Pandora’s monthly listening caps pulled down listener hours in April without resulting in a corresponding fall in unique listeners, the company’s CFO revealed Monday evening.

The Internet radio business’ listener hours dropped 12% to 1.31 billion in April from 1.49 billion in March, CFO Michael Herring said during a webcast of a Morgan Stanley Internet Bus Trip. The caps of mobile listening didn’t have the same impact on the number of people who listened to Pandora, however. Active listeners rose to 70.1 million in April from 69.5 million in March.

The drop in listener hours didn’t surprise Pandora. In fact, “the intended impact was actually realized” by limiting the amount of hours its users could listen on smartphones and tablets, Herring said. He added that the implementation of the caps had “almost the same impact” as the one placed on desktop listening in 2009 while droving subscription conversion, although he did not provide details on the latter point.

Investors had a different interpretation of the news, however. Shares of Pandora were down over 5% in Tuesday morning trading. Investors are clearly concerned that the drop in listening hours could have a negative effect on advertising revenue. The full impact of the listening cap won’t be known until Pandora releases its first fiscal quarter earnings on May 23rd. In spite of Tuesday’s drop, shares of Pandora are 9% higher than the close on the day the listening caps were announced in February and are up nearly 52% in 2013.

In order to moderate the amount of royalties it pays, Pandora implemented a 40-hour-per-month cap on mobile listening hours in March. Pandora pays a statutory royalty rate for each stream whether or not it is coupled with an advertisement and regardless of the platform on which the music is heard. The company decided to limit mobile listening because mobile advertising is less lucrative than desktop advertising. The company expected the monthly cap would affect 4% of monthly active users that represent the service’s heaviest listeners.

When the cap was announced in late February, founder Tim Westergren called the cap “a very unusual thing to do” but explained the company’s per-track royalties “have increased more than 25% over the last 3 years, including 9% in 2013 alone and are scheduled to increase an additional 16% over the next two years.” [Billboard.biz]