NEW YORK (CNN/Money) - A year ago, Pittsburgh radio station WXDX-FM was near the top of its game on weekday mornings. Today it's fighting hard for listeners.

What changed? Two words: Howard Stern.

A year ago next month, WXDX and five other stations owned by Clear Channel pulled the shock jock off the air in response to a government backlash against smutty programming. For Clear Channel (Research), the country's largest radio operator, the risk of losing valuable licenses to keep Stern on the air wasn't worth it.

But the Clear Channel radio stations have since paid a steep price. All six have lost a significant number of weekday morning listeners, according to new data from Arbitron, the country's top radio ratings service.

Arbitron tracks radio listeners by quarter and its latest available data, released this week, show that WXDX ratings during the morning hours have fallen by more than half from the ratings period before Stern was kicked off the air to mid-December. Third in the market a year ago, the station now ranks No. 11 during Stern's former 6 to 10 a.m. time slot.

Similarly, WTKS-FM in Orlando, Fla. has dropped from No. 2 to No. 8 and WNVE in Rochester, N.Y. has slid from sixth to fourteenth place on a ratings freefall. Once No. 5, San Diego, Calif.'s KIOZ is now tied for 20th place.

The lost market share only shows that Stern, love him or hate him, is "one of the biggest radio attractions in the modern era," said Michael Harrison, the publisher of Talkers, a monthly radio talk show trade magazine. "He's a phenomenon."

Ratings are important because they are used throughout the entertainment industry to establish advertising rates, which, in turn, affect revenues. For Clear Channel, the loss is minimal given that it owns more than 1,200 radio stations.

But the listener losses point to the big challenges confronting No. 2 radio station operator Infinity Broadcasting as it braces for Stern's much-publicized exit later this year. Stern said in October that he would join Sirius Satellite Radio, a subscription-based radio service, early next year in a five-year deal valued at $500 million.

In announcing the move, Stern expressed frustration with the indecency crackdown that led to his firing from Clear Channel and was putting pressure on Infinity, a Viacom (Research) subsidiary, to tone down his salacious show.

Stern, according to some industry analysts, has been a huge revenue generator for Infinity and the 27 stations that air his show.

Indeed, when Infinity added Stern to four stations in New York, California and Florida last summer, ratings jumped. In the Orlando market, Infinity's WOCL, for instance, has since shot from No. 17 on the eve of Stern's addition to No. 1, according to Arbitron. San Diego's KPLN is now ranked fourth, up from No. 17 before. Stations in Rochester and West Palm Beach also posted solid, albeit less dramatic, gains.

Infinity added Stern to five other markets last year, but Arbitron ratings data for them were not readily available.

Stern's looming exit from terrestrial radio has led to many grim -- arguably overblown -- predictions about what it portends for the future of free AM-FM radio. Radio operators have struggled to attract marketers amid a broader advertising rebound, but analysts agree that the industry's problems are bigger than Stern.

Infinity, however, is looking at a large revenue hole without Stern. While the company is publicly mum about its post-Stern plans, Infinity CEO Joel Hollander suggested in December that the company doesn't lack for interest among celebrities and other wannabe successors.

One name floating in industry circles is Erich "Mancow" Muller, a Chicago radio personality whose "Mancow Morning Madness" is headed for syndication. Mancow, himself a former shock jock, is now trying to position himself as Stern's heir apparent, but without the obscenity.

"He's taking it to the edge without going over the edge" and putting station licenses at risk, said Harrison.

Harrison thinks Mancow has a shot at replacing Stern, but not anytime soon. As Clear Channel and Infinity have both been reminded his year, wherever Stern goes the ratings quickly follow.

"If someone had come along and instantly maintained the ratings that Howard Stern had, that would have been looked at as the second coming of Christ," joked Harrison.