Like the running of the bulls in Pamplona, Spain, a large number of "thundering herd" brokers have been 'stampeding' out of Merrill Lynch.  Since January 1, at least 50 advisers who managed nearly $12 billion in client assets at Merrill have left, based on moves tracked by Reuters. Unfortunately for Merrill, many were veteran brokers who are hard to replace and who had managed large sums of client assets at the firm.  Reuters tracks the movement of advisers and teams managing a minimum of about $100 million in client assets - and such advisers or teams typically generate $1 million or more in revenue annually. Merrill Dismisses the Concerns. Merrill issued a statement on Monday downplaying the departures, claiming that "people have been calling 'the great exodus' for more than a decade now and the facts have not borne it out."  But data collected by Reuters tells a different story.  At least 70 advisers, managing over $16 billion in client assets have left since September 2011 - accounting only for those advisers or teams managing $100 million or more.  That would account for more than 1% of the brokerage's $1.5 trillion client asset base. "Even a couple of big teams leaving can rip a big hole because they are just so substantial," said Alois Pirker, a research director at the Boston-based Aite Group, which studies wealth management trends.  "When you're losing advisers and have to replace them and pay big signing bonuses, it's an expensive proposition." Cross Country. The defections are impacting Merrill's offices throughout the United States - from a Fifth Avenue office in New York, to a New Orleans complex in Louisiana and a Beverly Hills branch in California. Former Merrill adviser Brad Stratton, who left the firm 3 weeks ago after 20 years, said the departure of big names often leads other advisers at the branch office to consider similar moves: "When a senior producer leaves, (he is) usually not the last one to go." Some advisers who left in late February and early March did so after the arrival of a final stock award - the last of its kind tied to a guaranteed share price of around $52 in 2002 - delivered on 2/15/12.  Some senior Merrill advisers have also been riled by the pressure they say they are under to cross-sell Bank of America loans and services, which has led some to leave. Click to continue reading:  [Reuters, 3/13/12].