Mortgage securitization activity declined for the third consecutive quarter during the first three months of 2005, with the non-agency MBS sector continuing to outpace the agency side. Total issuance of mortgage- and asset-backed securities dropped to $562.48 billion during the first quarter of 2005, according to preliminary numbers compiled by Inside MBS & ABS. Although that was down 5.0 percent from the $592.37 billion issued over the last three months of 2004, it was 5.7…