Billionaire investor George Soros's hedge fund slightly increased its holdings of common shares in the SPDR Gold Trust (GLD), a gold-backed exchange-traded fund, and bought more shares of Delta Air Lines Inc. (DAL) in the fourth quarter, according to a regulatory filing late Monday.

Soros, who has said that gold is the "ultimate bubble," boosted his direct ownership stake in the SPDR Gold Trust by 24,800 shares to 4.7 million shares valued at $655 million.

However, Soros, who previously had call options on 75,000 shares of the exchange-traded fund, disclosed no such options in the Monday filing with the Securities and Exchange Commission.

Overall, the value of Soros's stock holdings rose to $7.7 billion at the end of the fourth quarter, up from $6.7 billion on Sept. 30, 2010.

Soros Fund Management more than tripled its stake in Delta Air Lines to 14.7 million shares, valued at $185.6 million at the end of the quarter.

Other boosted holdings include InterOil Corp. (IOC), in which the fund now holds a 4.4 million-share-position valued at $319.5 million on Dec. 31.

In a reversal from the third quarter, Soros reduced his stake in Monsanto Co. (MON) and AT&T Inc. (T). Soros disclosed owning 3.3 million shares of Monsanto and 1.1 million shares of AT&T.

Many investors that manage more than $100 million are required to file with the SEC Form 13-Fs with their stock holdings within 45 days of the end of a given quarter, giving the public its freshest possible glimpse into the portfolios of well-known money managers.

-By Brett Philbin, Dow Jones Newswires; 212-416-2173; brett.philbin@dowjones.com