Bill Gross, the bond manager who joined Janus Capital Group Inc. last year, said there’s “no liquidity in bond markets,” with small trades pushing around prices in the world’s biggest debt market.

“Treasury bonds move 2-3 ticks on even small trades,” Gross wrote in a Twitter message on Tuesday. A tick is an incremental move in bond prices.

While the U.S. government bond market has almost tripled since 2007 to $12.5 trillion at the end of last year, average annual trading volumes have fallen 11 percent in the period, according to data compiled by the Securities Industry and Financial Markets Association. Activity has slowed as central banks hoard the debt and Wall Street firms use less of their own money to facilitate bigger trades.

Regulators are growing increasingly concerned that the U.S. Treasury market, which sets benchmark rates for everything from mortgages to corporate debt, is more vulnerable to unpredictable swings. On Oct. 15, yields on 10-year Treasuries plunged the most since 2009, without an obvious catalyst.

After data was released today showing slower growth in the U.S. than analysts expected, yields on the debt rose 2.9 percent, much to the confusion of many analysts.

‘Just Because’

Jim Vogel, an interest-rate strategist at FTN Financial, explained the volatile selling as “just because” in a research note Wednesday.

Gross, who abruptly left Pacific Investment Management Co. in September, now manages the $1.46 billion Janus Global Unconstrained Bond Fund. The fund has returned 2.72 percent this year, beating 87 percent of comparable funds, according to data from research firm Morningstar Inc.

He’s previously cited a lack of liquidity -- or the ease to buy and sell securities -- in the corporate debt market, especially high yield. Investors have complained that regulation of banks since the financial crisis has reduced their ability to make markets.