BlackRock Inc. (BLK) - Get Report , the world's largest money manager, has replaced the head of its fixed-income division, a business that accounts for about 30% of the company's $6.29 trillion of assets under management.

Tim Webb, who has run the $1.86 trillion division for about two years, is leaving to take time off and care for his family in the U.K., according to an internal memo written earlier this month by his boss, President Rob Kapito, and obtained from the company by TheStreet. The fixed-income business has increased its assets under management by about 30% since the end of 2015, Kapito noted. 

Rich Kushel, a 27-year BlackRock veteran who currently oversees multi-asset strategies, accounting for 8% of assets under management, will take over the added responsibility for the fixed-income business, reporting to Kapito, according to the memo.

"Rich has had a wide variety of senior leadership roles across the firm during his 27 years at BlackRock, having led investment teams and businesses across a diverse range of asset classes, regions, and client channels," Kapito wrote.

Rick Rieder, chief investment officer of fixed income, remains in his role, reporting to Kapito.

According to the New York-based company's financial statements, fixed income strategies generated about $2.89 billion of revenue last year, compared with $1.15 billion for multi-asset strategies, so it's a major expansion of Kushel's mandate. 

Although BlackRock started as an investment firm primarily focused on bonds, the fixed-income business has been overshadowed in recent years by the runaway growth in index-tracking and exchange-traded funds, mostly concentrated in stocks.