Investment banks are riding out the roughest spell in Southeast Asia in six years.

Slowing economies have led to the scrapping of several initial public offerings, a drought in mergers and acquisitions and a plunge in local currencies that has discouraged investors from buying stock and bonds issued locally. Revenues from investment banking in the region, which encompasses countries such as Singapore and Indonesia, had dropped 14% to $621 million as of mid-August, according to data provider Dealogic.