The longest U.S. government shutdown on record is consigning currency traders to their worst year in decades as a dearth of economic data clouds the outlook for the dollar.

Foreign-exchange investors are on course for the poorest annual performance since 2005, according to a BarclayHedge index. The pinch was already being felt on Wall Street before the data vacuum, with Goldman Sachs Group Inc., Morgan Stanley and Bank of New York Mellon Corp. among those reporting a drop in currency trading revenues last quarter.

Amid the federal shutdown, crucial economic and market positioning statistics have not been published in weeks. That's made traders less willing to stake big bets on where the dollar is headed, computer-driven quantitative funds have less high-quality data to run on, and strate

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