Patrick Mortimer, head of Morgan Stanley's ( MS) U.S. prime brokerage business, resigned this week, the Wall Street Journal reports, amid declining assets and profits of hedge fund clients that make up the division's client base.

Mortimer, a 20-year veteran of the firm, left for personal reasons and doesn't appear to have been pushed out, the Journal reports, citing people familiar with the matter. Stuart Hendel, global head of the prime brokerage unit, and other executives will assume Mortimer's duties, and no direct replacement for Mortimer is planned, these people said.

The Journal notes Mortimer's resignation comes as Wall Street firms change their business models to accept the diminishing role of hedge funds as a driver of profits. For years, hedge funds sent large amounts of commissions and other business to Wall Street firms. But last year, many hedge funds ran into problems as the credit markets sputtered and the stock market followed.