U.S. exchange-traded fund assets fell 5.4%, to $575 billion, in the first half, according to a study.

State Street Global Advisors said the cause was falling equity valuations, as the number of ETF shares outstanding increased 9% from a year earlier. This indicates that "investors have continued to add shares of ETFs to their portfolios as the markets declined," the Boston firm said in releasing the study Monday.

Eighty-seven exchange-traded funds were launched in the six months, versus 167 in the first half of 2007.

The arrival of active exchange-traded funds was highly anticipated, but on average they are only trading just over 2,000 shares daily.

Assets in Treasury inflation protected securities ETFs rose 39% in the first half, and assets in commodity ETFs rose 34%.

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