Several years before becoming acting US interior secretary, David Bernhardt was a lobbyist for one of California’s largest water districts, where he sought to win more water for farmers, even if it came at the expense of imperiled fish that also needed it.

But despite joining the interior department in 2017, a new complaint alleges, he has continued to pursue policies that favor his old clients. According to the non-partisan Campaign Legal Center, he has taken steps to weaken protections for endangered fish, such as the diminutive delta smelt, in California and make more irrigation water available for prominent agricultural interests in the state.

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“It is difficult to discern where Mr Bernhardt’s private-sector lobbying activities end and where his public service begins,” the CLC writes in its complaint. “Mr Bernhardt’s participation in these particular matters appears to violate his ethics pledge and, at a minimum, violates his obligation to avoid the appearance that he is using his public office for the private gain of his former lobbying client.”

The White House ethics pledge, which Bernhardt signed shortly after taking office, explicitly prohibits executive branch officials from participating in any particular matters or specific issue areas that they previously lobbied on for two years after the date of their appointment. In a statement to the Guardian, an interior department spokesperson said Bernhardt had committed no violations.

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The CLC’s ethics complaint comes at a tumultuous time for the Interior Department, a massive agency that oversees approximately 500m acres of public land across the country.

In early February, Donald Trump announced that he would nominate Bernhardt to be the next interior secretary, a position that requires Senate approval. If confirmed, Bernhardt will succeed Ryan Zinke, a former congressman from Montana who stepped down from the Interior Department amid a swirl of scandals.

Last week, the CLC filed a separate ethics complaint with the Interior Department’s inspector general asking for an investigation into six top political officials at the agency who appear to have violated their ethics pledges. The complaint was based in part on The Guardian’s reporting.

Bernhard worked as a lobbyist for Westlands Water District, the largest agricultural water district in the country and a political heavyweight in the Golden State, from 2011 to November 2016. In that role, the CLC says, he lobbied or otherwise advocated for specific provisions that benefited Westlands to be included in the WIIN Act, a major law that Congress passed in late 2016 that partly governs water management in California.

Shortly after joining the Interior Department as Deputy Secretary, the complaint contends, Bernhardt would go on to direct his subordinates to begin a process that would effectively implement those same WIIN ACT provisions, again to the benefit of his former client.

The CLC’s complaint, which is based in part on a recent New York Times report, asks the Interior Department inspector general to investigate and issue a report on the acting secretary’s conduct.

On Wednesday, senators Elizabeth Warren and Richard Blumenthal also sent a letter to the department asking it to provide a range of information about Bernhardt’s involvement in California water management.

“The Interior Department has become the public arm of oil, gas, mining and other extraction industries,” said Congressman Raúl Grijalva of Arizona, the chair of the House Natural Resources committee, which conducts oversight of the Interior Department. “And I say that without reservations.”