Billionaire hedge fund investor William Ackman, whose funds have suffered double-digit losses this year, on Tuesday told investors that 2015 could be his firm’s worst ever, but said clients are generally sticking with him. “If the year finishes with our portfolio holdings at or around current values, 2015 will be the worst performance year in Pershing Square’s history, even worse than 2008 during the financial crisis,” Ackman wrote in a 17-page letter that was seen by Reuters.