Ackman's Pershing Square off 15 percent for the year: source

But on Tuesday, a nearly 10 percent rise in the stock of Valeant Pharmaceuticals, Ackman's biggest holding, helped pare the decline, leaving the flagship fund off 15 percent for the year, the person said.

Pershing Square, which invests for pension funds, endowments and wealthy investors, ended 2014 as one of the industry's top- performing funds, with a 40 percent gain. This year's sharp drop in Valeant and losses at Platform Specialty Products are weighing on performance and have, for a time, ranked the firm among the industry's biggest losers.

As an activist hedge fund, the $15 billion firm takes concentrated bets on a handful of companies, ensuring that performance could be volatile, investors have said.

The average hedge fund has lost 2.3 percent this year.

(Reporting by Svea Herbst-Bayliss, editing by Richard Valdmanis and Dan Grebler)