From April 1, 2010 to July 1, 2011, the county's population went from 130,044 to an estimated 128,200, a 1.4-percent decrease.
County government officials said a decrease may signal that people are moving out of the area in search of jobs. A population decrease would reduce local government's tax revenue which may lead to a decrease in services, said Mike Stark, the county operations officer.
"A combination of (decreasing population) and a lack of economic activity will ultimately lead to a decrease in services," he said. "It will translate to lower tax revenue, and we're already seeing that with a decrease in gross receipts taxes and production of gas."
The New Mexico Department of Health also tracks county population growth. The state agency estimated a bigger population decline in San Juan County than the Census Bureau.
The health department estimated the county's 2010 population was 130,501 and by 2011 it dropped to 128,247, a 1.7-percent decrease, according to the health department website.
The last time the Census Bureau officially released information that showed a drop in San Juan County's population was in 1970. The county's population went from 53,306 in 1960 to 52,517 in 1970, a 1.5-percent decrease over a 10-year period.
Census numbers are released every 10 years and the bureau releases population estimates every year.
"We take IRS tax returns and compare individuals who move from one county to another," she said.
Vidal said the county's 2012 population estimate will be released next month.
From 2000 to 2010, San Juan County's population grew 14 percent, from 113,801 to 130,044.
Stark said the county experienced rapid growth throughout the county in the early 2000s but the growth stopped around 2008 because natural-gas industry production started to decline and the county's population growth slowed and became stagnant
The Census Bureau population estimate is evidence that the local population is actually on the decline, Stark said.
Jay Peterman Jr., a Community Development Block Grant administrator for the city of Farmington, said he hasn't seen concrete statistics that show Farmington's population is on the decline.
There is evidence, however, that there are fewer jobs available to Farmington residents than there were seven years ago, he said.
According to three-year averages, from 2005 to 2007 there were 21,591 Farmington residents 16 years and older who were employed. From 2009 to 2011, there were 19,803 Farmington residents in that age range who were employed. That was a 8-percent decrease in Farmington's workforce, according to city documents.