What’s the Purpose of the Box?

These boxes were originally built for a Minnesota Fisher Den Research Project. The goal was to test a new habitat management tool that might help address the state’s declining fisher population.

Now installed at Wolf Ridge, the boxes serve a new dual role:

• Monitoring local wildlife

• Engaging learners of all ages in citizen science

Though they are not substitutes for natural denning sites, the boxes offer a rare window into the hidden lives of forest-dwelling animals, including American martens, flying squirrels, red squirrels, birds, and—occasionally—fishers.

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What is a Fisher?

Fishers are agile, tree-climbing members of the weasel (mustelidae) family and are Minnesota’s second-largest mustelid after the river otter. They weigh about as much as a red fox but have shorter legs and are skilled predators. Fishers are especially nimble in trees, often outclimbing martens and red squirrels.

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What Do Fishers Eat?

Their primary prey includes snowshoe hares and small mammals such as squirrels, chipmunks, mice, shrews, and voles. Fishers are uniquely skilled at hunting porcupines, repeatedly biting their faces to disable them. They also scavenge deer carcasses left behind by wolves or hunters.

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Fisher Reproduction Cycle

Fishers mate in April but use delayed implantation—a reproductive strategy where the embryo doesn’t implant right away. 

The pregnancy resumes months later, with females seeking large tree cavities to den in by February, and typically giving birth in March to 1–4 kits.

⚠️ Please do not disturb these boxes from February to April, as this could disrupt critical denning behavior.

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Why Are Fishers in Decline?

Since 2000, Minnesota’s fisher population has fallen from an estimated 16,000 to around 7,000—a nearly 50% decline. The main culprits are likely habitat loss and increased competition.

Fishers are highly forest-dependent and require wild, “messy,” structurally rich forests to thrive.

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What Makes a Forest Complex?

Fishers thrive in landscapes with:

• A mix of young, middle-aged, and old trees

• Standing dead trees (snags)

• Fallen logs, root wads, brush piles

• A layered canopy and rich understory

• Large cavities in living or dead trees

These features offer everything from thermal shelter and prey habitat to denning and resting sites.

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Old-Growth Trees Are Irreplaceable for Denning

In Minnesota, 96% of fisher dens are found in tree cavities: 67% in living trees, 29% in dead trees, and 4% in hollow logs

Fishers typically den in mature aspen trees aged 80–95 years, which provide the large, rotted cavities they need.

However, less than 2% of northern Minnesota’s forests contain aspen large and old enough for fisher denning. The timber industry usually harvests aspen at 40–60 years, long before such cavities can form.

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Do Fishers Use the Boxes?

Yes—but with a caveat. While fishers have been observed resting in artificial boxes, they have not yet been documented denning in them. These boxes are valuable research and education tools—but they are not a replacement for the protection of old-growth habitat.

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Why Does This Matter?

Fishers are indicator species—their presence signals healthy, resilient forest ecosystems. By protecting complex forest structures, we don’t just support fishers—we safeguard an entire web of life that depends on mature, biodiverse habitats.

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Call to Action

• Educate your community: Dead and dying trees (snags) provide critical shelter for wildlife like fishers, martens, flying squirrels, and many different birds.

• Get curious: Find a citizen science project in your area and help researchers collect data!

• Reach out: Contact Dr. Michael Joyce for more information on cavity-dwelling mammals in your area.