Fink was awarded $25.8 million in compensation in 2015, compared with $23.9 million in 2014, based on a calculation of his pay according to U.S. Securities and Exchange Commission guidelines.

But the company's own calculations of Fink's pay show his compensation flat for 2015. The calculations vary because BlackRock reports some incentive pay in a different year, according to the filing.

BlackRock's filing also shows the company will face an unusual test of its oversight of CEO pay at other companies, where its funds are often large investors and routinely vote in favor of their executive compensation.

Fink's pay included a $900,000 salary, a bonus of $8.7 million, nearly $16 million in stock awards and $193,000 in "other" compensation, according to that calculation.

BlackRock President Rob Kapito was paid $20 million, the filing said. Fink, 63, and Kapito, 59, were among BlackRock's founders in 1988.

Last year was difficult for money management firms, which were whipsawed by volatile capital markets.

The BlackRock board committee that analyzes compensation concluded among other factors that the company's share of key markets increased and that the company showed "strong and consistent financial results despite a challenging" environment, according to the filing.

BlackRock's stock traded nearly 5 percent lower at the end of 2015 than at the end of 2014, while a grouping of such companies measured by the Dow Jones U.S. Asset Managers Index <.DJUSAG> fell by 12 percent. Over the year, the stock returned negative 2.3 percent, a figure that includes dividend payouts.

Net income of New York-based BlackRock rose 2 percent in 2015 to more than $3.3 billion in a year of flat growth in assets under management for the company, which oversaw more than $4.6 trillion as of Dec. 31, 2015.

Separately, a proxy resolution on which BlackRock's investors will vote this year calls on the company to report on its own proxy-voting practices, citing among other things that BlackRock supports CEO pay more often than other investment managers.

In Friday's filing BlackRock's directors urged investors to vote against the proposal, calling it unnecessary since the company already discloses much about how its Stewardship Team determines votes and that such a proposal could "threaten the independence" of that team.

(Reporting By Sudarshan Varadhan in Bengaluru, Trevor Hunnicutt in New York and Ross Kerber in Boston; Editing by Anil D'Silva, Leslie Adler and Bernard Orr)

By Sudarshan Varadhan, Trevor Hunnicutt and Ross Kerber