(Adds fund manager comments, details on Pershing Square) By Svea Herbst-Bayliss and Richard Valdmanis BOSTON, Sept 2 (Reuters) - Hedge fund mogul William Ackman, one of last year's best performers, joined a string of high profile managers in reporting deep losses for August as global markets tumbled amid fears over slower growth in China. Ackman's Pershing Square Capital Management, which makes concentrated bets on a small number of stocks, told clients on Wednesday that its Pershing Square LP fund fell 7.7 percent last month, leaving it flat on the year with a 0.06 percent gain. The firm's Pershing Square Holdings portfolio dropped 9.2 percent, and is now down 0.1 percent for the year, erasing a 10 percent gain it had at the end of July, two sources familiar with the numbers said. Last year the fund gained 40 percent, beating the Standard & Poor's 13.7 percent gain. Ackman warned investors in a letter last week that... More