Oysters making themselves at home on reefs of concrete, granite

Oysters making themselves at home on reefs of concrete, granite

Kelton Clark, director of Morgan State University’s Patuxent Environmental & Aquatic Research Laboratory in Calvert County, stands amid bundles of oyster shells. Morgan researchers tested the suitability of old highway concrete for catching spat and raising oysters. “It may not be acceptable to you or me, but the Chesapeake Bay doesn’t care what we like,” he said. “There’s no scientific reason not to use this material.” Bay Journal photo by Dave Harp

By Timothy B. Wheeler

Bay Journal

An unremarkable thing happened in a remarkable way during the recently ended oyster season in the Chesapeake Bay.

Some Virginia watermen harvested bivalves from public oyster grounds in the Rappahannock River. There’s nothing unusual about that, of course, but these shellfish had settled as baby “spat” and grown to harvestable size on a thick bed of gravel-sized stones that had been put on the river bottom to provide an unconventional home for them.

Typically, shells of other oysters are the natural landing pads for recently hatched bivalve larvae, which need to attach to something hard as they begin sedentary lives of filtering algae from the water. But the Chesapeake is running short on shells; there aren’t enough to go around to sustain the traditional wild fishery — to say nothing of the growing aquaculture industry and an ambitious effort to restore the Bay’s depleted oyster population.

Leery watermen

Some watermen, particularly those in Maryland, remain leery of using anything other than oyster shells to provide habitat for bivalves. But the shell squeeze is prompting some oyster growers and fishery managers to try alternative “substrate,” the hard material on which baby bivalves live and grow.

Working with the Virginia Marine Resources Commission, W. E. Kellum Seafood, one of the state’s oldest and largest oyster businesses, has in the last few years tested the suitability of crushed concrete from a demolished bridge and ground-down stones taken from a dam on the James River.

“This past season, the oysters we harvested were from 2-year-old granite we planted,” said Tommy Kellum, the company president. “That worked extremely well. We got a terrific spat set on it, and it grew well.”

In the right conditions, oysters will settle and grow on practically any hard surface, not just other oyster shells. Bivalves can be found clinging to wooden docks, concrete bridge piers and riprap, the big granite rocks lining the shore to prevent erosion.

Nothing beats nature

Some watermen, particularly those in Maryland, remain leery of using anything other than oyster shells to provide habitat for bivalves. Maryland watermen and their supporters have protested the use of crushed granite, fossil shell from Florida and clam shells from New Jersey in oyster restoration projects in the Little Choptank and two tributaries of the Choptank — Harris Creek and the Tred Avon River — even though the work has been done in sanctuaries off-limits to commercial harvest.

The protests landed on sympathetic ears at the Maryland Department of Natural Resources, which blocked the further use of such materials in the Tred Avon. The watermen argued that the rocks interfere with crabbing and fishing. Based on their experience, they say, oysters will not settle and grow nearly as well on substitute materials as they will on shells. Some also noted that the Florida fossil shell used in Harris Creek and the Little Choptank was full of water-fouling silt.

“I think you should use the natural stuff that the good Lord put there,” said Ron Fithian, a Kent County commissioner and former waterman who is a member of Maryland’s Oyster Advisory Commission. “Nothing works better, and they shouldn’t substitute anything, especially stone. …You don’t get the concentration of spat on stones you do on oyster shell.”

Oyster shells are optimal

Scientists and other proponents of the rock and concrete alternatives acknowledge that oyster shells are optimal, but they insist there’s just not enough fresh shell to go around — thanks to the decades-long slump in the oyster industry, which rebounded a bit several years ago.

To make up for the shortage of fresh shells from harvested oysters, many watermen are pressing for the U.S. Army Corps of Engineers to permit the DNR to dredge 5 million bushels of fossil shell from an inactive oyster reef near the mouth of the Patapsco River called Man O’War Shoal.

The proposal is opposed, though, by conservationists, recreational fishermen and even some watermen, who fear dredging up the old shell will ruin the shoal’s value as habitat for striped bass and other species. The Corps has been reviewing the state’s application for nearly two years now, but recently indicated it may be ready to make a decision by this summer.

Watermen have also pushed for the state to resume the taxpayer-subsidized “shell repletion” program it ran from the early 1960s until 2006, planting shell on the bottom and “seeding” it with juvenile oysters transplanted from areas getting good natural spat set.

