Parts of the West are gaining population after years of decline thanks to the oil and gas boom, the U.S. Census Bureau said Thursday. The Midland, Texas, metropolitan area, where population rose 4.6 percent in the year that ended July 1, 2012, was the fastest-growing in the country, analysts said. Williston, N.D., with a 9.3 percent increase in population, was the fastest-growing micropolitan area, a term defined by the bureau as having a population less than 50,000 and above 10,000. "After a long period of out-migration, some parts of the Great Plains -- from just south of the Canadian border all the way down to West Texas -- are experiencing rapid population growth," said Thomas Mesenbourg, a senior adviser functioning as the bureau's director. "There are probably many factors fueling this growth on the prairie, but no doubt the energy boom is playing a role. For instance, the Permian Basin, located primarily in West Texas, and North Dakota accounted for almost half of the total U.S. growth in firms that mine or extract oil and gas, during a recent one-year period." While parts of the south are growing, four southern counties had the biggest losses in population. Bradford in northern Florida lost 5 percent of its residents followed by Henry in southern Florida and Macon and Perry in Alabama.