The ghosts of Christmas past can be found in some unusual places. The bottom of Lake Havasu, for instance.

There, thousands of Christmas trees sunk by wildlife biologists have found a second life as fish habitat in an ecosystem damaged by the damming of the Colorado River decades ago.

What nature once provided — a steady source of organic material such as brush and uprooted trees — disappeared when the once wild and muddy river was tamed. The lake is a reservoir behind Parker Dam on the California border with Arizona.

By the late 1980s, Lake Havasu’s now crystal clear waters harbored few places where newly spawned fish could find shelter from predators. Fish populations were a fraction of what they had been a generation before.

“There was no place for the young fish to hide until they matured,” said Kirk Koch, a fisheries program manager for the U.S. Bureau of Land Management. “Instead, they would be consumed by bigger fish.”

The solution was a gift that keeps on giving: Christmas trees.

More than 30 million farm-harvested trees are sold nationwide each year.

When it began in 1992, the effort at Lake Havasu was the largest fresh-water habitat recovery program in the nation, Koch said.

Over the next decade, $16 million and countless hours of work by volunteers created 875 acres of artificial reefs.

Structures were formed by sinking PVC pipe, concrete sewer pipe and cinder blocks in 42 coves. Then, discarded Christmas trees were lashed together, weighted down and dumped around the structures. Piles of brush were added.

As the trees and brush decomposed, the pipe and concrete structure grew a biological skin of mosses and algae that was then colonized by insects. In addition to providing shelter, the Christmas tree structures also became a source of fish food.

Scuba divers check sites annually and have found that fish are drawn to Christmas trees as much as Santa is.

“When they started, they could count all of the fish at any spot on their fingers,” Koch said. “Progressively, they found more fish — way, way more fish — than they can count.”

The project turned Lake Havasu into a popular sport fishing destination.

It takes a Christmas tree five to six years to decompose under water. So each year, volunteers toss in as many as 500 additional trees and a thousand brush piles to replenish the reefs.

Part of the benefit of creating habitat with Christmas trees is that it’s cheap — trash haulers are happy to unload onto others what they pick up at the curb.

This year, Riverside County supervisors approved a plan to transfer 2 tons of trees collected at county landfills to the California Department of Fish and Wildlife, which will dump them into two lakes that badly need them.

The load will make Quinn Granfors’ job much easier.

Granfors, a state fisheries biologist, has been tossing trees into Lake Elsinore and Lake Perris since 2006. Working under budget constraints, he was left to scrounge around on his own after Christmas in search of trees. Now they’ll be coming to him.

In the coming weeks, he and volunteers will send hundreds of weighted trees to the bottom of the lakes.

“I kind of joke with the guys that they’re now qualified to get a job with the mob,” Granfors said. “Because they know how to make organic material disappear.”