NEW YORK (Reuters) - Circulation fell at many U.S. newspapers in the six months to September, according to statistics released on Monday that for the first time include Internet readership in a bid by publishers to boost their attractiveness to advertisers.

Average daily paid circulation for newspapers printed Monday through Friday fell 2.6 percent and Sunday circulation fell 3.5 percent for the six-month period that ended September 30, 2007, compared with the year before, according to publishers' statistics released by the Audit Bureau of Circulations.

In the top 25 markets, average daily paid circulation fell 2.4 percent, while Sunday circulation fell 4.6 percent.

The declines come as readers move online, but they also stem from publishers' efforts to cut discounted copies from their subscription rolls, said a spokeswoman for the Newspaper Association of America.

Some papers, particularly in California and Florida, are dealing with the weak housing market, while others face their own regional trends, such as in Michigan where papers have cut jobs as they serve markets hurt by the slumping auto industry.

Most big city dailies reported that average daily paid print circulation fell. Dow Jones & Company Inc said daily circulation at The Wall Street Journal, including paid subscriptions to its Web site, dropped 1.5 percent, while The New York Times fell 4.5 percent.

The New York Daily News and New York Post reported circulation declines, with the Post down 5.2 percent and the Daily News down 1.7 percent.

Advertisers long have considered print circulation key to determining where they spend their dollars, but publishers hope the Web numbers will provide a better picture of the true reach of newspapers.

"We generally agree that we can now truly gauge the impact of newspapers across the variety of media platforms that they truly represent," said Dave Walker, chief executive of Newspaper Services of America, which buys ad space in papers.

Gannett Co Inc reported a 1 percent rise in daily paid circulation at USA Today, while the Philadelphia Inquirer said circulation rose 2.3 percent.

The Washington Post reported a 3.23 percent drop, while the Chicago Tribune fell 2.9 percent. Its parent company Tribune Co said circulation fell at Long Island, New York's Newsday, but rose 0.5 percent at the Los Angeles Times.

The new data includes the number of people estimated to read a paper, not just how many papers were sold.

Many also are reporting usage of their Web sites, as well as a figure that tries to count print and Web site use without counting people twice who use both.

Gannett's Asbury Park Press in New Jersey has average daily print circulation of 144,072. But it claims more than 650,000 print readers, more than 850,000 unique Web users and some 832,000 people, without counting people who read both editions more than once.

It is hard to say if advertisers will buy the new numbers.

Alan Mutter, a former newspaper editor who writes a blog on newspaper and media issues called Reflections of a Newsosaur, said many newspaper Web site visitors to not remain long enough to make them worthwhile for advertisers.

"I think it's certainly valuable in the sense that advertisers have something they can count on and make reasonable judgments about," he said. "The question is, are they going to like the numbers they see?"

Edward Montes, managing director for Havas's online media buying arm Media Contacts, said online-only ad buying at local papers has never been very effective.

"The increase in the data points allows us to more accurately project and measure, and make sure we're spending our client's money effectively," said Montes, whose clients include Sears Holding Corp, Fidelity Investments and Goodyear Tires. "They're saying print is declining, but oh, by the way, our online properties are increasing, but I don't think it will change the buying dynamic significantly."

(Editing by Maureen Bavdek/Tim Dobbyn)