More than 700 people have left the Environmental Protection Agency since President Trump took office, a wave of departures that puts the administration nearly a quarter of the way toward its goal of shrinking the agency to levels last seen during the Reagan administration. Of the employees who have quit, retired or taken a buyout package since the beginning of the year, more than 200 are scientists. An additional 96 are environmental protection specialists, a broad category that includes scientists as well as others experienced in investigating and analyzing pollution levels. Nine department directors have departed the agency as well as dozens of attorneys and program managers. Most of the employees who have left are not being replaced. The departures reflect poor morale and a sense of grievance at the agency, which has been criticized by President Trump and top Republicans in Congress. That unease is likely to deepen following revelations that Republican campaign operatives were using the Freedom of Information Act to request copies of emails from E.P.A. officials suspected of opposing Mr. Trump and his agenda. Employees say the exodus has left the agency depleted of decades of knowledge about protecting the nation's air and water. Many also said they saw the departures as part of a more worrisome trend of muting government scientists, cutting research budgets and making it more difficult for academic scientists to serve on advisory boards.