For people like Pete Alexander, the best gifts arrive after the holidays.

That is when he receives hundreds of unsold Christmas trees deemed dross by most everyone else and makes them useful again, turning them into habitats for fish in otherwise barren lakes.

“Christmas trees are perfect  just the right size and weight,” said Mr. Alexander, the fisheries program manager for the East Bay Regional Park District, which is based in Oakland, Calif. “And we get them free, because vendors want to get rid of them.”

The trees are taken to a different lake each year, where volunteers bundle them and secure them to the lake bed. Within days, the newly denuded branches become covered with algae, which attract aquatic insects, fish and, ultimately, fishermen.

Similar projects are taking place around the country this year, from Helena, Mont., to Hernando, Miss.