Sonoran Pronghorn Numbers Down
The Arizona Game and Fish Department, in coordination with the U.S. Fish and Wildlife Service, recently completed a range-wide survey of endangered Sonoran pronghorn in the United States. The results show an estimated 21 to 33 animals, which is a substantial decline from previous surveys.
The aerial survey took place from Nov. 30 to Dec. 4. Almost 2,000 square miles in southwest Arizona were surveyed including parts of the Cabeza Prieta National Wildlife Refuge, Organ Pipe Cactus National Monument, Barry M. Goldwater Range, and some Bureau of Land Management lands.
The department conducts the survey every two years. The December 2000 survey yielded a population estimate of 99 while the December 1998 survey showed 142 animals. However, in 2002 the entire range of Sonoran pronghorn, like most of Arizona, experienced a devastating drought.
Virtually no rain fell for more than 13 months over much of the range. The result was extremely poor forage conditions for pronghorn. While eight years of continuous pronghorn monitoring has shown that dry conditions often result in low fawn recruitment, this was the first year adult animals have died in large numbers due to drought.
From the first of June to mid-August 2002, 80 percent of the radio collared sample of pronghorn died from drought related causes. Recent rainfall throughout pronghorn range has provided substantial though temporary relief for the remaining animals.
Federal agencies, including the U.S. Fish and Wildlife Service, National Park Service, Department of Defense and Bureau of Land Management, along with the Arizona Game and Fish Department, are taking immediate actions to provide for water and high quality forage.
Data suggest drought-induced nutrient-and-moisture-poor forage has been a primary factor in the decline of Sonoran pronghorn. “We are digging wells and installing a sprinkler system to grow forage this winter in habitats used by pronghorn,” said Dale Hall, director of the US Fish and Wildlife’s Southwest Region. “Winter moisture is critical for growing the lush forage that nursing mothers will feed on in the spring when their fawns are born.”
Pronghorn are unique to North America. Sonoran pronghorn were listed as an endangered species in 1967. The listed pronghorn currently includes three sub-populations of Sonoran pronghorn: two in Sonora, Mexico, and one in Arizona. All three sub-populations contend with roads, fences, canals, and other barriers to movement. Border fencing and highways have divided the United State’s sub-population from the sub-population on Mexico’s El Pinacate Biosphere Reserve.
Further south, the largest sub-population is isolated from the El Pinacate sub-population by Mexico’s Highway 8. In December 2000, the population of Sonoran pronghorn in Mexico was estimated at approximately 350 animals. Biologists from the United States and Mexico are planning another aerial survey of the Mexican populations to get more current estimates later this month.
