Scientists on the trail of a soft-crab killer
Researchers in MD, VA investigate virus that's taking a bite out of lucrative 'peeler' catch before crabs can shed their shells
Patient Number 133 looked sturdy. Male. Blue. Admitted with all limbs.
On a warm July afternoon in a makeshift phlebotomy lab on Maryland's Tilghman Island, molecular biologist Eric Schott pressed a needle into 133’s squirming body, extracting a blood sample that he then placed into an ethanol solution. The blood would go back to his lab in Baltimore’s Inner Harbor. And patient 133, Callinectes sapidus, otherwise known as the blue crab, was returned to his shedding tank.
Patient 133 is part of Schott’s decade-long quest to learn more about a virus that is killing “peelers,” the term for crabs that are about to shed their hard, outer shell and become some lucky diner’s delicious soft-shell crab meal. Though they comprise only about 5 percent of the Chesapeake Bay’s blue crab harvest, which had a dockside value of more than $100 million last year, soft crabs are extremely lucrative. When sold, they fetch three to six times more per crab than hard crabs, and they are gaining in popularity nationwide as a delicacy.
About a decade ago, Schott, who works at the University of Maryland Center for Environmental Science, discovered that a virus was causing peelers to die in the tanks where they’re kept for days until they shed and are ready for market. Called CsRV1, it’s a reovirus that most likely passed through the gills or gut into the bloodstream and infected the immune cells. The virus is not communicable to humans.
Now, Schott and other researchers in Maryland, Virginia and Louisiana are trying to determine if they can predict which crabs are susceptible to the virus, and if there is a genetic component. The team is also trying to assess how long afflicted crabs take to die, and how the virus spreads from one crab to another. If they can figure out answers, they hope to save watermen the frustration and expense of losing valuable animals.
“Crabs have a very different immune system from us. They have antibodies, but no [immunological] memory,” Schott said. “That is why, when they are infected, it just kills them.”
The researchers so far have learned that the virus is present in 80 percent of crabs that die in shedding operations in Virginia. The rate is similar in Maryland, but lower in Louisiana, even though it’s the same blue crab species. Some suspect that temperature stresses the crabs, though Schott said 20-degree swings in the Chesapeake Bay from water to tank have not affected mortality. Other possible factors: crowding in shedding tanks, or too much nitrogen in the water or not enough oxygen.
“You have to handle these [soft crabs] with care. They are worth a lot of money,” said Jeff Shields, an invertebrate pathologist and disease ecologist with the Virginia Institute of Marine Science, who is working with Schott.
Shields has his own theory about what’s killing peelers. While crabs can survive wide temperature swings, bacteria increase when the mercury rises. He said he believes that this could be a major factor in their demise. The bacteria and viruses work in concert, but the bacteria might get them before the virus does. There are a lot of things that can kill a crab, he said.
Schott, in a joint bid with the Smithsonian Environmental Research Center, has tried to get funding for a crab tagging study that would track the virus in the wild. An earlier survey found it in 4 percent of wild crabs, but he suspects it’s more prevalent. If Schott and his team can determine what is causing the virus, they can advise crabbers how to prevent it.
Some watermen, particularly on Maryland’s Smith Island and Virginia’s Tangier Island, focus exclusively on peelers, scraping the soft grass beds for crabs seeking shelter to molt. Others use trotlines or pots to catch hard crabs, but keep any peelers they pick up.
In Virginia, the number of peeler operations has decreased from 318 in 2011 to 281 last year, according to the state’s marine resources commission. Maryland doesn’t keep similar data, but anecdotal information suggests that several soft-crab operations have gone out of business in recent years.
It’s unclear why soft-crab operations would be declining, especially with growing consumer demand. But it’s a labor-intensive business, requiring round-the-clock tending of shedding tanks to remove crabs from the water as soon as they shed. If the operator doesn’t, the crabs form new carapaces, becoming what watermen call a “paper shell” that’s almost worthless.
At Phillips Wharf Environmental Center, where Schott has set up his phlebotomy lab, aquaculture manager T. J. Himmelman monitors the test crabs at six-hour intervals. He records temperature, dissolved oxygen levels and salinity in the shedding tanks, as well as the crabs’ molt stage and overall health. On that warm July day, Schott worked with graduate student Matt Spitznagel and intern Nathaniel Alper to sample blood from more than 100 crabs.
What the team learns at Phillips Wharf will augment the work begun 10 years ago, when Schott first discovered the virus. After initially studying the problem in his lab, he found a partner with a soft-shell operation — Lee Carrion, who owns Coveside Crabs in Dundalk with her husband, Richard Young.
Carrion, a former teacher with a degree in biology, was seeing 80 percent mortality in her shedding operation when she met Schott. Since then, his findings have helped reduce her losses almost to zero.
On Schott’s advice and using “farming equipment and common sense,” Carrion said she and Young built a recirculating system and now bring in Chesapeake Bay water to run it, instead of piping it from their cove off Bear Creek in Dundalk. Young usually picks up the water in a 50-gallon barrel while he’s crabbing. The water quality is better in the open Bay, with more oxygen and less nitrogen than in the cove. The system streams the water through discarded oyster shells, which are used as a biofilter to remove nitrogen.
Carrion and Young also revised their harvest methods, taking only peelers with a telltale red or pink dot on their swimming fins. Those hues indicate the crab should be ready to shed in two or three days. In the past, they also took crabs showing white or green dots, not likely to shed for about a week. The longer the crabs stay in the Chesapeake instead of a holding tank, Schott suggested, the healthier they will be. VIMS’ Shields says two other precautions seem to be helping: shading shedding tanks to keep temperatures down and replacing at least some of the recirculating water once a day.
“We’re looking at anything we can do to improve survival,” he said.
The researchers have passed on what they know so far at two meetings: one in Galesville, MD, and the other in Hampton Roads, VA.
It’s a challenge to spread the word. Carrion said there’s a “paranoia” among watermen who are loath to trust scientists, fearing they’ll advocate curbs on harvest. But she said Schott and his fellow researchers only want to help.
“Eric has saved us a tremendous amount of money and heartbreak, because that is just devastating when you have all of those animals die and you don’t know why,” Carrion said. “Every time I have a crisis now, I’ve got him on speed-dial.”
- Category: Fisheries
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