EPHRATA — Biologists and volunteers released the last Columbia Basin pygmy rabbits produced in a captive breeding program to 6-acre enclosures of grass and sagebrush in central Washington on Monday, marking the latest step in restoring the endangered species to its native habitat.

Leanne Klinski drew a small rabbit from a carrier and carefully placed it inside a section of plastic tubing that acts as a man-made burrow. Everyone around her laughed as another rabbit promptly darted out the other end, one of several rabbits already inside the enclosures that scattered with the new activity.

“It’s a bittersweet thing for me, but it’s a feeling of success,” said Klinski, who oversaw the Oregon Zoo’s breeding program for the past four years. “To see them come out and contribute to the wild is the best ending you could have — and to be a part of the recovery of this very delicate species just makes my heart melt.”

The pygmy rabbit is America’s smallest native rabbit, weighing less than one pound when fully grown, and is the only rabbit that digs burrows. There are other species of pygmy rabbits across the West, but the Columbia Basin species has been geographically separated from them and is genetically distinct.

Its numbers have declined as its habitat was converted to farm land, and by 2002, only 15 rabbits remained in the region. The rabbit is listed as endangered by both the state and federal governments.

Captive breeding programs at the Oregon Zoo, Washington State University and Northwest Trek aided the species, but biologists could never get the tiny rabbits to breed as voraciously in captivity as they do in the wild. Only about 100 rabbits thrived on a consistent basis, which was not enough to release to the wild and still maintain a viable breeding population.

However, with a more hands-off approach, the rabbits so far have been breeding successfully in protected enclosures of old-growth sagebrush, man-made and natural burrows and areas of overhead netting to protect the rabbits and their offspring from predators.

The enclosures are designed to acclimate the rabbits to their natural environment before biologists release them to the wild.

WSU and Northwest Trek released the last of their rabbits last year. Zoo officials joined with the Washington Department of Fish and Wildlife on Monday to release 11 adult rabbits and five kits, each about 20 days old, to fenced-in enclosures near Ephrata.

Research scientists Penny Becker of the Washington Department of Fish and Wildlife estimates that 150 kits were born this year, vibrating the enclosures with their activity. Already 130 rabbits were caught for a DNA survey, and another survey will be conducted Monday.

“You’d walk around the ground and it would feel like it’s moving,” she said. “They definitely exceeded our expectations.”

Biologists have released 87 kits from the enclosures to the wild this year. Forty-two kits were released last year. In addition, some wild pygmy rabbits have been brought in from other states, including Utah, Idaho and Nevada, which also are breeding with the captive-bred rabbits.

The last purebred Columbia Basin pygmy rabbit, named Bryn, died in 2008.

Biologists now will monitor the rabbits — both inside and outside the enclosures — and continue to release new offspring to the wild, Becker said. If the population in this area continues to grow, the efforts will move to a new region.

“Delisting has to mean they’re successful in more than one location,” she said. “For rabbits, especially this species, it’s really a boom and bust species, so it’s important that we have more than one population that’s doing well.”