Oysters cover a piece of granite used by the Nature Conservancy to build a reef in 2015 at Iron Point in Virginia’s Piankatank River. Photo by Danielle McCulloch, Virginia Commonwealth University Center for Environmental Studies

What the science says

As adamant as the watermen are about the superiority of fresh shell, none of the research supports the notion. “Every single experiment we‘ve had, all the information we’ve looked at, is that alternate materials perform equally well [compared with] shell,” said Rom Lipcius, a researcher at the Virginia Institute of Marine Science whose work has focused in part on oyster conservation.

If reefs are properly sited and constructed, Lipcius said, even if made of chunks of concrete or granite rocks, “there is absolutely no doubt that they perform equally well, in some cases better than shell.” For one thing, the alternative materials, when deposited on the bottom as large pieces, are less likely than shells to be buried in the shifting sand and sediment.

Large rocks or chunks of concrete are also preferable to shells for building reefs in sanctuaries, Lipcius said, because it’s harder for the oysters growing on them to be poached. (Some suggest that watermen’s opposition to using alternate materials on sanctuary oyster reefs is based on their hope that they’ll eventually be permitted to harvest the bivalves there.)

The Virginia Marine Resources Commission has been dredging about 500,000 bushels of fossil shell annually from the James River for replenishing its public oyster grounds, but that’s only about half of what’s needed in the state. The commission has used crushed concrete, granite stone and gravel to enhance oyster habitat in many of its sanctuaries, said Andrew Button, head of the VMRC’s shellfish conservation and replenishment department.

‘Anything hard would work’

“Just about anything that is hard would work,” Button said. “Everything from shredded tires to ‘recycled bathroom fixtures’ has been tried with some success by someone at some point.”

Watermen and others have expressed concern that concrete from roads and other demolished structures might be contaminated with oil and other hazardous substances, which could be picked up by oysters and other marine life.

But in one recent study, Morgan State University researchers found no cause for concern. The Maryland State Highway Administration, looking for alternatives to landfilling old pavement, contracted with Morgan a few years ago to evaluate the feasibility and safety of using it in building oyster reefs.

Morgan scientists placed chunks of recycled concrete aggregate in tanks of Bay water at the university’s Patuxent Environmental & Aquatic Research Laboratory in Calvert County. They compared oyster spat survival on both concrete and shells and found no difference.

They also tested for chemicals that might leach into the water — and subjected it to even more rigorous analysis with a mass spectrometer. “There was less [pollution] in it than the EPA required of drinking water — orders of magnitude less,” said Kelton Clark, director of the Patuxent lab.

The researchers also set up demonstration reefs using the recycled highway concrete in two locations with different water salinity — one in the Patuxent River near the laboratory and the other in Fishing Bay on the Eastern Shore — to see if oysters on rubble would be any more vulnerable to predators. Again, no difference.

Failed hand-tonging test

There was one test that the highway debris flunked, when compared to shells: the hand-tonging test. Clark said researchers invited a hand-tonger to try harvesting the oysters growing on the concrete. The fist-sized chunks of rubble proved too heavy to lift using the tongs.

But for building oyster habitat in sanctuaries not open to harvest, Clark said, it’s just as good as the scarce shell. “It may not be acceptable to you or me, but the Chesapeake Bay doesn’t care what we like,” Clark said. “There’s no scientific reason not to use this material.”

In another study, the U.S. Army Corps of Engineers and the University of Maryland teamed up to see how alternative substrate performs in the Bay. In 2011, the Corps built seven reefs out of granite in the Cook Point sanctuary in the Choptank River, where the bottom consisted of sand, an area of flat shell and some large mounds of shells. The granite reefs placed nearby ranged in height from 1–3 feet off the bottom; some were covered with a layer of shells, while others were not.

The artificial reefs were planted with oyster spat produced by the University of Maryland Center for Environmental Science hatchery at Horn Point. After three years, UM researchers analyzed the growth, survival and reproduction of the oyster populations in the area, and also checked for other organisms living on or around the reefs.

They found more oysters on reefs made of both granite and shell than on those built of granite only, but both types had relatively healthy densities, averaging 91 oysters per square meter and 49 oysters per square meter, respectively. The granite-only reefs did have thicker populations of organisms such as anemones, which researchers suggested could be competing with oysters for space on the rocks.

