By Nathaniel Meyersohn, CNN Business

Gap chief executive Art Peck is stepping down ahead of the company's spinoff of its powerhouse Old Navy line, the company announced Thursday.

Peck, who has been at the company for nearly 15 years and has served as CEO since 2015, will be replaced after a brief transition by interim chief executive Robert Fisher, a son of the Gap's founders and currently the company's non-executive board chairman. Fisher has been with Gap for 35 years.

Peck, among the highest-paid chief executives in retail, has presided over several years of sales declines at the Gap brand. Gap's stock lost more than half its value during Peck's tenure and is trading at around $18 a share. Peck's compensation for 2018 was $20.7 million, filings show.

As the flagship name struggled and closed stores, Old Navy, Gap budget line created in 1994, continued to grow, topping $7 billion in sales last year.

Last year, Gap announced plans to spinoff Old Navy into a separate public company. Gap, which also owns Banana Republic, Athleta, Intermix, Hill City and Janie and Jack, and will hold onto those brands. It expects to complete the split in 2020.

Peck was expected to stay on with Gap through the spinoff, and Wall Street responded poorly to the announcement Thursday. Gap's stock plunged 12% during after hours trading.

The company also warned investors of weak sales during the third quarter and unexpectedly trimmed its guidance for the remainder of the year.

"This was a challenging quarter, as macro impacts and slower traffic further pressured results," said Teri List-Stoll, chief financial officer of Gap.

Several other top consumer brands have also announced leadership changes in recent weeks, including Nike, Under Armour and Bed Bath & Beyond. On Sunday, McDonald's parted with CEO Steve Easterbrook after he admitted to a consensual relationship with an employee.