The Bay has too few oysters. But it also has a shortage of what is needed to bring them back: oyster shells. And that will make the job of rebuilding the oyster population more difficult — though not insurmountable.

In the Bay, oyster shells are critical for oyster reproduction because larvae — or “spat” — require a solid surface to latch onto and grow. Historically, that has been old oyster shells on and around reefs.

But over the past few centuries, reefs have been knocked down by harvesting practices. Shells have been used for a host of other purposes instead of going back into the water.

“We’ve got hundreds of years of these shells going into roads and everything else,” said Jim Wesson, who heads oyster restoration efforts for the Virginia Marine Resources Commission. “There’s no way of getting them back.”

When scientists and managers met in January and agreed on a program to set aside 10 percent of historic oyster grounds as sanctuary, and to step up reef construction in those areas, they identified the shortage of construction materials as one of the chief issues needing research. “I can’t imagine it being something that can’t be solved,” Wesson said.

Right now, much of the shell used to cover bottom and build reefs is ancient material, thousands of years old, which has been dredged since 1960 from the upper Chesapeake. About 2.5 million bushels a year are extracted.

But that stockpile is limited and — like all dredging — the underwater excavation raises environmental concerns. Most of that shell already goes to existing programs aimed at improving existing harvest areas by covering them with fresh shell to improve spat sets.

Chris Judy, shellfish program manager with the Maryland Department of Natural Resources, said he expects the stepped-up restoration program envisioned over the next decade will at least double the demand for shells.

“The difficulty there is the current shell-dredging areas have a limited number of shells in them,” he said. “The shell program is going to be limited in its capacity to meet the need of this new initiative.”

In the Rappahannock, a project to restore eight small reefs and 200 acres of surrounding bottom will require about 2 million bushels of shell, Wesson said.

Although there are other known deposits of ancient shells, Judy said it is increasingly difficult to get permits to excavate them. The other main source of shells is from oyster shucking houses, which can yield 600,000 to 800,000 bushels a year.

Eventually, scientists agree that alternatives to oyster shell are needed. In Virginia, there is even talk of breaking up old toilets and using them in reef construction.

But not all alternative materials are created equal. Oysters need more than just solid materials to latch onto, they need structures that have lots of nooks and crannies to settle into. Those openings — called “interstitial space” by scientists — provide refuge from predators; oysters unlucky enough to settle on an exposed surface are likely to become fast food for a passing fish or crab.

In the past five years, Mark Luckenbach, a researcher with the Virginia Institute of Marine Science, has examined reefs constructed with three different materials off Fisherman’s Island near the mouth of the Bay. One was made of oyster shell, another was made of broken surf clam shells, and the other was pelletized coal fly-ash.

The fly-ash and surf clam reefs ended up having little interstitial space. Oyster larvae quickly fell prey to predators and survival was “near zero,” Luckenbach said. “By contrast, the reef that was originally in oyster shell I believe is the healthiest, prettiest looking reef that you can find in Virginia. They’ve been out there five years, and there are five year classes of oysters on it, and they are growing very nicely.”

On the other hand, too much space can be a bad thing. Experiments with large concrete chunks, which left openings large enough for fish and crabs to go through, provide poor oyster habitat as well. “We know what the extremes are,” Luckenbach said. “We know that we need some interstitial space, but I shouldn’t be able to swim through it.”

Determining what materials will serve as adequate substitutes will take more time, Luckenbach said. He and others say one way to stretch the shell supply may be to use other materials for the interior of the reef, then cover it with a layer of shell.

Maryland is experimenting with another idea: “recycling” shells in place. The idea is to go to bars covered with a shallow layer of silt, clean them, and return the shells to the bottom. But, Judy said, that approach has also raised some water quality issues and could face hurdles in securing permits.

The state also tries to spread shell over existing bars or hilly areas of the bottom, rather than build new reefs, which can take 8 to 10 times as much shell, Judy said.

While restoration efforts can help, the only practical way to restore 10 percent of the historic oyster grounds, Luckenbach said, is to get the oyster population on the road to recovery.

“There is only one way to get that much reef out there again, and that is to let the oysters do it,” Luckenbach said. “The are the only engineer that we’ve got that can do it because they sit out there and pull the calcium carbonate out of the water and build the reefs.”