Fannie Mae and Freddie Mac issued $261.59 billion in single-family mortgage-backed securities during the fourth quarter of 2011, a booming 47.6 percent improvement from a modest third quarter that followed two straight quarterly declines during the first six months of 2011.The recently completed October-December cycle represented the highest quarterly production level of the year, but it still came up 21.2 percent short of the volume generated during the fourth quarter of 2010.For the year, GSE single-family securitizations were down 12.7 percent from the volume generated during 2010.