YUMA, Ariz. — The population of desert bighorn sheep on the Kofa National Wildlife Refuge near Yuma in southwestern Arizona has plummeted, a recently completed survey shows.
The survey estimated the sheep population at 390, down by 230 animals from the last survey estimate of 620 in 2003 and fewer than half the 812 sheep estimated in 2000.
The numbers mean that the decline in bighorn sheep on the refuge seen in 2003 continues and that those numbers may have reached or are close to the lowest levels recorded.
U.S. Fish and Wildlife Service officials pointed to drought and possibly predatory mountain lions as the likeliest contributors to the decline.
But environmental groups say there's a much broader reason for the decline, and it's not just predators or lack of water.
"What they do is they look for what I consider is the easy answer, versus taking on the bigger issues," said Sandy Bahr, conservation outreach director for the Sierra Club's Grand Canyon chapter. "For bighorn sheep, certainly, habitat fragmentation is the biggest issue. But instead what they do is say 'well, lions are killing the sheep, so maybe we should kill some lions.'
"I would say that always pointing to mountain lions as a contributing factor is getting old. Clearly, lions and bighorns have evolved together.
The government report noted that the falloff in 2003 came after a severe drought year in 2002.
Fish and Wildlife, which is in charge of the 665,000-acre refuge, has conducted the surveys jointly with the Arizona Game and Fish Department every three years since 1992.
Between 1994 and 1997, a nearly identical decline was seen — from 811 to 600 sheep — with the falloff being noted one year after a severe drought in 1996.
The data reflects an overall downward trend in numbers of the bighorn sheep in southwestern Arizona that began with the drought year of 1996, Fish and Wildlife said.
"While drought is almost certainly the most significant limiting factor for bighorn sheep and other desert species, the continued decline in sheep populations in the face of somewhat improved rainfall conditions since the 2003 survey is disconcerting and is the subject of ongoing study," Fish and Wildlife said in a release.
Bahr said habitat fragmentation makes drought more difficult for the sheep to survive. That's because roads and highways like Interstate 10 prevent them from moving to other mountain ranges or into areas where Arizona's spotty rainfall may have provided enough forage for them to thrive.
Among possible factors for the decline being studied by Fish and Wildlife is predation by mountain lions, disease, availability of permanent water and public recreational disturbance in areas where female sheep give birth, the agency said.
Resident lions are rare historically on the refuge, according to data, but "recent reports indicate that at least five lions may be using the refuge for significant periods," the service's statement said.
Bahr said one thing Fish and Wildlife officials should consider immediately is seasonal closures of areas where lambs are born to prevent disturbances.
The Kofa population of bighorn sheep has been a significant source for repopulating other herds in Arizona and from Colorado and New Mexico to Texas, including 30 sheep captured and transported in 2005 to the San Andres National Wildlife Refuge in New Mexico.
Fish and Wildlife personnel are researching mountain lion predation, using cameras at water holes, searching for lion tracks, scat or kills, as well as trying to use radio collars to track lions and recording all sightings reported. Their impact on populations stressed by disease or drought can be substantial, the agency added.
Hunting will be adjusted and relocation minimized to reduce any more impact on the population, the agency said.
Separately, the Arizona Department of Game and Fish announced Friday two bighorn sheep transplant efforts to restore the animals to more of their historic range in central Arizona.
State biologists captured 28 desert bighorn sheep along the Arizona Strip in the northwest part of the state and moved them to a wilderness area about 100 miles to the south. They also captured 31 Rocky Mountain bighorn sheep from an area near the New Mexico border and moved them to central Arizona's Verde Valley.