Credit unions are a depository nightmare for commercial banks and thrifts, which rail against their preferential tax treatment and increasingly lax membership requirements. But one market in which credit unions pose no challenge at all is their investment in mortgage securities. Federally insured credit unions held just $24.01 billion of mortgage securities in portfolio as of the end of last year, according to data from the National Credit Union Administration. That was down 15.0 percent… [One data table included]