When fishermen reported seeing river otters in Lake Temescal earlier this year, park superintendent Doug Cantwell thought it might be an urban legend in the making.
He had never heard of the playful creatures being spotted before at the urban lake wedged between two Oakland freeways.
"Maybe we should call it the Loch Ness otter," he said with a laugh.
But Cantwell become a proud believer this fall when he saw them for himself, adding another small chapter to the comeback story of the sleek and frisky river otter to the urban Bay Area.
Otter watchers see the return as a good omen.
"We think this a hopeful sign for our water environments," said Megan Isadore of the River Otter Ecology Project, a Marin County-based nonprofit that monitors and studies otters in the region. "The otters are clearly expanding into areas where they were not seen before."
After decades of few or any sightings, in the past two years the group has recorded about 600 otter reports.
Either unnoticed or gone from the Bay Area three decades ago, the North American otter has been spotted in Los Gatos, San Francisco, Oakland, Fremont, Martinez, Walnut Creek, Richmond, Berkeley, Napa, Lafayette and elsewhere in the Bay Area.
In October, an observer reported seeing an otter plop itself on a dock at Oakland's Lake Merritt. Another was reported more recently off the Richmond shoreline. Otters have visited Walnut Creek's Civic Park during the last two summers, and ponds in Heather Farm Park in that city for longer than that, otter watchers report. They have also been in San Ramon Creek in Alamo and Danville.
One otter, named Sutro Sam, became a media sensation a year ago when he swam into the Sutro Baths at San Francisco's Ocean Beach and feasted on fish for several months before leaving.
Scientists can't explain the change fully because no one has extensively studied the river otters, which are smaller but more abundant than the threatened California sea otter. No river otter counts have been taken because the mammal is not considered endangered.
Experts, however, suspect the comeback is linked to a 1962 ban on otter trapping; a 1972 federal law to reduce pollution in rivers, creeks, and bays; plus a series of creek cleanup and environmental restoration projects.
Scientists also believe the otter, a sleek carnivore weighing up to 31 pounds that had been hunted for its pelt, is learning how to adapt to living in the territory of humans.
"They were persecuted for so long by people. The trapping really hit their numbers. Now they have been adapting to us," said Steve Bobzien, a regional park wildlife biologist who has studied large river otters in South America. "They are charismatic, playful, and smart, and they're so cute."
As river otters became scarce or nearly vanished from the urban Bay Area, they still were commonly seen in more rural, less developed areas like the Sacramento-San Joaquin River Delta. The otters easily swim in rivers, creeks, bays and canals, and are able to tolerate some saltwater.
Lake Temescal is regularly stocked with fish, making it a prize for any otter that can find it.
"It's wonderful to have them here. We're such an urban park," said Cantwell, the park superintendent. "We're also very curious how they got here."
The lake is some four miles away from San Francisco Bay -- four miles with many obstacles.
To discover the otter entry points into the lake, naturalists are installing cameras in culverts leading out of the lake.
"It's intriguing if river otters are using underground portions of the creek to get around," said Sharol Nelson-Embry, a park district naturalist. River otters also are known to walk between canals and creeks, making it easier to travel long distances in urban areas.
Traveling by both water and land likely enabled the river otters to reach the ponds in Heather Farm Park in Walnut Creek, where local creek advocates Lesley and Bill Hunt have seen them on water and land for years.
The otters fatten up on fish in those ponds, and then move on to other fishing and feeding grounds.
"The otters use their creeks and waterways as a network to get around," said Lesley Hunt, president of the Walnut Creek-based Friends of the Creeks. "The very charismatic otters provide a good reason why we need to protect these creeks and watersheds."
Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff
Northern American river otters are sleek, agile and playful members of the weasel family that live in and along rivers, creeks, marshes and coastlines. They feast on fish and crayfish in water, and frogs, small mammals, insects and even birds on land.
River otters can be up to 42 inches long and weigh between 11 and 31 pounds. Males average about 25 pounds. California sea otters weigh from 31 to 99 pounds and can reach about five feet in length.
Unlike the sea otter, which is slow on land, the river otter has a different skeletal structure which allows it to walk and run quickly on land.
Cute as otters are, wildlife experts warn not to corner them as they have sharp teeth.
Female river otters don't dig their own dens but use natural openings or dens dug by other animals. A mother otter typically has one to three pups in a litter.
Although it is called a river otter, this otter can tolerate both fresh and brackish water and a variety of water temperatures.
Seen an otter?
The River Otter Ecology Project advises citizen scientists on how to identify and monitor river otters and report their behavior. View their website at www.riverotterecology.org/ Take notice: it can take long watching periods to spot otters .