(Bloomberg) -- The $30.6 billion Vanguard Dividend Growth Fund has a problem a lot of mutual funds run by stock pickers would like to have: it is attracting too much money.

Vanguard Group, the world's largest mutual fund company, Wednesday said in a statement that it's closing Dividend Growth to new investors to protect the fund's returns. Existing shareholders will still be able to add money to the fund.

Dividend Growth, run by Donald Kilbride of Wellington Management, attracted $3 billion over the past six months and its assets have almost doubled over the past three years, Vanguard said. The fund, which invests in dividend-paying equities, beat 83% of peers this year and 86% over five years, according to data compiled by Bloomberg.

Vanguard Chief Executive Officer William McNabb said the decision to close the fund was made "to help ensure that the advisor's ability to produce competitive long-term results for investors is not compromised."

Several other Vanguard funds, including the $12.5 billion Vanguard Capital Opportunity Fund and the $44.9 billion Vanguard Primecap Fund, are also closed to most new investors.

Active managers have struggled to win assets as investors have migrated to mutual funds and exchange-traded funds that mimic indexes. In the 12 months ended June 30, actively run funds suffered redemptions of $317 billion, while passive funds gained $373 billion in deposits, data from Morningstar show.

Vanguard, which manages $3.6 trillion, attracted a record $148 billion in new client money during the first six months of 2016.

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