Survey: Bighorn sheep numbers up on Kofa refuge

A recently completed survey of the desert bighorn sheep population on the U.S. Fish and Wildlife Service's Kofa National Wildlife Refuge indicates the herd has grown slightly in size over the past two years, putting it at the highest it has been since a 2007 survey.

According to the survey's results, there are an estimated 428 sheep now on the refuge, slightly up from a 2010 survey which estimated the herd size to be at 402 sheep. The 2009 estimate was 410 sheep, while the lowest recorded estimated level was 390, from a 2006 survey.

“We have seen a slight improvement in the herd's size,” said Kofa Refuge Manager Susanna Henry. “I wish it had been more, but it's a species that takes a long time to recover.”

Based on the method biologists use to estimate this herd's size, and the scientific margin of accuracy involved, which has a 95 percent confidence interval, analysis of the past six surveys indicates there has been no significant decline or improvement in the herd's population, despite the increase seen over the past year.

Wildlife management agencies remain concerned about the low population levels on the refuge compared to the estimated 812 animals of the 2000 survey. The size of the herd on the refuge, once a very robust population, has dropped significantly in the years since then.

In 2003, the bighorn sheep population was estimated to be around 600, eventually falling to below 400 in 2009.

Henry said the biggest factor in why it takes the bighorn sheep herd so long to grow in size is that it is dependant on the survival of the lambs born each year. She explained that unlike other species, bighorn sheep ewes normally have only one lamb a year, and rarely have twins, so it is important for the lambs that are born to reach maturity.

The U.S. Fish and Wildlife Service (USFWS) and the Arizona Game and Fish Department (AGFD) have jointly conducted Kofa bighorn sheep surveys since 1981.

Wildlife experts attribute the decline over the years to a variety of potential factors including drought, predation, water availability, disease and human disturbance. Due to the significance of this sheep population, the USFWS and AGFD have instituted a comprehensive management program that addresses the suspected causes of the population's decline. Predation management, water improvement and disease monitoring are some of the ongoing projects.

Henry said she contributes this year's increase in the herd's size to a number of factors, such as favorable rainfall, maintaining water in several critical water sources, and predator management. She did add, however, that it was difficult to say which of the factors, if any, played a bigger role than any other.

For the past 50 years, the Kofa refuge has been an important source of desert bighorn sheep for the restoration and maintenance of bighorn populations across Arizona and throughout the southwestern United States, including New Mexico, Colorado, and Texas.

An extensive website dedicated to the Kofa refuge bighorn sheep is available at www.azgfd.gov/kofa. Launched in November 2007, everything from the latest updates, background information, frequently asked questions, past press releases, active management activities and more can be found at this site.

James Gilbert can be reached at jgilbert@yumasun.com or 539-6854. Find him on Facebook at www.Facebook.com/YSJamesGilbert or on Twitter @YSJamesGilbert.