Algicide Offers New Way
to Fight Off-Flavor in Catfish
By
Luis Pons
April 8, 2003
 Blue-green
algae give catfish farmers the blues.
Certain types of blue-green algae, or cyanobacteria,
produce off-flavor compounds that can give catfish a
muddy or earthy taste, which keeps them from being marketable.
Affected fish are held in ponds for costly weeks until
the compounds leave their bodies.
Currently, catfish farmers choose between two chemical
solutions for fighting the algal menace. Copper sulfate
is the only algicide approved by the U.S. Environmental
Protection Agency for such use on a permanent basis.
Diuron, an herbicide containing chlorine, is used with
government permission that must be renewed yearly.
These chemicals control the cyanobacteria, but can be
toxic to most phytoplankton, including green algae.
Green algae do not cause off-flavors, and they play
a significant role as a base for aquatic food chains
and as oxygenators for pond water.
Better help may be on the horizon for many catfish farmers
in the form of a natural-based algicide that kills blue-green
algae but is much safer for other pond life.
The new algicide, the subject of a patent application,
uses a product derivative based on the natural compound
anthraquinone, found in ryegrass (Lolium perenne). It
was discovered by scientists at ARS' Natural Products
Utilization Research Unit in Oxford, Mississippi, in
collaboration with biochemist Dhammika Nanayakkara at
the University of Mississippi's National Center for
Natural Products Research.
According to ARS microbiologist Kevin Schrader, the
project's lead scientist, the algicide is ready for
additional testing in catfish-filled ponds. It has shown
great potential against its target in laboratory tests
using microplates and in large fiberglass enclosures
known as limnocorrals placed in catfish ponds. Also,
catfish exposed to it in aquarium tests stayed healthy,
Schrader says.
ARS is seeking a commercial partner to fully develop
and commercialize the technology.
The algicide targets Oscillatoria perornata, a species
of cyanobacteria prevalent in Mississippi. (That state
is home to half of the nation's catfish farms.) O. perornata
produces the musty compound 2-methylisoborneol, which
in turn gives catfish the muddy type of off-flavor.
Pond testing against O. perornata will continue to take
place in four quarter-acre ponds in Stoneville, Mississippi,
at the Thad Cochran National Warmwater Aquaculture Center,
in collaboration with fisheries biologist Craig Tucker
of Mississippi State University's Delta Research and
Extension Center.
In laboratory tests, the algicide, which if approved
may be available in 2 to 3 years, was 40 times less
lethal to green algae than diuron. Also, it rapidly
dissipated from the pond water after application, whereas
diuron can be detected for days or weeks after application.
Off-flavor problems cause the $2-billion-a-year catfish
farming industry as much as $50 million in losses annually.
See the March 2003 issue of Agricultural Research magazine
for studies on another approach to controlling off-flavors.
This research is part of Aquaculture,
an ARS National Program (#106) described on the World
Wide Web at www.nps.ars.usda.gov.
Kevin Schrader
is in the USDA-ARS Natural
Products Utilization Research Unit, P.O. Box 8048,
Oxford, MS 38677; phone (662) 915-1144, fax (662) 915-1035.
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