The total assets held by the top 100 alternative investment managers globally reached $3.1 trillion in 2012, over 60 percent of the total $5.1 trillion alternative assets under management worldwide, a survey said on Monday.

The survey by Towers Watson said that almost 40 percent of the $3.1 trillion was held by the top 25 managers in each of the seven asset classes taken into consideration.

Alternative assets such as private equity, real estate or infrastructure appeal to investors looking to spread risks away from the standard classes of stocks, bonds and cash where assets under management total about $84 trillion, according to TheCityUk consultancy.

They have become increasingly popular over the past ten years with a wide range of investors, according to Craig Baker, global head of investment research at Towers Watson.

"It is therefore not surprising that allocations to alternative assets by pension funds, for example, now account for around 19 percent of all pension fund assets globally, up from 5 percent 15 years ago."

The largest alternative investment manager was the Sydney-based Macquarie Group, with around $95 billion of assets under management. In second and third place were U.S. firms Bridgewater Associates with around $84 billion and CBRE Global Investors with $80 billion.

Real estate managers held the largest share of the total assets of the top 100 at 34 percent, followed by direct private equity fund managers who held 23 percent and direct hedge funds at 20 percent.

The fewest assets were held in infrastructure and commodities, both at 4 percent.

Pension fund assets were the most commonly held, representing 36 percent of the top 100 alternative managers' assets. This was followed by wealth managers at 19 percent and insurance companies at 9 percent.

"Pension funds have always been and will remain a very large investor group for top alternatives managers, but the demand from non-pension fund investors, such as insurers, endowments and foundations, and sovereign wealth funds, is only going to increase in the future," said Baker.