Spunky, fish-chomping river otters who faced extinction in Colorado and other Western states are bouncing back — evidence that pollution control and wetland protection bring results.

Colorado Parks and Wildlife biologists say that, after completing the reintroduction of about 120 young male and female otters 25 years ago, otters are multiplying with a statewide population now numbering in the hundreds.

The recovery here, mirrored in other states that reintroduced otters decades ago, stands out in the struggle for wildlife survival because biologists consider otters a “sentinel species” that is highly sensitive to pollution. It shows how a relatively modest state effort to keep an imperiled species off the federal endangered species list — the U.S. ecological equivalent of an emergency room — can lead to a comeback.

State-run reintroduction “has made a significant contribution to the conservation of river otters throughout the state of Colorado,” U.S. Fish and Wildlife Service biologist Leslie Ellwood said. “We’re now seeing river otters in streams and lakes where they had not been seen for the past 100 years.”

Although Colorado officials could not produce current population survey data or a peer-reviewed study, and while otters still are classified as “threatened” in the state, CPW tables indicate otters exist in 38 of 64 counties.

This means habitat suitable to otters in Colorado is relatively healthy, said Reid DeWalt, assistant director of wildlife and natural resources for the state. “CPW is vested in the long-term sustainability and balance of wildlife for future generations.”

Colorado residents supported the comeback, using the Checkoff Colorado box on state income tax forms to raise up to $240,000 per year — money that funds work to save 750 “non-game” species, including otters.

Otters eat practically any animal that moves in riparian corridors: fish, frogs, snakes, turtles, crayfish, birds, salamanders. They grow to be 3- to 4-feet long, with males weighing about 25 pounds and females about 18 pounds. When predators such as coyotes lurk, otters have the ability to chase them away. And they energetically chase one another, sliding crazily along muddy banks — activities biologists describe as pure play for fun.

The prospect of a widening recovery has emerged as a success at a time when conservationists warn that scores of other plant and animal species are vanishing.

“In general, we humans screw a lot up. This is something we’re doing right, bringing these animals back. Success stories are hard to find,” said LynnLee Schmidt, mammals curator at Denver’s Downtown Aquarium, which is home to three otters.

Schmidt worked close-up with an orphan otter. She kept Olive, an otter pup rescued from a Florida gas station at 11 weeks old, at her home for eight months last year. The otter scooted and bounded around energetically, splashing water from the bathtub and biting, before eventually settling into an aquarium exhibit. (The pup was originally named Oliver, but the name was changed when handlers verified its gender.)

In the wild, biologists regard river otters as a sentinel species, similar to frogs, which disappear when the natural environment is compromised.

“If, indeed, Colorado’s river otter population is growing and they are expanding their geographic distribution, it means we are maintaining sufficient flows in high-elevation streams and that the water quality is good because it is supporting fish, which is otters’ primary food source,” said Barry Noon, a Colorado State University professor of wildlife ecology who ran a National Biological Services office under President Bill Clinton in the 1990s.

A recovery would mean many measures are working, including the 1972 Clean Water Act, bans on building in buffer zones along waterways and law enforcement work to prevent the illegal killing of otters, Noon said.

“Animals as indicators of the integrity and health of our environment are really powerful. They tell us if our environment is at risk,” he said. “If animals experience problems, we eventually will experience problems too. The welfare of the natural environment and human welfare are linked.”

In the 1800s, otters swam in waterways across North America until fur-trappers hunting for pelts nearly obliterated the species. Urban development later destroyed otter habitat and water pollution worsened the harm. Otters need clean wetlands, rivers and streams with plenty of prey and secluded spots to build dens, often near beaver lodges and along riverbanks.

Starting in 1976, Colorado wildlife crews reintroduced river otters at five sites: Denver Water’s Cheesman Reservoir, Rocky Mountain National Park, and the Gunnison, Piedra and Dolores rivers. Similar efforts were happening elsewhere in the United States. The International Union for the Conservation of Nature estimates that more than 4,000 otters were reintroduced across 21 states.

Colorado kept up the reintroductions until 1991.

In 2003, members of the Colorado Wildlife Commission decided to down-list the status of the river otter to “threatened” from “endangered.” It’s still illegal to hunt them.

Colorado wildlife officials contend that the reintroduced otters are thriving with established populations around the Western Slope in waterways, including the Colorado, Green, Yampa, Dolores, Gunnison and San Miguel rivers, as well as tributaries in each basin.

Yet state monitoring is limited. CPW relies on Colorado residents to serve as eyes and ears — filling out and submitting forms giving details when they spot otters. Mink, beavers and muskrats share the same habitat as otters and, sometimes when seen swimming, are confused with otters.

U.S. Fish and Wildlife Service officials, charged with enforcing the federal Endangered Species Act, welcome state efforts to head off crises, said Marjorie Nelson, the USFWS regional chief of ecological services.

“Implementing conservation efforts before species are listed and their habitats become highly imperiled increases the likelihood that simpler, more cost-effective conservation options are available,” she said, “and that the conservation efforts will succeed.”