SANTA FE — Lea County, in the heart of southeastern New Mexico's bustling oil patch, was the fastest-growing county in the state last year, while two-third of the counties lost population, according to the Census Bureau.

With New Mexico's economy still weak, the statewide population increased by a meager 0.3 percent from 2011 to 2012. All of the growth came from births, because more people left New Mexico than moved into the state, according to the federal agency's latest population estimates.

"I am very disturbed about this," said Jack Baker, the state demographer and senior researcher in geospatial and population studies at the University of New Mexico. "What worries me most is that now we're looking very negative on migration. That's the first time we've seen that in 20 years."

The Census Bureau estimated a net loss of about 5,200 people from migration into and out of the state from July 2011 to July 2012. The loss was somewhat smaller — about 1,100 people — when viewed from April 2010 to mid-2012. Births and deaths are other components of population change.

"More people are leaving than are coming. That's a problem. They're probably exploring their economic opportunities elsewhere. That's probably the driver of that but that's not good for us," Baker said in a telephone interview this week.

Lea County's population grew by 1.8 percent last year, and Baker attributed much of that to the booming oil field economy. There's also a uranium enrichment plant in the county near the small community of Eunice. The county ranked fourth in the state in population growth in the past decade.

New Mexico's population increased by 13 percent from 2000 to 2010, when about two-fifths of the counties lost population. However, Baker said the state's growth has been flattening since the economy soured.

The state has lost nearly 43,000 jobs since the recession began in late 2007, according to Gerry Bradley, an economist and research director for New Mexico Voices for Children, a social services advocacy group.

The state has started to slowly gain jobs and Bradley said one of the economic bright spots is in the Santa Teresa area in Dona Ana County because of an industrial development near the international border with Mexico.

Among larger communities, several in southern New Mexico led the state in the rate of population growth from mid-2010 to mid-2012, according to estimates released Thursday by the Census Bureau.

Sunland Park's population increased 3.3 percent, followed by Rio Rancho at 3.2 percent, Las Cruces at 2.9 percent, Alamogordo at 2.8 percent and Hobbs at 2.7 percent.

Rio Rancho, which flanks Albuquerque, was the fastest growing city in the past decade — increasing by 69 percent.

Sandoval County, which includes Rio Rancho, followed Lea County in population growth at 1 percent last year, and Eddy County was up 0.8 percent. Otero, Curry, Santa Fe and Dona Ana counties each grew by 0.7 percent from 2011 to 2012.

Bernalillo County, the state's largest that includes Albuquerque, grew by 0.5 percent. Only three other counties had population growth: San Juan, 0.4 percent; Chaves, 0.2 percent; and Union County, 0.1 percent.

In the past decade, Sandoval County was the fastest-growing. Its population increased by 46 percent from 2000 to 2010, but Baker said that was fueled by a dramatic growth in housing in Rio Rancho.