Morgan Stanley reported a surprising third-quarter loss, suggesting the bank is losing hard-won ground in the battle with Goldman Sachs for Wall Street supremacy.

The firm’s $91 million loss, on weak volumes during one of the most difficult trading quarters in recent memory, came a day after Goldman overcame those same conditions to beat Street estimates with a $1.9 billion profit.

Morgan Stanley cut the investment bank’s bonus pool 8 percent this year. Pay at the firm’s institutional securities unit, which includes traders and bankers, dropped to $5.3 billion in the year’s first nine months as revenue there rose 32 percent to $12.7 billion, according to its Web site.

As expected, Morgan Stanley is restructuring its ownership of hedge fund unit FrontPoint Partners LLC, in part to comply with the reform law’s “Volcker rule” on hedge fund assets.