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NEW YORK (CBS NEWS) – Jana Partners has upped its stake in Whole Foods Market Inc. as the hedge fund looks to shake up the board and senior management of the high-end grocer and push for a sale of the company that has been losing ground in the natural and organic niche it popularized.

Whole Foods’ shares, which have lost nearly half their value since early 2015, jumped almost 10 percent to close at $34.17 on Nasdaq.

Jana announced a 8.3 percent stake in Whole Foods in a regulatory filing on Monday, up from the 7.15 percent it held at the end of last month, according to Thomson Reuters data.

The New York-based activist investor, which usually works behind the scenes, is mounting an increasingly public campaign for change at the Austin-based grocer.

Jana said in its filing that it had started a review of strategic options for Whole Foods in light of the company’s “apparent unwillingness to engage in discussions with third parties regarding such alternatives.”

Like other hedge funds mounting proxy contests, Jana has assembled a group of industry experts it feels can add hands-on knowledge to running the business.

Former Gap Inc Chief Executive Glenn Murphy, former Harris Teeter Supermarkets CEO Thomas “Tad” Dickson and former Barclays stock analyst Meredith Adler have agreed to be on Jana’s slate of nominees, the hedge fund said in its filing.

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