Students from James Madison High School release brook trout fingerlings into Catharpin Creek near Haymarket, Va. Teens from several schools are conducting water-quality studies in the area to determine the viability of introducing the fish. The brook trout has been decimated because of ecological stresses. (Dayna Smith/For The Washington Post)

On a sunny Saturday, 20 high school students thrash through thorny brambles to descend upon a burbling little stream in Jackson Hollow, north of Haymarket. One student kneels in the water with a thermometer, another gauges the current with a flow meter, a third measures the water’s oxygen content.

Others maneuver a net to catch and catalogue the stream’s insects. Still others make plans to plant saplings along the banks to create shade. Others stand by to enter their findings into a database map.

Hanging out at a mountain stream on a fine spring day should be nothing but fun for a gang of high school kids.

These students are on a mission, however. They’re budding scientists trying to revive the brook trout — the freshwater fish symbol for Virginia. The brook trout was once ubiquitous in the state, but it has been decimated because of environmental stresses.

The students crunch the data they collect through a computer program and send the results to state and federal officials.

“It’s kind of like giving the stream a physical,” said senior Julia Hakeem, president of the Students for Environmental Action (SEA) club at James Madison High School in Vienna.

In this case, the exam will tell whether this stream is healthy enough to support brook trout. And they have already had some success.

It was here in Catharpin Creek in spring 2017 that they placed about 50 brook trout fingerlings, or young fish. While the stream’s data looked good, there was no guarantee the trout would live through the summer’s warmer waters, and certainly no guarantee they would reproduce.

In December, seven months after the trout were released in the creek, a small group of biologists and students visited the stream. Junior Luke Pohlman stared into the water and spotted a distinctive torpedo shape, about five inches long.

“I saw the brook trout,” Pohlman said with a grin. “It was one that had survived. We were expecting to see zero.”

The sighting of just one trout filled the club with excitement.

“This was a truly historic event,” the students’ faculty adviser, Kirk Smith, told the students in January. “That was the first time in 100 years that trout were released in Catharpin Creek.”

The native brook trout’s dark-green-speckled back and bright orange belly make it easy to identify. The small mountain streams in the national parks and forests of the Blue Ridge and Appalachian mountains are the largest stronghold of these fish south of the Mason-Dixon Line, as they provide cold and clean water, an adequate food supply and a stream bottom that offers places to hide and to spawn.

Virginia’s mountain “brookies” are small — a nine-incher is a whopper — but they are prized by naturalists for their beauty and by fishermen for their smarts and their fight.

In precolonial days, they populated a much greater range than they do now, according to the Eastern Brook Trout Joint Venture, an alliance of conservationists based in Sanbornton, N.H.

But today, because of agriculture, foresting, mining, grazing and development pressures, the fish have disappeared from 38 percent of the streams they once called home in Virginia. And their presence has been reduced in 34 percent more. Biologists are eager for new streams where the trout might be reintroduced and thrive.

But there is a problem finding them.

Trout Unlimited, a nonprofit dedicated to preserving and improving fish habitat, sponsors an education project in more than 200 Virginia schools. Under the program, known as Trout in the Classroom, students get fish eggs in the fall, nurture them in aquariums, release them into streams in the spring and learn science.

Most of the fish get released into the same creeks for two reasons: The state frowns on genetically commingling aquarium-raised trout with native brook trout in their mountain streams, and only a few other streams have been identified as good habitat.

Enter the students in the SEA club, which is nicknamed the Stream Team.

The club was born in fall 2013, when two students told Smith, the James Madison science teacher, they wanted to do hands-on science.

By luck, Smith, 57, had written a doctoral dissertation in 2011 for George Mason University, in which he assessed the characteristics of the best brook trout streams in Maryland, using state-collected data. He concluded there were five key indicators of a good brook trout stream: cold water, good oxygen content, a rocky and gravelly bottom, minimal nearby agriculture and remoteness from roads. He devised a way to assess and quantify all these indicators and created an index score based on the numbers. A score of 80 or above indicated a potentially good habitat for the fish.

Until the students approached him, however, he thought he had created just another academic theory.

By spring 2014, the Stream Team had expanded to six girls, and they set out to put Smith’s methodology to the test. They checked Wildcat Hollow, a cold stream north of Markham, where many classrooms in Virginia release their trout. The index score indicated that this was, indeed, a good stream.

They checked out Fiery Run, a small burbling creek that flows out of Shenandoah National Park, 10 miles southeast of Front Royal. The index scores were good, but summer water temperatures were too high and the stream held other fish that would make life too hard for the little brook trout.

More study suggested that the best streams were farther west, so they headed into the mountains.

Stream Team co-director Hania Abboud, a senior, contacted Dawn Kirk, a U.S. Forest Service fisheries biologist, who suggested they assess two streams that had no trout now — Narrow Passage Creek near Woodstock and Mill Run in Fort Valley. They did, and both scored in the mid-80s.

“I was really pleased with what they found there,” Kirk said.

On April 21, they checked Cove Run in Shenandoah County and then nearby Sulfur Springs Gap. Sulfur Springs did not look good, but Cove Run scored an 83.6, a promising number that they will also send to Kirk.

A week earlier, Hakeem and junior Mary Waclawski had presented their stream work at the state science fair, where they won a first prize. But for all the students, finding a new home for reproducing brook trout would be the holy grail.

Meanwhile, Catharpin Creek is their adopted child. Because it lies in the piedmont, where waters tend to run slower and are warmer, state fish biologists rate it as an “iffy” habitat. And while its index score was 87.5, its summer water temperatures indeed were a little warm. But Smith diverted a spring to cool it down, and students from area high schools are teaming up to plant trees to shade the waters.

So state officials gave permission to the James Madison students to release their classroom-raised trout there last spring. On Saturday, they released 55 more. Will they survive? Maybe. Could this become a new home for self-reproducing wild trout? Too soon to tell. But the students have found a new home for trout raised in their classroom — and that’s a good start.