Study on soft-shell and razor clams delivers more questions than answers
Causes of both population decline and recent rebound remain murky
Though they once served as important filterers and foodstuffs in the Chesapeake Bay, relatively little is known about the long-term population dynamics of soft-shell clams — and even less about razor clams.
That was among the findings of a study by the Virginia Institute of Marine Science and Smithsonian Environmental Research Center, for which researchers took a holistic look at the species and the factors that could be keeping them to a small fraction of their historic abundance.
The study states that clams find protection in the sort of complex habitats — seagrass and oyster beds — that are often at risk as much as the clams themselves. A domino effect of factors seems to be working against a fuller recovery of the clams, researchers found, and more work is needed to ensure their presence into the future.
The soft shell fishery hit its stride in the 1950s, with harvesters bringing in an average of 460,000 bushels per year until about 1971. Some watermen observed that three or four times as many were available but not harvested because of restrictions, according to Maryland Department of Natural Resources records on the fishery.
The Bay’s population of both clam species began to decline in the 1970s, followed by a dive in the 1990s and early 2000s, and never bounced back.
Along with having historic market value, both species provide food for blue crabs and cownose rays as well as filter pollutants from the water, perhaps as well as oysters. In the Baltic Sea, where soft shells are abundant, for example, they are said to filter the entire water column in less than a day.
The soft shell, in particular, “is a very important species to the ecology of the Chesapeake Bay and has been a big fishery in the past, but it's received very little attention regarding management or science or stock assessment or anything like that,” said Matthew Ogburn, an ecologist in SERC’s fish and invertebrate ecology lab and one of the authors of the study.
That could be because the clams are now valued locally as little more than crab bait or shipped to the Northeast as fodder for New England-style stews and dishes.
Soft shells (Mya arenaria), named for their comparatively brittle, elongated shells, were so abundant and such popular fare in the Bay region during the 1950s and ’60s that the state of Maryland crowned a “clam queen” to promote the vibrant fishery.
A string of diseases and worsening water conditions decimated the soft-shell population in the 1990s and 2000s, but in recent years their numbers have rallied — and so has the number of clammers dredging for them. As recently as 2013 and 2014, according to DNR data, there were just eight watermen bringing in 278 bushels of soft shells in the state. But that number rose to 31 watermen harvesting 17,468 bushels two years later.
Still, the population seems to suffer from a boom-and-bust cycle that never reaches historic levels, and researchers want to better understand the reasons.
This last year, heavy rain deluged some of the clams’ brackish habitat with fresh water and buried some of them in sediment. The combination was blamed for an abysmal harvest. But researchers found that a number of other factors could be stalling their recovery: Habitat loss, increasing pressure from predators and low reproduction numbers rose to the top of the list, but warming water conditions and harvest methods could play a role, too.
When soft-shell clams are less abundant in Bay waters, many watermen pivot to harvesting razor clams (Tagelus plebeius), which can be sold as crab bait. But even less is known about that species — razor clams lack the historical landing data and surveys that would help develop a framework for their previous abundance. The first major decline in population was recorded in 2004.
“Without knowing what the population is doing over time, it’s hard to assess how well it’s doing and what the next steps should be,” said Rochelle Seitz, a research professor at VIMS and one of the study’s authors.
Population assessments are needed for both species to properly assess their health and identify the factors that might be affecting their numbers, researchers said.
Potential impacts of soft-shell harvests on the aquatic ecosystem were nearly a moot point when there were hardly any clams to harvest, but no longer. The technique used to harvest clams is called hydraulic dredging: A suction tube vacuums sediment onto a conveyer belt from which watermen pick out the clams and release the rest back to the water. The process disturbs the bottom and can generate a sediment plume, both of which have the potential to harm the underwater grasses that are often located nearby.
Researchers and river advocates are looking to strike a balance between the ecological and economic demands on these clams. But doing that takes data — and time.
Researchers do have the numbers from a long-term survey conducted by SERC scientists on the Rhode River near Edgewater, MD. The center has been sampling soft-sediment habitats there since 1980, when there were close to 2 million soft-shell larvae per hectare. That dropped to 100,000 per hectare in 2005, but, since then, “it’s basically zero.”
“The amount of filter feeding we’ve lost with that is pretty incredible,” Oglund said.
The soft shells’ recovery could also be hampered by current and future changes in water temperature associated with climate change. Already, soft shell clams in the Bay are near the southern extent of their range. But Seitz said that range currently extends into Georgia, so warmer water isn’t likely to shift them north during the next decade, though it could drive them into deeper, cooler portions of the Bay.
The simplest way to support the clams in the meantime — short of conducting sweeping population assessments — would be to protect and increase the habitats where they seek refuge from predators and harvesters. The study showed that both species were found more often in complex habitats, such as a combination of seagrass and shells.
Seitz also pointed to work in other states where the fisheries for these species are thriving and the research more robust. Maine is figuring out how to grow soft-shell clams through aquaculture.
“They deal with a lot of the same threats we do here,” Seitz said. “There’s a lot we can learn.”
- Category: Fisheries
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