Charter Communications' $78 billion proposed takeover of Time Warner Cable was recently approved with conditions by the U.S. Department of Justice. But not all mega-mergers may be so lucky in 2016.

The Federal Trade Commission and other governmental agencies are scrutinizing M&A deals more closely than ever. That has already stymied or delayed a few high-profile transactions, including Halliburton-Baker Hughes and Pfizer-Allergan. This trend could dent investment bankers' high fees. 

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Acting as financial advisors to many proposed mergers, Wall Street banks have traditionally earned millions of dollars as consultants. This occurs, regardless of whether a deal closes. Blocking or withdrawal of a deal only reduces the percentage of advisory fees. On the target side, banks may receive high fees for a deal that closes at a higher price than initially expected or if a company is able to ward off a hostile bid. With the economy on solid ground, investment banking fees have been on an upswing in recent years.

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According to Thomson Reuters, the value of mergers and acquisitions deals (M&A) reached a record high of $4.7 trillion in 2015, roughly 42% higher than in 2014.  

The New York Times reported that on one day in 2015, Goldman Sachs (GS) - Get Report advised on approximately $100 billion worth of merger deals. In the fourth quarter of 2015, the investment bank reported $879 million in M&A advisory revenue, its highest quarterly level since 2007 according to The Wall Street Journal.

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Such exorbitant fees are no longer restricted to big banks. Boutique banks have also been earning big money. 

Based on the estimates provided by Freeman & Co., Business Insider reported that Wall Street banks, including many smaller banks, earned about $342 million in fees for advising on big merger deals that were closed on just one day.

Consider the boutique Centerview Partners, which ranks amongst the top advisory firms in closing many mega M&A deals. Business Insider reported that NBC Universal's proposed $3.8 billion acquisition of DreamWorks Animation could potentially earn Centerview $20-to-$25 million in advisory fees. 

But in the current, more closely monitored era, such fees may decline significantly. This has already started to take place. 

A Hailburton purchase of Baker Hughes could have earned bankers about $350 million. The deal was worth about $35 billion and the advisory fees ranged from 1%-to-2% range. Credit Suisse was the lead advisor to Halliburton, joined by Bank of America Merrill Lynch, while Goldman was the lone advisor to Baker Hughes. Pfizer's decision to back off its offer for Allergan cost Centerview, JPMorgan and others about $250 million. 

It's possible investment bankers' best earning opportunities have already passed -- at least for the time being -- although they will still be doing fine. 

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.