“We can’t be certain as to when or whether the volume is going to recover,” said Lee Shavel, chief financial officer at the Nasdaq OMX Group.

The recent slowdown has occurred not only on the nation’s 13 official exchanges and trading platforms. Dozens of off-exchange operations have captured a larger proportion of all stock trades in recent years, but even their overall trading numbers have been trending down.

The decline in trading has not sent the prices of stocks down. Though there is less buying and selling, the people who have remained in the market are willing to pay higher prices, driving the value of the benchmark Standard & Poor’s 500-stock index up 102 percent since the market hit a bottom in the spring of 2009.

But the recent falloff in trading is striking because data from the New York Stock Exchange shows that volumes have not declined for three consecutive years in records going back to 1960. For an explanation of the lower trading volumes, many market-watchers have looked to the high-speed traders, who use computers algorithms to take advantage of small price discrepancies and who have accounted for an increasing share of all trading in recent years.

These firms have been curtailed slightly by recent regulations aimed at making the markets less volatile. But more fundamentally, industry participants say high-speed traders rely on transacting with slower, traditional traders like retail investors and mutual funds. When those groups pull back, the high-speed firms have little choice but to scale back as well.

“On a typical trade, two high-frequency trading firms will not trade against each other,” said Manoj Narang. His New Jersey high-speed trading firm, Tradeworx, is still growing, he said, but for most established firms, if ordinary investors “don’t want to trade, there’s really simply nothing for us to do.”

Among retail investors, the most reliable source of trading volume has been the day traders who were given access to cheaper trading by discount brokers like E*Trade and TD Ameritrade.