Macau’s top gaming stocks lost a record $18.4 billion in combined market value on Wednesday after officials said they would change casino regulations to tighten restrictions on operators, including appointing government representatives to “supervise” companies in the world’s biggest gaming hub.

The Bloomberg Intelligence index of the six big casino operators fell a record 23%. American operators saw the worst selloffs, with Sands China Ltd. sinking as much as 33%, while Wynn Macau Ltd. plunged 34%, both the steepest declines ever. Galaxy Entertainment Group slumped 20%, its sharpest drop in a decade.

The sector also led declines in China’s dollar bond market.