Goldman Sachs Group (GS - Get Report)  said first-quarter profit fell less than expected, as fees from advising on mergers and acquisitions helped to mitigate an abysmal performance from the Wall Street bank's juggernaut trading business.   

But the Wall Street firm's shares tumbled after analysts said the outperformance was driven by accounting decisions that might not be repeated in future quarters, including a low tax rate and less-than-estimated compensation costs.

Net income fell by 21% from a year earlier to $2.25 billion, the New York-based bank said Monday in a press release. Earnings per share were $5.71, beating Wall Street analysts' average estimate of $4.89.

Investment-banking fees climbed 1% from a year earlier to $1.81 billion, though revenue from trading bonds, commodities and currencies declined by 11% to $1.84 billion, and stock-trading revenue tumbled 24% to $1.77 billion.

"We are pleased with our performance in the first quarter, especially in the context of a muted start to the year," CEO David Solomon said in the press release. 

The trading decline stemmed from lower revenue in government bonds, corporate bonds, foreign exchange and stock derivatives, partly offset by an improved performance in mortgages and commodities. 

"Commissions and fees were lower, reflecting lower market volumes," according to the press release. 

Goldman Sachs shares fell 3.2% on Monday to $201.10.

Brian Kleinhanzl, an analyst at the brokerage firm Keefe, Bruyette & Woods, wrote in a note to clients that Goldman Sachs beat the earnings estimates partly because of lower-than-expected compensation expenses, which were 37% of revenue, compared with his forecast for 41.5%.

Analysts monitor quarterly compensation expenses closely, since many Goldman Sachs employees are paid via year-end bonuses, which means the bank could just make up for the shortfall later in the year.  

The company's effective tax rate during the quarter, at 17.2%, was well below the official U.S. corporate rate of 21%, resulting in another source of lower-than-expected expenses, Kleinhanzl wrote.

"GS posted a beat, but it was driven by lower expenses and a tax rate," according to Kleinhanzl. "Management did not change full-year guidance for the tax rate, which was 22% to 23%, so there is no go-forward benefit from this quarter's low tax rate."

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