Shares of Affiliated Managers Group Inc. /zigman2/quotes/209583132/composite AMG -1.31% tumbled 11% in midday trade, enough to pace all of the S&P 500's /zigman2/quotes/210599714/realtime SPX -1.54% decliners, after the asset management company swung to a first-quarter net loss, and said its chief executive was stepping down after a year in the role. The company reported before the open a net loss of $200.8 million, or $3.87 a share, after a profit of $153.0 million, or $2.77 a share, in the same period a year ago. The company said the net loss includes a non-recurring expense of $415 million to reflect the reduction in carrying value of an investment in a U.S. credit alternative manager. Economic earnings per share fell to $3.26 from $3.92, reflecting the "significant market declines" in the fourth quarter, but matched the FactSet EPS consensus of $3.26. Revenue fell to $543.1 million from $612.4 million, topping the FactSet consensus of $542.3 million. Assets under management declined to $774.2 billion as of March 31 from $830.9 billion as of March 31, 2018. Separately, the company named Chief Financial Officer Jay Horgen as CEO, with current CEO Nathaniel Dalton will become senior advisor and remain on the board of directors. Dalton became CEO in May 2018. The stock has gained 1.4% year to date, while the SPDR Financial Select Sector ETF /zigman2/quotes/209660484/composite XLF -0.26% has run up 17% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.54% has gained 16%.