Christmas is over — so now what do you do with that massive Fraser fir that’s been decorating your living room? Well, you could mulch it, or just toss it in a lake. Seriously.

Around the US, many park districts and wildlife areas collect used or unsold Christmas trees to recycle as fish habitats. It works this way: at the end of the holidays, real Christmas trees, stripped of all their ornaments, are bundled up together and thrown in lakes to create reefs. These reefs give young fish a place to hide from larger fish, provide new nesting grounds, and also allow for the growth of algae. And that helps the ecosystem as a whole: the algae feed aquatic bugs, bugs feed little fish, and little fish feed large fish.

“You’re essentially creating a whole ecosystem there.”

“If you imagine a lake bottom that has no vegetation, is really bare, it’s kind of two dimensional. And by putting these reefs in, you create a kind of three dimensional habitat for fish,” says Joseph Sullivan, the fisheries program manager at East Bay Regional Park District in Oakland, California. “You’re essentially creating a whole ecosystem there.”

For decades, the East Bay Regional Park District dumped tens of thousands of unsold Christmas trees in Quarry Lakes in Fremont to help fish populations. Beginning in the mid-1800s, the lakes were mined for gravel, which killed the natural underwater vegetation, Sullivan says. So back in the 1990s, Sullivan’s predecessor Pete Alexander began collecting unsold Christmas trees from vendors to create 100-foot-long reefs in the barren lakes, Sullivan says. The last Christmas trees were sunk in 2014, he says. Right now the natural vegetation has grown back, making the holiday-themed reefs obsolete. “We don’t see a need for it anymore,” Sullivan says.

After 20 years, however, Christmas trees are still refurbished as reefs at Carlyle Lake in southern Illinois. Every season, the Illinois Department of Natural Resources and the US Army Corps of Engineers collect around 1,000 Christmas trees or more to reuse as fish habitats. Around two or three trees are bundled up together and secured to the bottom of the lake with 25-pound weights. (If the trees float, they can become a boat hazard.) Here, the trees will last three to five years before rotting away, says Doug Wasmuth, a natural resource specialist with the US Army Corps of Engineers. “I think it’s a great way to make good use out of an old Christmas tree,” Wasmuth says. (The trees are also recycled outside the lake as “habitat piles” — places for rabbits to hide, he says.)

The same happens regularly at Lake Havasu, in Arizona, where firs and spruces are sunk in 20 to 30 feet of water with biodegradable sandbags as weights to help a variety of sunfish, bluegill, and bass, says Robert Stapp, a maintenance worker for the Bureau of Land Management’s Lake Havasu Field Office. The Bureau also uses tree branches and palm fronds taken from local landscapers, which get a tax credit at the end of the year for donating their scraps. “It’s totally green,” Stapp says. “It’s biodegradable, everything breaks down so it’s good for the environment.”

In North Carolina, the state’s Wildlife Resources Commission has shifted from Christmas trees to plastic tree-like structures in several lakes, because they don’t rot as wood does, says Mark Fowlkes, the Piedmont aquatic habitat coordinator at the NC Wildlife Resources Commission. The Christmas trees lasted only about two years, so they took time and resources to replenish. “From an economic standpoint, using artificial material was beneficial,” Fowlkes says. But several local lakes still deploy real Christmas trees occasionally, he says.

So if you’re looking for creative ways to recycle your Christmas tree, here’s one way to do that. Just don’t go dump the tree yourself, Fowlkes warns. Check with your state’s wildlife agency or reservoir managers instead.