On a cool winter evening just before sunset, birdwatcher Helen Daley spotted something entirely unexpected slithering in the waters of Los Gatos Creek.

“I turned the binoculars on it,” Daley said. “It was moving, and the water was shaped like a ‘V.’ It dived under, and its tail slipped up. It was a tapered, long tail. It wasn’t like that of a rat or beaver.”

Daley, a nurse who lives in the Cambrian Park area of San Jose, rushed home and confirmed online that the animal she saw was a North American river otter.

While most people are familiar with their famed seafaring cousins, river otters in the Bay Area are seldom noticed or talked about. The sleek water weasels — about a quarter the size of sea otters — had long been nonexistent on Bay Area wildlife maps. But a few years ago, naturalist Megan Isadore and ecologist Paola Bouley started noticing river otters poking their noses out of the water in Lagunitas Creek in Marin County.

“The surprising thing about river otters in the Bay Area is that no one really knows anything whatsoever about them,” Isadore said. “So we decided to start figuring out where they were.”

Fourteen months ago, Isadore and Bouley launched the River Otter Ecology Project. The group’s first order of business was to create a website with an interactive map where local “Otter Spotters” could chart their sightings. Since then, the once empty map has exploded with 374 sightings throughout the Bay Area — from Cloverdale in Sonoma County to Los Gatos. The most famous river otter of all — “Sutro Sam” — has made quite a splash in the abandoned Sutro Baths of San Francisco, although no one knows where he came from.

Second only to sea otters in the plushness of their pelts, the Bay Area’s river otters were hunted to local extinction by the turn of the 20th century. But river otter trapping was banned in California in 1962, and the passage of the Clean Water Act a decade later might laid out a “Welcome Back” mat for the otters by reducing toxic chemicals in the waterways. Still, river otters have taken their time to come back, or just gone unnoticed until recently.

Efforts to improve the health of local waterways through creek restorations, water quality control efforts and public education have ramped up in the past 20 years, said Josh Collins, chief scientist at the San Francisco Estuary Institute. But, he said, weighing the relative success of all of these measures is no simple task.

“You start looking at a watershed the same way a physician would look at a patient,” Collins said. “You can’t measure everything, so what do you need to measure? What’s the pulse of the watershed?”

For Collins, that pulse is the flow of the river, and the sediments and chemicals it carries along with it. If the pulse is healthy, “the ecological part of it will probably come along also,” he said. “And I think that’s what we’re starting to see.”

Regardless of the reason for the uptick in river otter sightings, the otter project is taking advantage of the deluge to study the lifestyles of the secretive water lovers.

With the cooperation of Point Reyes National Seashore, Muir Woods National Monument, the National Park Service, local and state parks, the otter project is monitoring 100 miles of coastline. The group’s growing team of volunteers has placed 20 infrared, motion-sensitive cameras throughout the territory to monitor the otters’ activities.

In the collection of videos on the otter project’s website, the sinewy weasels hunt, frolic and, above all, roll around in their own poop.

The researchers collect the otter scat to study their diet, which has been shown to include insects, fish, crayfish and even birds. “You could say I’m a member of the pooperazzi,” said Robert Aston of San Francisco. The retired paramedic is one of a legion of volunteers who check cameras and collect the scat on a weekly basis.

Aston enjoys his frequent walks in Point Reyes National Seashore, even though he interacts with the otter’s excrement far more than with the otters themselves. He claims he can smell the pungent poop long before he lays eyes on it.

“Otter scat is kind of dark and oily and distinctively smelly, so it wasn’t that hard to find,” Isadore said. “And otters are weasels, so they are pretty musky and smelly animals.”

Another odd substance — called “otter jelly” — provides a source of pure otter DNA that the researchers, in collaboration with the California Academy of Sciences in San Francisco, will use to chart out the family relationships between the otters. The jelly, an anal secretion thought to protect the otters’ intestines from the sharp bones and shells of the prey they eat, could hold the key to charting out where individual otters came from.

For example, by comparing Sutro Sam’s DNA to that of otters from across the Golden Gate in Marin, the otter project might be able to determine if he swam across the bay to seek residence in the city. Until then, his origins will remain a mystery.

While sea otters are awkward on land and spend their entire lives in the ocean, river otters spend two thirds of their time on terra firma and navigate easily between fresh and salt water. They do require a healthy supply of fresh water to thrive. So, Isadore said, they’re unlikely to be found south of Monterey, where the land becomes more arid.

For now, birdwatcher Daley’s sighting in Los Gatos is the southernmost one. She keeps returning to Los Gatos Creek, but so far hasn’t seen the elusive otter again.

Still, Daley figures she has a pretty good chance of catching another glimpse — certainly better than all those people around her absorbed in jogging or walking their dogs.

“A lot of people aren’t in tune to their surroundings,” Daley said. “There’s a lot of wildlife, but they’re really not aware it’s out there.”

Contact Jessica Shugart at 408-920-5782. Follow her at Twitter.com/ JessicaShugart