Bivalve’s dramatic demise is as ‘quiet as a clam’

Large clam species are collapsing throughout the Chesapeake.

By Karl Blankenship
Bay Journal News Service
It seems a familiar Chesapeake Bay story: A shellfish racked by disease, preyed upon by cownose rays, is at historic lows, depriving watermen of a valuable fishery and removing a key filter feeder from the ecosystem. Biologists describe its loss as “catastrophic.”
“It’s amazing,” biologist Mark Homer said of their dramatic decline. “You feel sorry for the animal.”
Homer wasn’t talking about the Bay’s best known beleaguered bivalve, the oyster, but rather the soft-shell clam. And his worry goes beyond this one species of clam. Scientists and watermen are witnessing a collapse of several key clam species all around the Bay.
Unlike oysters, which build elaborate reefs, soft-shell clams burrow into the sediment where they are out of sight – and seemingly out of mind. So much so that few people seem to have noticed their near total collapse in Maryland over the last two decades, and their even earlier collapse in Virginia.
Their decline around the Bay has been as dramatic – perhaps even more so – as that of oysters. After peaking with harvests of nearly 700,000 bushels in Maryland during the early 1960s, the total catch in several recent years has been zero.
“We are hesitant to write the obituary of soft-shell clams in the Chesapeake Bay, but recent landings, surveys and discussions with commercial clammers paint a gloomy future for this species,” Homer and other Maryland scientists wrote in a recent report.
But the scientists went on to paint an even grimmer tale. While studying the demise of soft shells, populations of another species, the stout razor clam, crashed as the researchers watched helplessly.
While other smaller clams may be more numerous, the larger soft shell and razor clams were once the dominant clams in Maryland in terms of biomass. Their numbers, and water-filtering ability, once may have rivaled that of oysters.
The situation in Virginia appears no better. Its soft-shell clam fishery collapsed before Hurricane Agnes in 1972. Neither soft-shell, nor razor clams, are sampled in routine surveys, but anecdotal evidence suggests that populations of both are greatly reduced, said Jim Wesson, a scientist with the Virginia Marine Resources Commission. “It doesn’t look like there are very many razors or soft shells there, but nobody has ever followed them.”
In addition, wild populations of the once-abundant hard clam in Virginia – the major commercial clam species in that state – have declined by more than half since the 1970s for reasons that are not fully understood. “It is a very, very small fishery that is still harvesting wild clams,” Wesson said.
The type of clams found in the Bay varies from place to place. The hard clam is largely absent in Maryland waters because of the lower salinities there. The soft-shell clam has always been less abundant in Virginia waters because of warmer temperatures. The Bay is near the southern limit of its range.
But wherever one looks in the Bay today, there will almost certainly be fewer large clams than would have been the case a generation ago. It’s a change that scientists worry could have a profound – if poorly understood – impact on the Bay ecosystem.
Like oysters, most clams feed by filtering the water. While oysters sit above the surface and filter the passing water, clams dig into the sediment and feed through siphons they stick out of their burrows and into the water like straws.
“Obviously, if you have a good population and they are pumping away, they are taking phytoplankton out of the water, and if [clams] are not there, then what takes their place?” asked Vic Kennedy, a professor at the University of Maryland Center for Environmental Science who studies bivalve ecology.
Soft-shell clams actually have a higher filtering rate than oysters, and their filtering capacity may once have rivaled or surpassed that of the oyster population. “When you look at a harvest of more than 600,000 bushels, there had to have been a tremendous population of these things at one time,” Homer said.
Homer, a biologist with the Maryland Environmental Service, has been studying the evolving clam situation along with two other scientists, Mitchell Tarnowski, a shellfish biologist, and Christopher Dungan, a research scientist, both with the Maryland Department of Natural Resources. Their work has been funded by the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office.
Research indicates that the razor clams also filter the water, although the species is poorly studied and it’s uncertain whether it feeds primarily by sucking plankton out of the water column, or by vacuuming detritus off the bottom of the Bay.
Of potentially greater concern is the impact on the food web. Soft-shell clams are a major source of food for blue crabs, spot, croaker, flounder and other bottom feeders, as well as sea birds and some waterfowl.
“In terms of the Bay’s food web, I think that at one time, soft-shell clams were more important than oysters,” Homer said. “It is like a gift to predators. They don’t have to spend a lot of energy to get it. There is just this little shell hiding this giant lump of protein and glycogen.”
The demise of clams could ripple through the system in somewhat unexpected ways, such as increasing cownose ray predation on oysters, Wesson said.
“Rays used to eat the soft-shell and razor clams,” he said. “Since they are gone, that’s why rays are such a problem for us with oysters right now. There’s probably not that much food for them.”
Some speculate the clam loss could make it harder to sustain the robust blue crab populations that fishery managers want to restore. Studies by scientists at the Virginia Institute of Marine Science show that clams are a major food source for crabs; and clams become a more important part of the diet as crabs grow. Razor clams appear to be popular blue crab food, especially in Maryland – the small harvest is used primarily for crab bait.
Howard Townsend, a scientist with NOAA’s Chesapeake Bay Office who is developing ecosystem fisheries models, has been working to incorporate clams into modeled food webs. Reaching conclusions in such models is difficult because they rely on predation studies, which are often limited. Nonetheless, when he runs the model with reduced clam populations, blue crab numbers also drop.
“In a modeled ecosystem, based on the data, it fits fairly well,” Townsend said, though he cautioned the lack of adequate monitoring makes it difficult to ascertain how well the modeled systems mimic real ones. Nonetheless, it illustrates how tweaking one part of the Bay food web can have ramifications elsewhere, Townsend said.
