They may be fickle, but short-sellers took pressure off all kinds of ETFs in December, save for GLD.

Short-sellers, in contrast to November, appeared to be swept up by the “risk-on” trade that prevailed last month, reflecting a respite from worries about the eurozone debt crisis and the health of the world economy.

Every fund on IndexUniverse’s “Big Bets” list saw declines in short interest, with the exception of the SPDR Gold Shares (NYSEArca: GLD), the physical bullion ETF. And even the 2.5 percent increase in short interest on GLD was an indirect example of investors taking on risk. When fear abates, not only does the appetite for gold fall, but some investors take the next step and start shorting gold as well.

The number of PowerShares QQQ Trust (NasdaqGM: QQQ) shares shorted fell by 43 percent in December, the biggest percentage decline of any ETF on our “Big Bets” table last month. In the end, just about all the big funds had drops in shorting, including the SPDR S&P 500 ETF (NYSEArca: SPY). The number of SPY shares being sold short fell by more than 37 percent.

The ultimate example of how much investors’ moods changed in the period of just one month is clear in the sharp drop in short interest on the Financial Select Sector SPDR Fund (NYSEArca: XLF). In December, the number of XLF shares short fell by more than a quarter, just a month after they jumped by more than a third.

The big shift may reflect rekindled optimism that the world economy is stabilizing, or simply relief that worst-case scenarios, such as the demise of the euro, weren’t going to play out. Bond yields on sovereign debt by and large came off their highs last month. But given that today S&P cut France’s debt rating one notch from a risk-free “AAA” to “AA+,” it’s clear that the eurozone crisis is still playing out.

But even if it was last month’s story, short-sellers also took pressure off the CurrencyShares Euro ETF (NYSEArca: FXE). It’s one of the most heavily shorted ETFs on the market, which is why it’s on IndexUniverse’s “Really Really Short” table. But last month, the number of FXE shares being sold short fell by more than 35 percent.

Another ETF on the “Really Really Short” table, the Market Vectors Bank and Brokerage ETF (NYSEArca: RKH), saw the number of its shares being shorted fall almost in half last month. RKH, for those who may not recognize it, used to be the Merrill Lynch-sponsored Regional Banking HOLDRS.

Data is believed to be accurate; however, transient market data is often subject to subsequent revision and correction by the exchanges.