Private-equity firm Blackstone Group L.P. BX, +0.95% plans to trim its ownership stake in SeaWorld Entertainment Inc. SEAS, -1.11% by 15 million shares, following the theme-park operator's public listing in April.

The new stock sale would cut Blackstone's ownership in the company from 63% to 46%, not including an underwriter option, resulting in SeaWorld no longer being a "controlled company" under New York Stock Exchange governing standards. This change will trigger a requirement that SeaWorld appoint a majority of independent directors within a year after it is no longer a "controlled company," among other requirements.