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WASHINGTON — On the morning of Aug. 21, 2018, David Bernhardt, then the deputy interior secretary, wanted to attend a White House meeting on the future of a threatened California fish, the delta smelt — an issue upon which Mr. Bernhardt had been paid to lobby until he joined the Trump administration a year before.

It was a sticky ethical issue, seemingly exemplifying the revolving door that has separated lobbying from policymaking in the nation’s capital for decades. So Mr. Bernhardt, who had, as a lobbyist, pressed to loosen delta smelt protections for a California water district, personally approached the Interior Department’s ethics referee.

“I see nothing here that would preclude my involvement,” he wrote ahead of the meeting in an email to Edward McDonnell, a career ethics lawyer at the department, effectively clearing himself of wrongdoing hours before the event in question. Mr. McDonnell agreed.

More than 900 pages of Interior Department emails, obtained by the environmental group Friends of the Earth through a Freedom of Information Act request, show the ethical tangle that has come from elevating a lobbyist to head a cabinet department that oversees the issues upon which he once lobbied. The oil and gas extraction, water allocation and wildlife protection issues that are central to the Interior Department’s mission were also central to Mr. Bernhardt’s lobbying portfolio. Because of that, he has repeatedly reached out to an Interior ethics lawyer to seek guidance on his work.