The state’s population of fishers — the weasel often mischaracterized as a “fisher cat” — has dwindled significantly in recent years and wildlife experts say there are multiple factors behind the shift, from disease to a losing turf war with other animals.
How we know
Conservation officials don’t have any hard data on the fishers’ population or any means of arriving at a reliable estimate, like they have for some other species, according to Fish and Game wildlife biologist Patrick Tate.
“It’s very costly to acquire population estimates,” Tate said.
Instead, the best data regarding changes in fisher population are derived from trapping numbers. Looking just at how many animals are trapped is misleading on its own since that can fluctuate based on market demand for furs and the number of people engaged in trapping. To overcome this limitation, Tate uses a formula to arrive at what’s called the catch per unit effort index, which counts catches per traps per 100 nights.
Based on this model, fisher numbers have decreased steadily for most of the past decade.
In response to this, Tate said, the limit on trapping licenses was lowered to 450 for the 2015-2016 season. It was about 550 for the past few years.
The reduction is also evident anecdotally to folks who’ve been observing local wildlife for the past several years, like naturalist Dave Anderson with the New Hampshire Forest Society.
He said the biggest decrease can be seen in the northern parts of the state.
“The decrease in New Hampshire fisher populations has not been as marked in southeast New Hampshire,” Anderson said.
Anderson believes one of the biggest contributing factors to shrinking fisher numbers in the White Mountains area and north of the mountains is the success of other animals like bobcats and the fisher’s weasel cousin, the pine marten.
Bobcats and martens are antagonistic to fishers. While fishers hunt for similar prey as martens — mostly small mammals like rodents and hares — martens prefer more forested and higher-altitude areas and, unlike fishers, they can hunt beneath the deep snow in the White Mountains.
While martens are native to the area, they had largely left the state when much of the forests were cleared for agriculture, preferring places like Maine instead. Since the state’s forests have rebounded, martens have returned over the past 15 years or so, according to Anderson.
Similarly, bobcat numbers have rebounded thanks to conservation efforts that banned bobcat hunting and a better environment for prey species.
Tate thinks the bobcat relationship may play a bigger role in determining the fate of fishers. Studies in Maine that shows fishers were the No. 1 killer of Lynx kittens (a species similar to bobcats) and where bobcat populations were highest in Minnesota, fisher numbers were lowest. What we know about fisher and bobcat populations in New Hampshire follows a similar model.
While fishers might be eating the occasional bobcat kitten, bobcats eat fishers too, but they tend to focus on the smaller females, which can have an exponential impact on the production of future generations.
Add to that the spread of diseases like canine distemper among the state’s fishers, and bobcats have an edge over the weasels.
Distemper was positively identified in two of three fishers tested by Fish and Game. Tate said distemper is a sort of population control mechanism. When the population of an animal species grows too dense, food grows scarce and starvation lowers the animal’s immune system. Right now, this is a natural period of decline in a system of cycles, according to Tate.
He said marten numbers are growing in the state because fisher numbers are shrinking, not the other way around.
“Marten are doing so well because the fisher numbers are so low,” Tate said.