Thrift institutions let their portfolios of mortgage-related securities dwindle during the third quarter of 2003, content to feed on the burgeoning whole-loan market instead. According to data from the Office of Thrift Supervision, industry holdings of mortgage-related securities dropped from $148.0 billion as of the end of June to $143.5 billion at the end of September. The sharpest decline was in agency pass-throughs, which