Spring 2000 Table
Disease in Aquaculture: Scientists, Managers, and Growers Take on
by Tracey Crago, WHOI Sea Grant
Shellfishing is big business in the
Commonwealth. The commercial value of wild shellfish harvested in
1996 was over $21 million. Shellfish grown by aquaculturists added
another $4.5 million. As an industry, shellfish aquaculture has
seen steady increases over the last decade. According to the Scott
Soares, the Commonwealths aquaculture coordinator, the number
of acres under cultivation and the number of shellfish aquaculturists
is increasing at a rate of approximately 10 percent a year.
As with agriculture and other industries
dependent on hard-to-predict factors such as weather and product
demand, aquaculture can be a tricky business. Disease is one variable
that can devastate a crop in agriculture and aquaculture, translating
to substantial losses for the farmers.
A recent workshop co-sponsored by the
WHOI Sea Grant Program, the Southeastern Massachusetts Aquaculture
Center (SEMAC), the Cape Cod Cooperative Extension Program, and
the Massachusetts Aquaculture Association, and hosted by the Cape
Cod Museum of Natural History, focused on shellfish disease, particularly
as it relates to the aquaculture industry.
Cape Cod, where virtually all of the state's cultured shellfish
originates, quahogs (Mercenaria mercenaria) and oysters (Crassostrea
virginica) account for 95 percent of the total. Other species
include bay scallops (Argopecten irradians), surf clams (Spisula
solidissima), and soft-shell clams (Mya arenaria). Razor clams
(Ensis directus) may soon be added to the list. Photo: Dale
Leavitt, WHOI Sea Grant/SEMAC
Speakers, including pathologists, growers,
and industry representatives, delivered a recurring message: shellfish
diseases are not only here, they are here to stay. And, most likely,
they concur, additional diseases will be discovered in the future.
The good news? These diseases are not harmful to human consumers.
Roxanna Smolowitz, a pathologist at
the Marine Biological Laboratory, studies oyster diseases and QPX,
the relatively new quahog parasite that derives its moniker from
the term quahog parasite unknown. She says that the potential for
organisms to become pathogenic is always there, given the right
For diseases to occur, says Smolowitz,
"there has to be an interaction of three factors: host susceptibility,
agent virulence, and environmental conditions." Without the right
combination of each of these, the disease can either decrease in
amount or severity or be eliminated. But, Smolowitz was quick to
point out, "[getting a disease] is not that hard."
Robert Hillman, pathologist at the Battelle Memorial Laboratory
in Duxbury, echoed Smolowitzs sentiments. "There probably
isnt a species out there for which disease cant become
virulent at some time."
Joining Smolowitz and Hillman and presenting
data from several years research on shellfish disease in Massachusetts
and Delaware Bay was Robert Barber, a pathologist with the Haskins
Shellfish Research Laboratory of Rutgers University. Barber has
studied MSX (multinucleated sphere unknown), dermo (short for Dermocystidium
marinus, the initial classification of the parasite that causes
the disease; the parasite was later found to be of the genus Perkinsus),
and JOD (juvenile oyster disease) in oysters and QPX in clams.
In the U.S., MSX and dermo were first
observed in the mid-Atlantic region. These, along with JOD and QPX,
are now present in Massachusetts. Although health examinations are
done to prevent the spread of disease and to identify populations
at risk of significant disease impacts, pathologists concede that
the spread of disease is virtually inevitable.
When disease is prevalent throughout
a region, what explains total devastation in one shellfish grant
and not in another-even an adjacent plot? According to Smolowitz,
that might be explained, at least in part, by one or a combination
of two factors: the direct effect of environmental parameters on
the organism or the parasite, or the result of a generalized stress
on the host.
Nearly as important as understanding
shellfish diseases and the ways disease affects an animal (at the
individual, population, or species-specific level), is knowing how
to manage disease. Not to be mistaken with solutions or cures, management
options simply offer growers a way to work around disease.
As evidenced by the recent workshop,
interaction between scientists, industry, and regulators is essential-not
only in the identification of disease and understanding its impacts,
but in order to move forward with management options. A panel discussion
focusing on the future ended with a call for shellfish disease monitoring
programs throughout the Commonwealth.
Long-term monitoring programs in the
Delaware Bay and Chesapeake Bay regions have proven extremely beneficial.
Rick Karney, shellfish biologist and director of Marthas Vineyard
Shellfish Group, says monitoring programs offer "an early warning
to growers and managers." If an area tests positive for a disease,
says Karney, "growers and managers can plan what to do with their
beds. There are economic options they can take that may lessen their
losses." Recently, Barnstable County announced that it had funds
available to begin a sampling and monitoring program for a number
of sites in Massachusetts.
While scientists continue to study
these diseases in hopes of providing workable solutions for the
industry, many growers remain committed to the promise aquaculture
holds. Richard Kraus, hatchery owner and operator of Aquacultural
Research Corporation in Dennis, seemed to echo the bittersweet plight
of growers and scientists in the room when he said, "The number
one question people always ask me is, Why are you still in
the business? Well, I guess its because Im an