fishers, which are native to North American forests, were heavily trapped for their fur and pushed out of their habitat by logging, farming and human settlement. They were ranked the carnivore with the fourth greatest contraction in geographic range among 32 North American species. But the adaptable fishers have launched a comeback, extending their range by 57 percent from its nadir.

However, regional differences are striking. Fishers in the eastern region, which includes the Atlantic Coast, increased their range by 119 percent. Fishers in the central region (Minnesota, Wisconsin, Michigan and Ontario, Canada) expanded by 46 percent. But fishers in the Pacific and Northwest regions increased their ranges by only 15 and 18 percent, respectively.

Kays says the research points to evidence that fishers in the East have been freed from the pressure of competing with larger predators, a phenomenon known as mesopredator release.

“Within a couple of generations, eastern fishers have adapted as suburban animals,” Kays says. “They’re better adapted to New York than coyotes, which have been spotted in Central Park. Fishers are smaller, they’re better ratters than coyotes, and they’re nocturnal, so they can hide out during the day and then make their way through tunnels and drainage pipes to hunt after dark.”

Results suggest that reintroduction of fishers will be more likely to succeed in areas with fewer apex predators, he said. Researchers would like to gain a better understanding of how fishers avoid predators, including where they take refuge.

The research was funded with National Science Foundation grant #0756920 to Kays and National Geographic Society Waitt Grant Program #W157-11 to S.D. LaPoint, the lead author, who is with the Max Planck Institute for Ornithology in Radolfzell, Germany. Co-author J.L. Belant is with the Carnivore Ecology Laboratory at Mississippi State University.