SAN DIEGO — A private equity firm that includes Honolulu Star-Bulletin owner David H. Black, is buying the San Diego Union-Tribune for an undisclosed price, the newspaper announced today.

Copley Press Inc. agreed to sell the dominant newspaper in the nation's eighth-largest city to Platinum Equity, based in Beverly Hills, the newspaper reported on its Web site. Copley put the newspaper up for sale in July 2008.

Platinum said its team includes Black, whose company, Black Press Ltd. of Canada, owns dozens of community newspapers, mostly in western Canada, and has expanded in the United States with acquisitions of the Honolulu Star-Bulletin in 2000 and the Akron (Ohio) Beacon Journal in 2006. Black's role was not specified.

"The Union-Tribune is a premier media property in an outstanding market," Black said in a statement.

Louis Samson, a principal at Platinum Equity, called the newspaper "a good fit."

"The Union-Tribune is more than a business. It's an institution in San Diego," Samson said.

"But it faces enormous challenge in a period of tremendous upheaval for the newspaper industry. We will bring a strong operational focus that helps ensure the Union-Tribune not only survives in this market, but thrives."

Like much of the newspaper industry, the Union-Tribune's fortunes have sagged as readers have migrated to the Internet.

Publisher Gene Bell told employees in January that the Union-Tribune's advertising revenue had fallen 40 percent since 2006. He said then that the newspaper faced a "challenging" environment in its search for buyers, forcing unpaid furloughs and other cost-cutting measures.

The Union-Tribune won a Pulitzer Prize in 2006 for its reporting on the bribery scandal that landed former Republican Rep. Randy "Duke" Cunningham in federal prison. Since then, it has made drastic cuts to its staff that included closing its Washington, D.C., bureau.

The sale is expected to be completed during the second quarter.

Some newspaper buyouts by private equity firms haven't panned out well.

The Star Tribune of Minneapolis filed for bankruptcy protection in January, burdened by heavy debt it took on when it was bought by Avista Capital Partners LP in March 2007.

The owner of Philadelphia's major daily newspapers filed for bankruptcy protection last month. Philadelphia Newspapers LLC publishes The Philadelphia Inquirer and Philadelphia Daily News.

The Union-Tribune reported that other companies known to have considered buying the newspaper include Tribune Co., publisher of the Chicago Tribune and Los Angeles Times, and MediaNews Group Inc., publisher of the San Jose Mercury News and The Denver Post, and Los Angeles billionaire Ron Burkle's Yucaipa Cos.

The sale ends the Copley family's 80-year run in the San Diego news business.

In 2006, Copley sold the Torrance, Calif., Daily Breeze and three Los Angeles-area weekly newspapers to Hearst Corp. for $25.9 million. Hearst transferred the papers to MediaNews. Copley sold nine newspapers in Illinois and Ohio to GateHouse Media Inc. for $380 million in 2007.