Algicide Offers New Way to Fight Off-Flavor in
By Luis Pons
April 8, 2003
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
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
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
ARS is seeking a commercial partner to fully develop and commercialize the
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.