Natural gas output in the U.S. is hitting record levels, but there is mounting evidence that the best days of the shale boom are over. Government data released Tuesday show production climbed to 70.21 billion cubic feet in 2013. Output grew by just 1.5% from 2012, the slowest rate since output fell in 2005, before widespread use of new drilling technologies began to unlock massive quantities of natural gas from shale rock formations. The EIA is projecting output to rise by 2.1% in 2014 and 1.3% in 2015. The slowdown has major implications for gas prices. As abundant supplies flooded the market, benchmark U.S. gas prices have fallen, averaging $3.20 per million British thermal units over the past five years, down from close to $9/mmBtu in 2008. Sustained bouts of cold weather this winter have eroded supplies, spiking prices to two-and- a-half-year highs near $4.50/mmBtu recently. […]