SPOKANE — The U.S. Fish and Wildlife Service on Wednesday released its final recovery plan for the endangered Columbia Basin pygmy rabbit.

The plan recommends continuing many recovery efforts already under way including releases of captive-bred animals into the wild, relocating pygmy rabbits from places outside the Columbia Basin, and semicontrolled field breeding measures.

It also calls for surveys to determine if pygmy rabbits may exist in areas not covered in earlier surveys.

Pygmy rabbits are the smallest rabbits in North America, with adults weighing about 1 pound and growing to less than a foot.

Columbia Basin pygmy rabbits are believed to be extinct in the wild. The last known animals were captured in 2004 for a breeding program intended to boost numbers for eventual reintroduction.

Threats to the species include large-scale habitat loss and fragmentation, mainly from past agricultural development, plus fire, invasive plant species, recreational activities and livestock grazing. Other threats include extreme weather, predation, disease and loss of genetic diversity.

“All these influences have impacted the Columbia Basin pygmy rabbit and together led to the population’s endangered status,” the agency said.

Pygmy rabbits rely on sagebrush to provide food and shelter and are one of only two rabbit species in North America that digs its own burrows. They usually are found in areas with dense sagebrush cover that have relatively deep, loose soils.

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.

They were historically found throughout Central Washington, including portions of Douglas, Grant, Lincoln, Adams, Franklin and Benton counties. The last known wild subpopulation occurred on state lands in southern Douglas County. This site also is the location for state reintroduction efforts that were resumed in 2011.

Due to dramatic declines in the number of Columbia Basin pygmy rabbits during the 1990s, the state of Washington started a captive-breeding program for the population in 2001.

The captive-breeding program was conducted in cooperation with Washington State University, the Oregon Zoo and Northwest Trek Wildlife Park. Numerous rabbits have since been released into the Columbia Basin, and appear to be doing well.

The released rabbits have been breeding successfully in protected enclosures of sagebrush, man-made and natural burrows, and areas with overhead netting to protect the rabbits and their offspring from predators.