The wildlife scene at the beaver pond in downtown Martinez continues to surprise observers. Last year, in addition to the beavers, muskrats, river otters, turtles, and herons, someone spotted a single mink. Now there’s an entire mink family.
Beaver advocate Heidi Perryman and photographer Cheryl Reynolds observed them on a recent July evening near the beavers’ main dam, where Escobar Street crosses Alhambra Creek. If you visit the Martinez Beavers site (www.martinezbeavers.org), you can watch four young mink cruising around like a paddling of ducklings in Reynolds’ video. Furry ducklings with sharp little teeth. The mink appeared to be using a muskrat burrow on the side of the pond.
My own mink-watching experience is limited to a couple of chance encounters: one in a marsh in North Dakota, the other in the Delta near Thornton. But I remember sitting in church as a kid behind women wearing mink stoles and not being quite sure whether the critters were dead, or alive and exceptionally well trained. The idea of wearing defunct weasels, heads, tails, and all, to church struck me as strange, and still does.
Not much is known about the mink in the wild; there hasn’t been a lot of field research on the species. Lacking radiotelemetry studies, it’s not clear how large their home ranges are or how far they travel to find food and mates. That’s partly a matter of priorities: North American mink are neither endangered nor economically important pests, so budgets for mink studies have been limited.
We do know that they inhabit a broad swathe of northern and Central California: interior marshlands, mountain streams, even ocean beaches. They’ve been found from the Oregon border south along the coast to San Francisco Bay, in the Central Valley to Mendota in Fresno County, and east of the Sierra to the Owens Valley. Mink are or were widespread in the northern Sierra and the Coast Ranges.
Less committed to an aquatic lifestyle than otters, mink lack webbed feet and are not very agile swimmers. Physiological adaptations include a lowered heart rate when diving and a less acute sense of smell than their terrestrial kin. They hunt by sight and sound, and can detect ultrasonic mouse vocalizations. Like most mustelids, mink have paired anal glands used for scent-marking and sometimes to repel predators, although without a skunk’s volume or range.
A typical mink’s diet consists of fish, frogs, crayfish, and rodents, notably mice and muskrats. Salmon and trout can outmaneuver them, unless the fish are preoccupied with spawning. In one study, only three of 174 trapped mink had trout remains in their stomachs. Lesser game may include aquatic insects, snails, and earthworms. As opportunistic predators, they’ll also go after waterfowl—including those crippled but not retrieved by duck hunters—and domestic poultry.
“Few people realize that minks are good mousers,” wrote Joseph Grinnell in the classic Fur-bearing Mammals of California. “The stomach of a mink trapped at Pittsburg, Contra Costa County was found to contain an entire family of three or four young meadow mice.” It’s more likely that what attracted them to Alhambra Creek was the muskrat population, though. Male mink, larger than females, take a disproportionate share of muskrats. The youngest muskrats, still confined to their burrows, are the most vulnerable to mink predation.
Recent coverage in the Chronicle implied a connection between the arrival of the mink family and the disappearance of this year’s beaver kits. Perryman says that’s unlikely: “The size of the kits when [they were filmed] was already unmanageable for a mink.”
The mink’s reputation has suffered because of its predilection for killing more prey than it can immediately consume, and its supposed impact on game birds and sport fish. Grinnell disagreed:
“With respect to food, the mink appears to subsist chiefly on kinds that are of no value to man; even its levies upon domestic fowl and the like—depredations regarded by man as ‘harmful’—are fully offset by its consumption of mice, carp, and the like…The case for the mink is clear—it should be conserved (not wastefully destroyed) for its value to the greatest number of people…”
What Grinnell didn’t know was that mink can also be important environmental sentinels. “Mink are extremely sensitive to environmental pollutants,” writes biologist Serve Larivière in the authoritative Mammals of North America. “Because of their position at the top of the food chain in aquatic environments, mink accumulate many chemical compounds and heavy metals in their systems, and for this reason they are often used as bioindicators of pollution.” Larivière says they’re particularly sensitive to mercury and polychlorinated biphenyls (PCBs.)
Thriving mink, then, are a hopeful sign of a healthy wetland—one more piece in the living mosaic that has assembled around the Alhambra Creek beaver pond.