Net short bitcoin contracts rose to 1,801 in the week that began Dec. 26, from 1,507 short contracts the previous week.
During the last two weeks of December, bitcoin in the spot market slumped nearly 28 percent, but still ended the year up about 1,300 percent.
On Tuesday, bitcoin recovered from recent losses to trade up 3.4 percent at 13,900 <BTC=BTSP> on the Bitstamp platform.
Trading in bitcoin has tended to be volatile. In November, it tumbled almost 30 percent in four days from $7,888 to $5,555. In September, it fell 40 percent from $4,979 to $2,972.
ARK Invest, a New York-based asset manager which oversees exchange-traded funds focused on technology, said in a newsletter that investors had displayed "irrational behavior" with respect to bitcoin, including speculators going on margin to invest in the digital currency.
"Historically, these signs have marked the top, either cyclically or securely, for an asset class," said ARK Invest.
Still, it said the launch of a futures market for bitcoin by CBOE and CME Group Inc last month potentially heralded another era of financial innovation.
It said that if blockchin technology, an open ledger that has underpinned bitcoin, is as fundamental a breakthrough as the internet was when it was first launched, then institutions will start investing significantly alongside retail investors over the next six to nine months.
(Editing by Bernadette Baum)
By Gertrude Chavez-Dreyfuss