A plan to relicense two hydroelectric dams in Ellsworth would threaten Union River water quality and fish habitats because the dam operators would allow water levels in a lake north of Ellsworth to get too low, according to 14 commercial fishermen, sportsmen and environmentalist groups. Those groups wrote a letter dated Monday that petitions the Maine Department of Environmental Protection to hold a public hearing on the plan proposed by Black Bear Hydro, a Maine-based subsidiary of Canadian firm Brookfield Renewable, so they can air objections to it, said Brett Ciccotelli, a fisheries biologist with the Downeast Salmon Federation. In a plan revision submitted on April 11, Black Bear sought to limit the amount it is allowed to reduce water levels in Graham Lake to less than six feet, from roughly 104 feet above sea level to 98.5 feet above sea level.