Nov. 15 (Bloomberg) -- The net asset value of structured investment vehicles, companies that borrow short term to buy higher yielding securities, has fallen to 69.7 percent as the credit slump erodes their holdings, Fitch Ratings reported.

The amount that would be left after selling SIV assets and repaying debt dropped from 71 percent in the past month, data compiled by Fitch show. SIV holdings of nonprime U.S. mortgages and debt guaranteed by so-called monoline insurers fell the most, Fitch said.

``The worst offenders have been monoline-wrapped bonds, as well as investment bank debt and commercial mortgage-backed securities,'' said Patrick Clerkin, a senior director at Fitch Ratings in an interview.

SIVs have been forced to sell about $75 billion of investments since July as record U.S. home foreclosures caused investors to withdraw from asset-backed commercial paper. Citigroup Inc., the largest manager of SIVs, is working with the U.S. Treasury, Bank of America Corp. and JPMorgan Chase & Co. to create an $80 billion fund to help SIVs avoid dumping their holdings and further roiling credit markets.

The average market value of the assets held by SIVs fell to 97.7 percent from 97.8 percent, based on the SIVs Fitch rates, including three run by New York-based Citigroup.

Bonds guaranteed by monoline insurers fell to 94.6 percent from 95.8 percent on concern the companies may lose their top credit ratings because of losses on subprime mortgages rose.

Securities sold by investment banks declined to 96.5 percent from 97.5 percent.

The data doesn't include Rhinebridge Plc, the SIV set up by Dusseldorf, Germany-based IKB Deutsche Industriebank AG that was forced to sell assets to repay debt. Fitch, based in New York, said it hasn't received information on Rhinebridge since the SIV was forced to appoint Deloitte & Touche LLP as a receiver on Oct. 23 to protect investors. The SIV had a zero net asset value last month, Fitch said.

To contact the reporter on this story: Neil Unmack in London at nunmack@bloomberg.net

Last Updated: November 15, 2007 09:51 EST