"Pimco Total Return has been a large driver of all bond fund flows," Timothy Strauts, an ETF analyst at Morningstar, said in an interview. "The poor performance of that fund may have something to do with it, but also as yields have fallen, lower fees become more and more important."

Gross, who runs the $244 billion Pimco Total Return Fund, has advanced 1.9 percent this year through Nov. 25 at less than one-third the pace of the benchmark, in part because of his avoidance early this year of U.S. Treasuries. The $18.8 billion Loomis Sayles Bond Fund, co-managed by Fuss and Kathleen Gaffney, rose 0.8 percent, according to data compiled by Bloomberg. The Loomis Sayles fund holds no Treasuries.

Gross earlier this year announced plans to open his first actively managed ETF, the Pimco Total Return Exchange-Traded Fund, as Newport Beach, California-based Pimco seeks to expand in the fastest-growing segment of the industry. The new ETF would charge annual fees of 0.55 percent, the firm said in a July filing. The institutional share class of Gross's mutual fund, available in some retirement accounts, carries an annual expense ratio of 0.46 percent, while the firm's most widely used retail share classes charge fees ranging from 0.75 percent to 0.85 percent.

Industrywide, the number of bond ETFs has grown from just six in 2006 to more than 160, according to data from Washington- based ICI, including those that invest in government bonds, emerging markets, high-yield bonds and other specific categories.

"Bond index funds have been the unsung heroes in the index wars," Dan Wiener, chairman of Adviser Investments in Newton, Massachusetts, said in an interview. "The worry for active managers should be whether their cost structure is too high and whether they are adding value."

The average intermediate-term bond fund has returned about 4.7 percent this year through Nov. 25, according to Morningstar. That compares with the 6.8 percent advance in Barclays Capital U.S. Aggregate bond market index in the same period.

Many of the bond-fund managers were hurt this year as the sovereign-debt crisis in Europe combined with concerns that the U.S. economy is slowing sent investors to the safety of government-backed securities.

"I think you're going to see a huge movement into indexation of bonds, much lower fees," BlackRock's Chief Executive Officer Laurence D. Fink said during a Oct. 19 conference call with investors and analysts. "If you don't believe interest rates have that much further to go down over the next few years, you're only earning a few hundred basis points of return."

Legg Mason's CEO Mark Fetting also noted that clients increasingly shifted to passive bond products. Legg Mason, based in Baltimore, manages active stock and bond funds. In an Oct. 27 call with investors and analysts, Fetting said he noted a "couple of clients moving to passive."

"Although, if we get into a more sustained, less volatile environment, I think clients will appreciate the value that comes from active strategies," Fetting said.