VIMS scientists who studied the blue crab diet caution that while soft-shell and razor clams may be popular food, crabs are omnivorous and eat almost anything they come across – including other blue crabs as they get older. And the most common clam found in crab guts, at least in Virginia, is the smaller but common Baltic macoma.
Crabs may like clams because they are especially nutritious – clams may live nearly a foot beneath the surface, but crabs find it worthwhile to dig that far to get them. “They are such a good, nutritious food source for crabs,” said Rochelle Seitz, a VIMS scientist who studies benthic ecology. “And if they can get a large clam, then that is sort of a large bang for your buck.”
Finding substitutes may be getting harder, at least in Maryland, Homer said. Soft-shell clams were once the dominant species in the large barren or sandy bottom areas where they easily burrowed into the sediment. When they disappeared, there were plenty of razor clams filling that habitat niche – but now, those are largely gone, too.
But the full impact of that change isn’t known because clams have never received anywhere near the attention as oysters – either for research, or as a fishery. “Oysters have always been king,” Homer said. “If you had a certain amount of resources, you never bothered with clams.”
Soft-shell clams, while popular in New England where they are known as “steamers,” were not a major harvest item in the Bay until the 1950s, when demand increased as clam beds in New England were overharvested.
Harvesting in the Bay got a boost in 1951 when Fletcher Hanks of Oxford, MD, invented the hydraulic escalator clam dredge – a contraption that scoops sediment and clams loosened by jets of water onto a conveyor belt, which hauls it onto a boat.
Soft-shell clam harvests dramatically increased around the Bay in the 1950s and 1960s – for a while, Maryland even had a “clam queen” to promote the fishery.
But the fishery in Virginia collapsed in the mid-1960s. In Maryland, catches declined after peaking at 680,000 bushels in 1964.
A multitude of factors may have contributed to the clam decline. In deeper waters, low-oxygen levels may have made some areas uninhabitable. Soft-shell clams, and possibly hard clams, may have been overfished; but razor clams, which were never a major commercial species, were not. Baywide, Tropical Storm Agnes devastated clam populations in 1972, smothering them under a layer of sediment.
But the most persistent problem, at least for soft-shell and razor clams, appears to be disease.
While the soft-shell fishery rebounded in Maryland by the late 1970s, it never attained pre-Agnes levels. And then waves of disease began sweeping through the population.
Christopher Dungan, a DNR scientist at the Cooperative Oxford Laboratory, said Dermo first turned up in soft-shell clams in Virginia in the 1950s. It’s possible it contributed to the population collapse in that state prior to Agnes, but no one knows.
Another disease, disseminated neoplasia, a leukemia-like disease that is typically fatal within 6 months of occurrence, turned up in Maryland soft-shell clams in 1984. Biologists were on the lookout for that disease since it was identified devastating clam populations in New England in the 1970s. At the time, Dungan said, it was thought that pollution allowed the disease to take hold in the animals – ironically, the Bay was considered too clean for DN.
Then, Dermo-like parasites turned up in Maryland clams in the early 1990s. Harvests – which had reached post-Agnes highs of around 300,000 bushels in the late 1980s – crashed and never recovered. Commercial landings in recent years have been at or near zero. More recently, a virus disease has been detected in the gills of soft-shell clams.
Today, finding soft shell clams in bare areas in Maryland is rare, Homer said. With their numbers greatly reduced, soft-shell clams are quickly eaten by predators.
The remnant soft-shell clam population has retreated to “trashy” bottoms such as remnant oyster reefs or piles of man-made refuse, where they have some protection from predators.
When soft-shell clams began to decline, clammers began turning to stout razor clams, which had been sold largely as crab bait.
But in late 2003, biologists were contacted by clammers who reported massive razor clam die-offs. What biologists found was a situation even worse than the reports: By late 2004, their surveys indicated that 70-80 percent of the razor clam population in Maryland had died, apparently from disease.
Since then, the scientists describe the razor clam population as being in a downward spiral. The magnitude of the decline is hard to gauge because populations were never assessed—even now, razor clam catches are not reported. Scientists said that in areas where they once collected 12-13 bushels of razor clams an hour, they now get only two to three bushels.
It’s possible the diseases could bring more problems for the Bay’s clams. Dungan has detected Dermo in at least three other species of Bay clams, although its impact on those species is unknown.
Further perplexing Dungan and other scientists is the relationship between Dermo, oysters and clams. The type of Dermo infecting the clams, Perkinsus chesapeaki, is related to the Dermo parasite that has contributed to the devastation of the Bay’s oyster population, P. marinus. Oddly, P. marinus does not commonly infect clams in the wild, nor does P. chesapeaki infect oysters in the Bay, even though, as Dungan pointed out, “infected oysters and clams are living side-by-side, cheek to jowl.”
Yet in the lab, clams and oysters acquire both infections.
Such unexplainable variables lead some to hope that the diseases, which seem to have arrived so suddenly, may one day be vanquished in the Bay.
While the outlook is gloomy, scientists have not given up: Seitz said soft-shell clam restoration is being considered as part of a conservation project in Virginia’s Lynnhaven River. The clams would be returned in conjunction with underwater grass beds, which would offer some shelter from predators.
If nothing else, the demise of the clams is a sobering reminder of how profoundly the Bay ecosystem has been altered – perhaps irreversibly – in recent decades.
“Unfortunately, we’ve seen it so many times now,” Tarnowski said. “In my career in Maryland, I’ve seen the decimation of oysters, and then soft clams and now razors. What’s left? It’s like all the major shellfish species have been decimated by disease and one wonders what is going on in a broader context. Why are these animals becoming so vulnerable, and what does that bode for us as a species?”