DEXTER, N.M. — Thousands of endangered Rio Grande silvery minnows reared at a national hatchery in New Mexico were being prepared Wednesday to be trucked to Texas, where the tiny fish will be released into the Rio Grande near Big Bend National Park.

The release is part of a five-year experiment by the U.S. Fish and Wildlife Service to establish a population of minnows in the river’s southern reaches. More than 400,000 silvery minnows were released at Big Bend last year and surveys have shown that there are still minnows in the area.

“It’s a little early to really say that this is a successful program, but we’re definitely learning and we’re hopeful,” Fish and Wildlife Service biologist Aimee Roberson said.

Hatchery staff members on Wednesday were loading 500,000 minnows into several trucks for the overnight drive from southeastern New Mexico to Big Bend. About 60,000 minnows bound for Texas came from the city of Albuquerque’s breeding facility.

Manuel Ulibarri, director of the Dexter National Fish Hatchery and Fish Technology Center, said it’s a big effort involving many agencies.

The silvery minnow once was abundant in the Rio Grande from New Mexico to the Gulf of Mexico. Due to environmental pressures on the river and changes in habitat, the minnow now occupies only a stretch in central New Mexico, just 5 percent of its historic range.

The minnow’s fate is shared by dozens of other aquatic species in the West. The Fish and Wildlife Service says for many of these species, populations are small and concentrated in fragmented portions of rivers that are vulnerable to drying and low flows.

The agency says hatcheries like the one at Dexter have become increasingly important as both short-term refuges and long-term breeding facilities.

More than 30 species of listed fish, wildlife and plants are in the care of hatcheries in the Southwest, including 15 fish species at the New Mexico facility.

Dexter’s 72 ponds, raceways and laboratories are dedicated to holding, studying and culturing endangered fish for restocking.

“Most of the species we’re working with are remnants of larger populations, and that’s all there is that’s left. What we help do is preserve those species,” Ulibarri said.

Hatcheries, however, are only one tool for species recovery.

Mike Oetker, assistant regional director of the Fish and Wildlife Service’s Fisheries Division, said habitat restoration and construction of fish passages in some cases must be done to ensure that captive-bred fish can survive once released in their native waters.

“It’s definitely not as simple as just throwing the fish in the river,” Roberson said.

Many agencies have been working for years to improve the ecosystem along the middle Rio Grande in New Mexico, and Big Bend’s status as a national park ensures that miles of the river are protected. Roberson said that means the minnows released in those areas should fare well.

Noah Greenwald, endangered species director for the environmental group Center for Biological Diversity, said the danger is when wildlife managers use hatcheries as substitutes for protecting habitat. He said that has happened in the Northwest with salmon and the Kootenai River white sturgeon.

“For some species it really is a last resort,” Greenwald said. “There are species that get down to one or two populations and it could potentially mean the difference between survival and extinction.”

When the minnows arrive in Texas, they will be placed in holding pens in the river to acclimate. Biologists plan to open the pens Friday.

The minnows will be considered a nonessential, experimental population, meaning they will not have the same federal protections as those found in the middle Rio Grande in New Mexico.

As part of the silvery minnow’s recovery plan, the Fish and Wildlife Service hopes to establish at least three stable populations — including the one in New Mexico.