Ackman's Pershing Square plunges 13% in August

Hedge fund mogul William Ackman's Pershing Square Holdings portfolio fell 13.1 percent this month, leaving the fund down 4.3 percent for the year, the firm said on Thursday.

The performance numbers cover returns through Tuesday, Aug. 25.

On Wednesday, Ackman told investors that his firm's roughly 10 percent gain through July had turned into a loss amid the market rout. He did not say exactly how much the fund had lost.

Read MoreAckman posting a loss after market decline

On Thursday, when stocks rose steeply for a second day in a row, several of Ackman's holdings saw strong gains including Valeant and Canadian Pacific, suggesting to some investors that his portfolio's performance may still improve before month-end numbers are finalized next week.