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WASHINGTON — A previously undisclosed invoice indicates that David Bernhardt, President Trump’s choice to lead the Interior Department, continued to lobby for a major client several months after he filed official papers saying that he had ended his lobbying activities.

The bill for Mr. Bernhardt’s services, dated March 2017 and labeled “Federal Lobbying,” shows, along with other newly disclosed documents, Mr. Bernhardt working closely with the Westlands Water District as late as April 2017, the month Mr. Trump nominated him to his current job, deputy interior secretary. In November 2016, Mr. Bernhardt had filed legal notice with the federal government formally ending his status as a lobbyist.

Westlands, a powerful California agribusiness group, was one of Mr. Bernhardt’s main lobbying and legal clients between 2011 and 2016. During that time, Westlands paid Mr. Bernhardt’s firm $1.3 million for lobbying services.

A New York Times investigation this year revealed how Mr. Bernhardt made it a priority at the Interior Department to promote policies long sought by Westlands, including a weakening of Endangered Species Act protections for a rare fish. Those changes would have led to the release of vast amounts of water from the delicate ecosystem of the San Francisco Bay Delta for irrigation of commercial farmland.