Invesco confirmed on Thursday it would buy active asset manager OppenheimerFunds, a subsidiary of Massachusetts Mutual Life Insurance, in a stock deal shoring up its position, amid a trying period for the fund industry.

Asset management firms have been grappling with a shift in investors’ preferences as they flock to cheaper exchange traded and index funds and away from active funds, which have lagged during much of the nine-year bull market. That has sparked a price war that is denting margins.