By Michael Flaherty

NEW YORK (Reuters) - Starboard Value, the activist hedge fund behind shake-ups at Yahoo Inc <YHOO.O> and Marvell Technology Group Ltd <MRVL.O>, has notched an investment return of more than 10 percent this year on the back of boardroom deals and takeovers.

The Starboard Value and Opportunity Fund has produced a net return for its investors of 10.21 percent through Sept. 30, according to a document seen by Reuters.

The performance is slightly below that of the Russell 2000 index in the same period and above the 6.71 percent posted by peers in the HFRI Event-Driven Index.

Starboard's year-to-date "top winners," according to the document, are network security company Infoblox Inc, which was recently bought by private equity firm Vista Equity Partners, and Depomed Inc <DEPO.O>, a drugmaker whose board the activist investor threatened to overthrow.

Starboard and Depomed struck a truce in October, with the hedge fund getting three seats on the board. At the time of the deal, Depomed's stock was up more than 40 percent from a year before, at $27 per share.

Starboard's net returns to investors was a loss of 8.55 percent in 2015 and a gain of 21.45 percent in 2014, according to the document.

The fund's performance through the third quarter stands out given the struggles of other activist investors this year. The HFRI Activist Index has a return of only 2.33 percent year to date.

Starboard's so-called tear sheet uses the HFR Event-Driven Index as its peer comparison.

The hedge fund's most recent quarterly filing shows an increase in smaller bets it has placed on companies already under attack by activists pushing for a sale.

The New York-based fund, led by Jeffrey Smith, does not publicly disclose its investment returns. Starboard, which manages around $4 billion, did not reply to requests for comment.

The Starboard document shows that it underperformed the Russell 2000 index in 2013 and 2012, and beat the index in 2011 with a 7.65 percent net return.

Data from Lazard's Corporate Preparedness group show that Starboard has launched five campaigns this year and won 14 board seats through September, the most seats gained by any activist hedge fund.

Four of the seats came from an agreement it struck in April with Yahoo, before the internet company agreed to be bought by Verizon Communications Inc <VZ.N> in July for $4.8 billion. Starboard's Depomed deal in October took its board seat total for the year to 17.

Another Starboard target company, Infoblox, struck a deal in September to be bought for $26.50 per share, or $1.6 billion overall. Filings show that Starboard paid around $15 per share for its 7.1 percent stake.

Another winner for Starboard this year is semiconductor company Marvell, where it gained three board seats in April. Its stock has risen 60 percent to $13.44 since last November.

Starboard's negative return in 2015 appears to stem largely from share losses in some of its largest positions, including Yahoo and auto parts maker Advance Auto Parts Inc <AAP.N>.

Before Yahoo's stock began its climb this year, ahead of the Verizon deal, the shares dropped by a third to $34 in 2015.

Advance Auto Parts, where Starboard is represented on the board and owns 3.8 percent of the company, saw its shares slide 5 percent in 2015.

(Editing by Carmel Crimmins and Bill Rigby)