Billionaire investor Bill Ackman tells investors he will no longer engage in activist short-selling

• Hedge fund boss Bill Ackman told investors Tuesday his firm has "permanently retired" from activist short-selling. 
• The Pershing Square boss said the firm's "quieter constructive engagements" aren't as high-profile like its battle with Herbalife. 
• Ackman outlined his thoughts on the matter in his annual letter to investors. 

Hedge fund manager Bill Ackman told investors Tuesday is throwing in the towel on short-selling activism. 

Known for his crusade years ago against nutritional supplement maker Herbalife, Ackman in his annual letter said Pershing Square Capital Management is stepping back from such bets going forward. 

"Despite our limited participation in this investment strategy, it has generated enormous media attention for Pershing Square. In addition to massive amounts of media hits, our two short activist investments managed to inspire a book and a movie," he wrote in the annual letter. "Fortunately for all of us, and as importantly for our reputation as a supportive constructive owner, we have permanently retired from this line of work." 

Ackman said some of its investors over the last year have asked whether Pershing's approach has changed again as they perceive the firm to be a "quieter" investor as it has been five years since Pershing's last proxy contest. 

"To set the record straight, we have always sought to have positive, constructive engagements with our portfolio companies and their boards and management teams. Fortunately, we have largely succeeded, other than in the initial stages of a handful of proxy contests over the years," Ackman said. 

"Unfortunately, our quieter constructive engagements which characterize the substantial majority of our investments are less widely known because quiet, constructive engagements do not generate media attention, certainly when compared with proxy contests."