Better than clam shells; surface area matters

Most of the artificial reefs built in Harris Creek and the Little Choptank River as part of those sanctuary restoration projects are too new yet to evaluate their performance as hosts for oysters, but preliminary analysis of reefs finished three years ago in Harris Creek shows that those with a stone base have nearly three times the density of oysters, on average, as those with a base made up of clam shells. All were planted with spat on shell produced by the UM hatchery.

Scientists say the shape and size of the materials used can matter in determining how well oyster spat settle and survive on artificial reefs. The granite stones used to build reefs in Harris Creek, for instance, have more than three times as much surface area as do the reefs made of clam shells. That’s important, according to Jay Lazar, field operations coordinator for NOAA’s Chesapeake Bay office, because it gives oyster spat more places to latch onto as they settle to the bottom. The spaces between rocks also offer more protection from predators.

Smoother sailing elsewhere

Elsewhere in the Bay, alternative substrate isn’t as controversial. Crushed stone was used to build a reef near Baltimore in the lower Patapsco River, on which hatchery-spawned baby oysters were planted. Jim Mullin, executive director of the Maryland Oystermen Association, said his members didn’t mind because the Patapsco has never had any productive oyster bottom, at least in recent times. The river is closed to harvest.

In Virginia, the Army Corps began building more than 25 acres of artificial reef in the Piankatank River in May, using crushed stone for the $2 million project. The Piankatank project, which is also being supported by the Virginia Marine Resources Commission and The Nature Conservancy, is one of five Virginia Bay tributaries targeted for large-scale oyster restoration.

Nonprofit groups doing their own oyster restoration projects also have had to make do with shell alternatives. Jackie Shannon, the Chesapeake Bay Foundation’s oyster restoration manager in Virginia, said the group has had success with alternative substrate in building reefs in the Lafayette River. They used crushed stone, covered with a veneer of shells, and surrounded them with “reef balls” — concrete igloos with holes in them. At low tide, the tops of the reefs are visible above water, literally covered with oysters that settled there naturally.


Tommy Kellum said his seafood company cooperated with the Nature Conservancy and the VMRC in building reefs out of crushed concrete in the Piankatank River sanctuary.

“I’ve been amazed at the recruitment and viability of that reef,” he said.C. Hudgins, a waterman from Mathews County, said watermen in his area did get a nice little harvest off the experimental reef in the Rappahannock River, made of granite rocks milled down to gravel size.

“Oysters love to strike on it,” Hudgins said, referring to larvae settling and attaching themselves to a hard substrate. “The biggest thing [with the granite] is to get it small enough” so it can be hoisted easily in the dredges watermen use.

Unless something changes, the industry may have no choice but to look for alternatives to shell, such as granite, Hudgins said, adding: “I think it is a viable option.”


Bay Journal is published by Bay Journal Media, a 501(c)(3) nonprofit, to inform the public about issues that affect the Chesapeake Bay. A print edition is published monthly and is distributed free of charge. News, features and commentary are available free online at bayjournal.comMarylandReporter.com is partnering with the Bay Journal by publishing one of its articles every Friday.

1 Comment

  1. Clean Chesapeake Coalition

    There is no dispute that natural indigenous shell is the best surface upon which baby oysters will strike and grow – after all, that’s what Mother Nature intended. Anybody concerned about the paucity of natural shell for oyster restoration and propagation needs to support the State of Maryland’s permit application to the U.S. Army Corps of Engineers to dredge fossil oyster shells in the upper Bay (Man O’War Shoal) for planting and seeding throughout the Bay. This was done for more than 4 decades with measurable success. That program ended in 2006 at the urging of the Coastal Conservation Association (CCA) on behalf of recreational fishermen…and at the expense of the commercial seafood industry. The lack of natural oyster shell is a self-imposed crisis. Just look at how Virginia is mining shell in the James River and far exceeding Maryland in harvested bushels. Defaulting to the dumping of construction debris in our Bay when there is no shortage of natural oyster shell buried under sediment in the upper Bay is lunacy. Note, the 2006 permit to dredge for oyster shell in the upper Bay was supported by the Chesapeake Bay Foundation and the Maryland Watermen’s Association. Why do they oppose essentially the same permit today?

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