The chief investment officer of Doubline Capital told the Sohn Investment Conference in New York that interest rates cannot maintain the low volatility they’ve experienced in the past eight years. His investment recommendation: buy interest-rate volatility on long-maturity U.S. Treasuries via a put-call straddle on TLT. The $12.2 billion iShares 20+ Year Treasury Bond ETF, known by its ticker TLT, is the most popular exchange-traded fund that tracks longer duration bonds, according to data compiled by Bloomberg Intelligence. So far this year, investors have poured more than $3 billion into the fund as yields have been tempered.