BlackRock CEO Laurence Fink says the large trading profits big banks have reported for the second quarter aren’t as impressive as they sound.

The banks are simply taking advantage of reduced competition to charge their customers more for executing trades.

“There are fewer players. There is very little capital being committed by these dealers,” Fink told the Financial Times.

“They’re just taking the spread between the bid and the asked (the price gap between buyers and sellers), and they are making very luxurious returns.”

So it’s not a matter of banks coming up with brilliant trading strategies, he says. Rather, it’s just a matter of banks ripping off their customers.

Fink says that at BlackRock he seeks ways to lower the spread and thus save money for the firm’s clients.

Trading profits have been a boon for major banks such as Goldman Sachs and JPMorgan Chase this year, offsetting growing losses in their consumer credit businesses and sagging activity in mergers, acquisitions, and private equity.

Fink says that, in the past, Wall Street was relied upon “to be the safety guards to the capital markets.”

“In many cases they have dropped that responsibility, and now it is falling on some of the large investment management firms to make sure the securitization market is being protected.”

Others, however, remain bullish on bank stocks.

Goldman Sachs strategist David Kostin, for example, wrote to customers: “We expect these sources of earnings strength to continue.”