PARIS—French billionaire and Vivendi SA chairman Vincent Bolloré ousted the longtime chief of Canal Plus on Thursday, further tightening his grip over the Vivendi-owned pay-TV group amid falling subscriber numbers.

Vivendi said Thursday that Jean-Christophe Thiery, a long-term lieutenant at Mr. Bolloré 's family holding company, will take the top role at Canal Plus, replacing long-serving Chief Executive Bertrand Meheut. Mr. Bolloré also named himself chairman of Canal Plus.

The changes underscore the speed with which Mr. Bolloré is exerting hands-on control of Vivendi's businesses since taking the helm of the media conglomerate last year. He has expanded his stake in the firm to nearly 15% and, unlike many chairmen, he has started diving more deeply into the day-to-day operations of units that once had more of a free hand.

Vivendi announced the changes in a statement in which it said Mr. Bolloré would replace Vivendi CEO Arnaud de Puyfontaine as chairman of Canal Plus. Mr. de Puyfontaine will continue in his role at Vivendi.

Mr. Meheut, who joined the company in 2002, will remain as special adviser to Mr. Bolloré "on the significant transformation" that Vivendi wants to achieve with Canal Plus, the company said.

For Vivendi, a once sprawling conglomerate that has slimmed down dramatically in recent years, much is at stake in shaking up Canal Plus.

The pay-TV unit is Vivendi's largest business, accounting for more than half of its revenue and profit in the second quarter, according to Vivendi earnings released on Wednesday.

Mr. Bolloré has made clear since summer that he wanted to shake things up at the unit, where subscriber numbers in France have been declining for two years. Vivendi wants to change the funds it allocates to programs in an attempt to turn around the decline, according to people familiar with the matter.

In July, Vivendi announced that Rodolphe Belmer, Canal Plus's No. 2 executive, had been relieved of his duties. He was replaced by Maxime Saada, formerly Mr. Belmer's right-hand man.

Mr. Thiery, the new Canal Plus chief, has worked on Mr. Bolloré 's media businesses for more than a decade. He is currently head of the unit that runs the Direct Matin free daily newspaper as well some of Mr. Bolloré 's telecom holdings.

Vivendi has been a source of intrigue in recent months as asset sales have left the group with a roughly €9 billion ($10.1 billion) war chest, leading analysts and investors to speculate about what the company will do with the cash.

Write to Nick Kostov at Nick.Kostov@wsj.com

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(END) Dow Jones Newswires

September 03, 2015 08:55 ET (12:55 GMT)