BlackRock Inc.'s Jason Miller has fielded more calls from Japanese investors in the past few weeks than ever in recent memory. Among the main concern for clients: How can they safeguard returns in a new era of below-zero rates?

Miller, who heads BlackRock's exchange-traded fund unit in Japan, is not alone in grappling with a new reality for the nation's money managers. Diam Co., which manages $144 billion, said it has been conducting daily internal meetings since the Bank of Japan made its announcement Jan. 29 to discuss new ways to eke out returns. Japan Post Insurance Co., the nation's largest insurer, said it may seek to replace low-yielding Japanese bonds with foreign holdings.

The BOJ's surprise decision to move to a negative-rate policy has roiled the nation's $14.1 billion money-market fund industry, with the 11 main firms in the business halting new investments into their funds. Managers with the flexibility to invest across asset classes may accelerate a shift away from domestic government bonds with yields on 10-year debt hovering near record lows below zero.