The foreign-exchange market proved its resilience to challenges ranging from tumbling profits to a rate-fixing scandal in figures published once every three years by the Bank for International Settlements.

Average daily trading dropped to $5.1 trillion in April, from a revised $5.4 trillion in the same month in 2013, the BIS said Thursday in its latest triennial survey of the currency market. If it hadn't been for the dollar's appreciation, that volume would actually have risen about 4%, according to the report. The use of swaps increased, widening its lead over spot trading, or dealing in the currencies themselves rather than derivatives.

The size of the drop in currency trading is potentially a surprise for an industry blighted by losses from the Swiss National Bank's shock decision to scrap its exchange-rate cap last year, as well as accusations of price-rigging that have led to more than $10 billion in fines and penalties. While individual central banks recently reported a recovery in trading in the six months through April, that followed a steady drop-off in activity since 2014.